Magnetism reminders Question 10W: Warm-up Exercise Teaching Notes | Key Terms | Answers
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1. What is the difference between a magnetic material and a magnet?
2. How might you magnetise and demagnetise a bar of steel?
3. How might you investigate the magnetic field around a magnet?
4. Draw the magnetic field pattern you might expect to find if a north pole is brought up to a north pole.
5. Draw the magnetic field pattern you might expect to find if a north pole is brought up to a south pole.
6. What forces act in each of the previous cases?
7. How is a stronger magnetic field represented with field lines?
Electromagnetism reminders
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8. The wire is moved down through the magnet? What does the meter do?
9. The wire is moved more quickly. What happens?
10. The wire is held still in the magnet. What happens?
11. The wire is moved up through the magnet. What happens?
Magnetic flux Question 20W: Warm-up Exercise Teaching Notes | Key Terms | Answers
This is the field of a bar magnet:
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1. Label a point X where the field is strong and a point Y where the field is weak. 2. Label a pair of points P and Q where the field directions are opposite to each other. Here is the magnetic field produced by passing a current through a solenoid:
current
3. Write down two ways in which you can change the amount of magnetic flux in the solenoid.
Here are four different solenoids generating magnetic fields:
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A
B
C D
4. Which coil creates the greatest magnetic field?
Drawing magnetic circuits Question 30S: Short Answer Teaching Notes | Key Terms | Answers
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The diagrams show magnetic flux generated in a variety of different ways. Examples include a permanent magnet, two different transformers and a wire carrying a current. 1. Draw lines (or loops) for each one to show the magnetic circuit.
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N
S
A B
D
C
2. Is there a magnetic circuit for the field of a bar magnet? Draw it.
N
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S
Sketching flux patterns Question 40S: Short Answer Teaching Notes | Key Terms | Answers
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Learning typical shapes of magnetic fields Use the examples and guidelines suggested below to learn how to make a rough sketch of the expected shape of the magnetic fields of magnets and coils. Flux goes with the flow Inside a magnet or a piece of magnetised material, the flux just follows the direction of magnetisation. It emerges from, and enters into, the iron at the poles. So start sketching at the poles, all flux lines are continuous. A line which emerges (conventionally at a north pole) enters the material again at the south pole. Flux lines never cross. Think of flux as like a fluid pumped out of N poles and sucked into S poles. Here is a sketch of the flux from a short bar magnet:
1. Sketch the flux from a longer magnet, like this:
S
N
2. Sketch the flux from a thin flat magnet, such as a magnadur magnet, like this:
N S
3. Sketch the flux from a horseshoe magnet, like this:
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Use symmetry Magnetic fields are usually very symmetrical. Think about which parts must be just like others, or perhaps their mirror reflections when drawn in two dimensions. For example, the field of the coil below can be divided into four quarters, each a copy (reflected or inverted) of the others. So you only need to draw one bit of the field.
4. Identify the similarly shaped regions of the field between a N and a S pole.
N
S
5. Identify the similarly shaped regions of the field around a pair of coils with currents going in the same direction round them. Sketch the field around and in between them.
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N and S poles of coils Looking at a coil face on, if the current goes anticlockwise that face is like a N pole and flux emerges from it. If the current goes clockwise that face is like a S pole and flux goes into it. Arrows drawn on the letters N and S help to remember this rule.
6. Identify N and S poles of this long coil: –
+
7. Identify N and S poles of this electric motor winding:
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Same environment, same flux If the pattern of current turns around one place is the same as that around another, the flux pattern in those places will be the same. 8. State how this principle tells you that the flux in a long narrow coil will be straight and uniform, like this:
9. Sketch the flux inside this doughnut shaped coil:
Put it all together Use all these ideas together to guess the shape of the flux. 10. Sketch the flux in the air and in the iron of this electric motor: 9
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pole with winding
stator
rotor
pole with winding
Magnet down a tube Question 50S: Short Answer Teaching Notes | Key Terms | Answers
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In a laboratory demonstration, a magnet is dropped down the a copper tube about 1 m long and the time of fall is measured. The experiment is repeated with the magnet timed over the same distance in free air. The results show that it takes longer for the magnet to fall through the copper tube than through the open air.
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N S
1. Use your knowledge of the laws of electromagnetic induction to explain this result.
2. Outline how you would attempt to do the experiment, suggesting how you would collect the data needed to demonstrate the effect.
3. What result would you expect if you used a glass tube of the same dimensions as the copper?
4. A student proposes cutting a thin vertical slot running the length of the tube, so as to be able to see the magnet fall. What is the objection to this idea for doing the experiment you described in answer to question 2?
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5. What result would you expect if the tube was made up of a series of short tubes each separated by a very small horizontal plastic ring?
Changes in flux linkage Question 60S: Short Answer Teaching Notes | Key Terms | Answers
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1. This coil is positioned in a magnetic field. The orientation of the coil in the field can be changed. The coil is connected to a sensitive ammeter.
A
B
Which of the following can result in the production of an induced emf? A: moving the coil up in the direction of the arrow so that it leaves the magnetic field. B: moving the coil sideways in a direction parallel to the magnetic field. C: rotating the plane of the coil through 90. D: rotating the plane of the coil through 180.
2. Coil 1 is connected in series with a switch, a rheostat and a battery. Coil 2 is connected to a sensitive ammeter.
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coil 1
coil 2
There are a number of ways in which a change can result in a current on the ammeter connected to coil 2. Write down as many of these ways as you can think of. For each way that you suggest, illustrate your answer with a diagram or a few words of explanation.
Electromagnetism Question 70S: Short Answer Teaching Notes | Key Terms | Answers
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Questions 1. Describe an experiment to show that there is a magnetic field around a long straight wire carrying an electric current and that the direction of the magnetic field is determined by the direction of the current. Use diagrams to help your description.
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2. Complete the diagrams to show the pattern of the magnetic field around each coil when a current passes through it in the direction shown.
The circuit which operates the starter motor in a car is similar to this:
C1 fixed key K metal plate spring S
soft-iron plunger P
M
C2
starter motor
solenoid contact plate
3. When key K is turned, it completes the connection between C1 and C2. Explain why this causes the soft iron plunger to move.
4. Explain why the movement of P causes a current in the starter motor.
5. When the car engine starts, key K is released so that C1 and C2 are no longer connected. What is 14
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the function of spring S?
A Hall probe measures a steady magnetic field directly by detecting the effect of the field on a slice of semiconductor material. A student sets up the circuit below to investigate, using a Hall probe, the factors which determine the magnetic flux density within a long solenoid.
A
+
–
Blu-Tack to hold Slinky
Blu-tack to hold Slinky
Slinky
probe
6. Suggest and explain two ways of varying the magnitude of the flux density in the solenoid.
7. A solenoid similar to that shown in the diagram has 100 turns connected in a circuit over a length of 0.50 m. Calculate the flux density at the centre of the solenoid when a current of 10 A flows. Use the relationship between flux density, current and number of turns N in length of coil L NI B o L 0 = 4 × 10–7 N A–2
The following questions are about the use of electromagnets to produce the large magnetic fields needed by some hospital whole-body scanners. Magnetic resonance imaging (MRI) typically requires the patient to be surrounded by a magnetic field of strength between 0.20 T and 2.0 T. In this equipment the patient lies inside a 2.5 m long solenoid consisting of 750 turns of thick copper wire carrying a current of 1000 A.
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2.5 m coil windings (750 turns) solenoid
area = 1 m2
to power supply
8. Show that the magnetic field strength inside the solenoid is approximately 0.40 T.
9. Calculate the electrical power required to maintain this field if the solenoid has a resistance of 3.0 10–2 .
10. Explain why a built-in cooling system is necessary.
Rates of change Question 80S: Short Answer Teaching Notes | Key Terms | Answers | Key Skills
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Here a coil is connected to a resistor R. The emf output from the coil is measured by a data logger connected across the ends of the resistor.
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Magnet
Grap h
Grap h
R
printer
computer
data logger
coil
These data are processed to give a graphical printout of the results obtained when a bar magnet is dropped so that it falls freely through the coil. p.d.
1 mV per cm
cm cm
B
A
timebase
C E
12 ms per cm
D
1. Explain how the trace arises.
2. Explain why the curve gets steeper from A to B.
3. Explain why the emf shown at B has a smaller magnitude than the emf shown at D.
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4. Explain why the graph has a positive and a negative section.
5. The areas under the two segments of the curve are the same. Explain why this is so.
The uniform magnetic field inside an MRI scanner has a flux density of 0.40 T. A patient inside the scanner is wearing a wedding ring. A finger movement can rotate the axis of the ring through an angle of 90 as shown in the diagram below:
6. Calculate the average emf induced in the ring if the ring diameter is 20 mm and the finger movement is completed in a time of 0.30 s.
7. Describe how the ring must move if there is to be no induced emf.
The next questions are about a student’s investigation of the magnetic flux in an iron rod. An iron rod passes through a coil which carries alternating current.
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probe coil to oscilloscope
coil carrying alternating current
A detector consisting of a probe coil wrapped around the rod is connected to an oscilloscope which displays the output trace shown in the figure below:
Sketch and explain the effect on the oscilloscope trace of each of the following changes. Each change is made separately and starts from the situation shown above. Assume that the emf and time scales on the oscilloscope remain unchanged. 8. The number of turns on the probe coil is doubled.
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9. The probe coil is positioned at the top of the rod.
10. The frequency of the alternating supply to the magnetising coil is doubled, the amplitude of the alternating current remaining unchanged.
Bugging Question 90S: Short Answer Quick Help
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The signal in a phone line is essentially an oscillating current that passes backwards and forwards, altering with the frequencies of the spoken voice of the user. A one-way telephone system microphone earpiece speaker
carbon granules listener
diaphragm diaphragm
The system shown is used to relay orders to the battlefield during a battle. As a spy behind enemy lines, your task is to intercept these orders by using a bugging device. Q Department (the Ministry’s espionage device experts) have suggested a very simple bugging device, just using some wire connected to a pair of headphones. 1. Explain how you would go about bugging the telephone system above using only the wire and headphones. Your explanation should include a detailed diagram to show exactly how you expect your bugging device to work. 20
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Transformers Question 100S: Short Answer Quick Help
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This arrangement is an experiment to investigate electromagnetic induction.
S
B
centre-zero galvanometer
1
2
State, giving a reason for your answer in each case, what is observed as: 1. The switch S is closed.
2. Switch S remains closed.
3. Switch S is reopened.
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The battery is replaced by an a.c. supply. The voltmeter can register either a.c. or d.c. S V a.c. supply
4. State what will be observed with switch S closed.
5. Give a reason for your answer.
6. Door bells in houses are often connected to the mains electricity supply through a step-down transformer. Here are two circuits to do this. A
mains input
transformer
on / off bell push
output
bell
B mains input
transformer
output
bell
magnetic bell-push operating normally-open reed switch
Which circuit do you consider to be less expensive to operate once it has been installed? Explain your choice.
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7. In petrol engines, the fuel is ignited by a spark across a gap which must be less than about 0.60 mm. To establish the field necessary for this spark, emfs of up to 40 kV are needed, using only a 12 V battery. To achieve this, two coils are wound around the same iron core. 12 V spark gap contact breaker
secondary coil
primary coil
iron core
The secondary coil is in series with the spark gap. The primary coil is in series with the battery and a contact breaker. When the primary circuit is broken, a spark is produced. The capacitor prevents sparking across the contact breaker. Explain why a large emf is induced when the contact with the battery is broken.
The circuit breaker Question 110S: Short Answer Teaching Notes | Key Terms | Answers
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1. This diagram shows the positions of the magnetic compasses when no current flows through the coil.
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magnetic north
Show on this diagram the directions in which the different compasses will point when a large current flows through the coil in the direction shown. Draw the magnetic field around the coil due to the current.
A coil is wound round a soft-iron ring.
A supply B
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2. Draw lines to indicate the direction of the magnetic flux through the iron when a direct current is passed through the coil. A second coil is wound round the opposite side of the ring.
3. State and explain how you expect the flux to be different from that produced by the single coil in question 2.
A third coil is wound round the ring and connected to a voltmeter.
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V
The voltmeter can register either d.c. or a.c. voltages. State, giving a reason for your answer in each case, what you would expect to observe on the meter when the current through the other coils is: 4. Direct.
5. Alternating.
An arrangement similar to this can be used to cut off the power to an appliance if it develops a fault, e.g. an accidental connection of the appliance to Earth.
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soft-iron rod pivoted at P springy copper live
coil with soft iron core
P
C neutral D1
D2
appliance
This is a circuit diagram, not a physical design. 6. If the effect of the fault is that the current in D1 is different from the current in D2, the power supply will be cut off. Explain how this occurs.
Eddy currents and Lenz’s law Question 120S: Short Answer Teaching Notes | Key Terms | Answers
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These questions are all about induced currents. In some circumstances, these are called ‘eddy currents’. Here is a diagram of the well-known ‘jumping ring’ experiment. If you have not yet seen it done ‘live’, 27
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ask your physics teacher to show it to you. Al ring
solenoid
1. When the circuit is closed, the aluminium ring jumps and falls back down again. Explain why this happens.
2. What happens when the circuit is broken? Explain your answer.
3. The demonstration is a lot more effective if the coil has an iron core that extends some way above the end of the coil. Why is this?
4. It is possible to make the ring hover dramatically above the coil if an alternating current is passed through the coil. How can this happen?
5. Discuss the probable effect on the demonstration of using rings of different materials and dimensions.
Here is an aluminium vane that swings between the poles of a powerful magnet. When pulled back and released, it comes to rest very quickly. When slots are cut into the vane, it swings for a long time in the same magnetic field.
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6. Explain the difference between the two results.
A solenoid is connected to a source of alternating current. Two pieces of iron, A and B, of identical dimensions are treated to look the same. When iron core A is inserted into the solenoid and the current switched on, the iron heats up rapidly and quickly reaches a temperature of 50 C. When iron core B is inserted into the solenoid and the same current passed, there is no detectable heating effect.
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Iron core
a.c.
7. Suggest and explain how the two pieces of iron differ.
Flux or flux linkage? Question 150S: Short Answer Teaching Notes | Key Terms | Answers
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Before attempting these questions, you need to be clear in your mind about the distinction between flux and flux linkage. Here are six sets of flux lines, each one passing perpendicularly through an area. The area enclosing the flux is shown by a circle. Each circle is perpendicular to the flux.
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A
D
B
E
C
F
1. Which of A to F has the most flux associated with it?
Each of these coils is situated in a magnetic field and therefore has magnetic flux linked with it. Be sure that you understand the difference between the situations in questions 1 and 2.
4 turns A
3 turns B
6 turns C
5 turns D
2. For which coil is there the greatest flux linkage? Explain how you arrived at your answer.
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Question 170S: Short Answer Quick Help
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How quantities vary together The flux in a coil is proportional to the current in that coil. The emf across a coil is proportional to the rate of change of flux linkage. The flux change in a coil is proportional to the sum over time of emf time. Check out your understanding of these relationships by completing the following sketches of graphs of one quantity against another. Graphs of flux and current producing flux 1. As shown below, the current in a coil was slowly increased to a maximum. Sketch on the graph alongside how the flux through the coil varied.
time
time
2. As shown below, the flux through a coil decreased from one steady value to another. If this happened because the current in the coil changed, sketch how the current varied with time.
time
time
Graphs of flux and emf 3. The next graph shows flux increasing steadily. Sketch next to it how the emf varies with time.
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time
time
4. Now sketch the emf if the flux grows sharply from one steady value to another.
time
time
5. Now think the other way round. The next graph shows an induced emf growing steadily with time. Sketch how the flux causing it must change with time.
time
time
6. How must the flux vary to produce a sharp ‘spike’ of emf like this?
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time
time
Sinusoidal variations 7. The current varies sinusoidally, as shown next. Sketch how the flux varies.
time
time
8. The flux varies sinusoidally, as shown. Sketch how the emf varies.
time
Alternating current generators Question 180S: Short Answer 34
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time
Teaching Notes | Key Terms | Answers | Key Skills
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Thinking about generators and induced emf An emf is induced in a coil when the magnetic flux through the coil changes. The emf in volts is numerically equal to the rate of change of flux linked, in weber turns per second. Use these principles to answer the following questions. A cycle dynamo stator with pole pieces magnet rotates
N
S
coil
A cycle dynamo has a permanent magnet which spins inside a cylindrical stator made of iron. On the inside of the stator there are pole pieces wound with coils, connected in series. 1. As the magnet turns, when is the flux through the coils greatest? When is it least?
2. The magnet rotates 10 times a second. In approximately what time does the flux through the coils go from its least value to its largest value?
3. At this speed the dynamo produces an alternating emf just enough to light a lamp, and so of the order of magnitude 2 V. Estimate the largest flux through the coils, if the coils have 500 turns in all.
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4. If the coils have cross section of 20 mm 20 mm estimate the flux density in the coils when the flux is a maximum.
Spinning a coil in a magnet A large Alnico horseshoe magnet produces a uniform flux density of 0.5 T between its poles. The poles have a cross section of 50 mm 20 mm. search coil flux density 0.5 T
50 mm
20 mm
5. Estimate the flux between the pole pieces.
6. A circular search coil of radius 10 mm is placed between the poles. How must it be placed so as to have the maximum flux through it? Estimate that maximum flux.
7. The search coil is rotated through 180, in a time of 1/10 s. Estimate the magnitude of the emf which would be detected briefly across each turn of the coil.
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8. The search coil has 500 turns. What emf is expected across the coil in the conditions of question 7?
9. If the coil is rotated continually like this, how does the emf across it vary?
A large alternator A large alternating current generator (alternator) is to be designed. It is to produce an alternating emf of maximum value 10 kV at a frequency of 50 Hz. The design has a cylindrical stator with fixed coils in which the induced emf is generated. The emf is generated by rotating a cylindrical d.c. electromagnet rotor inside the stator, at 50 rotations per second.
rotor coils
50 Hz 10 kV
stator coils
10. Calculate the maximum rate of change of flux linkage through the stator coils, in weber turns per second.
11. If the emf and flux are varying sinusoidally at frequency f, the maximum rate of change of flux is 2 f times the maximum value of the flux. Calculate the maximum flux linkage through the coils of the stator.
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12. The stator coils are wound with 300 turns. What is the magnetic flux through them, in webers?
13. The largest flux density which can be achieved in the air gap between rotor and stator is 0.1 T. What must be the design area of the stator coils?
Sketching field patterns and predicting forces Question 230S: Short Answer Teaching Notes | Key Terms |
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Answers
Combining magnetic fields Complex magnetic field patterns can be made by combining simpler ones. The result is found by superposing the fields. These questions show you how, and how to use field shapes to predict forces on currents. Fields add like vectors Magnetic flux emerges (by convention) from a N pole. It enters magnetic material at a S pole. Here is a sketch of the fields near the two kinds of pole.
N
S
The next diagram shows how the fields combine. You just choose a few suitable places, and add the vectors for the two fields. For example, in the middle the two fields are equal and point in the same direction. Their resultant is just twice as large as either alone.
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+ =
S
N
+ =
+ =
1. If lines of flux tend to shorten, what force will there be between the two magnets? How does this depend on whether one pole is N and the other is S? 2. Now sketch the field between N and S poles facing one another but offset, as below. If lines of flux tend to shorten and straighten, what forces do you expect on the magnets? Try it for two magnets at right angles, too.
S
N
N
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3. The diagram below shows the fields of two coils with current in the same direction in each. Put the two coils close together as shown and sketch the combined field.
put coils together:
Use symmetry Magnetic fields are usually very symmetrical. Think about which parts must be just like others, or perhaps their mirror reflections when drawn in two dimensions. For example, the field of the coil below can be divided into four quarters, each a copy (reflected or inverted) of the others. So you only need to draw one bit of the field, as has been done in the diagram below.
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4. Complete the field in the rest of this diagram, using symmetry. Simplify 5. Very close to a wire in a coil, the wire is nearly straight. So you can sketch the field round a straight wire by knowing what the field of the coil looks like. Sketch the field round the short section of the coil picked out in the diagram below. Describe the shape of the field near any more-or-less straight wire.
sketch the field close to the wire
Field between two long straight wires The diagram below shows the fields around each of two long straight wires. Imagine the wires completing their circuits a long way off.
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current
flux
now put the wires close together
6. Bring the wires close together and sketch the combined field of the two. If lines of flux tend to shorten, what forces will you expect between the two wires? Do ‘like currents repel’? ‘Catapult’ force on a wire in a field 7. The diagram below shows the field of a long wire carrying a current, and the uniform field between the poles of a large magnet. Put the wire in the uniform field, as shown. Draw vector 42
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diagrams to get the resultant field directions at points A, B and C.
S
N
put the wire in the uniform field (plan view) current down into the screen (paper)
A N
B
wire, current down
C
S
8. Use the answers to question 7 to explain why the combined field of wire and magnet has the shape shown in the diagram below. Why is the resultant field at a point like X in the diagram equal to zero? wire
N
S
9. If flux lines tend to shorten or straighten, predict the direction of the force on the wire. Draw an arrow to represent the force on the diagram above. Twisting forces on a coil in a field 10. The diagram below shows a rectangular coil in a uniform field. On the plan view, draw arrows for the forces on the two sides of the coil where current goes down into or up out of the screen (paper). Sketch the field round the coil if you can.
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plan view S
N
current down
current up
11. Remembering that the face of a coil where the current goes anticlockwise is like a N pole, add poles for the coil to the diagram. Do these predict the same twisting direction of the coil as the forces you have drawn?
Forces and currents Question 240S: Short Answer Teaching Notes | Key Terms | Answers
Here are two parallel wires each carrying a current in the same direction.
1. Do these wires attract or repel?
2. How can you reverse the direction of the force?
This is a loop of thin aluminium foil carrying a current and hanging vertically.
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Quick Help
aluminium loop
3. What happens when switch S is closed? Explain your answer.
4. If the foil were perfectly flexible, what shape would it take up?
5. If you can, explain your answer to question 4.
6. The arrangement generates some magnetic flux. Which way does the flux go?
This is a demonstration you have probably seen before
N
_ +
S
and this is a side view of the magnet and the wire. 45
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N
wire with current into screen / paper S
7. Draw the magnetic field of the wire and the magnet separately.
8. Now draw the combined field you will get if the wire carrying the current is positioned between the poles of the magnet. You will need to draw quite a large diagram to show this clearly. Use your answer to explain which way the wire will move under the combined effect of the two fields.
An ancient demonstration, now no longer carried out (why?).
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I
pool of mercury
Current passes into the centre of the star via the axle and out though whichever star point is in contact with the mercury trough. 9. Explain how this arrangement produces continuous rotation.
Thinking about the design of a simple d.c. motor Question 250S: Short Answer Teaching Notes | Key Terms | Answers
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1. Describe a simple experiment to show that a magnetic field exerts a force on a wire carrying an electric current.
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Here are the relationships between the force and the current and between the force and the length of the conductor in the field.
current in conductor
length of conductor
A magnetic field exerts a force of 0.25 N on an 8.0 cm length of wire carrying a current of 3.0 A at right angles to the field. 2. Calculate the force the same field would exert on a wire 20 cm long carrying the same current.
3. Calculate the force the same field would exert on three insulated wires, each 20 cm long and held together parallel to each other, each carrying a current of 3.0 A in the same direction.
4. Use this diagram to help you relate the answers you have calculated to the design of a simple electric motor.
N
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S
What are the design features which will make a good motor?
Emf in an airliner Question 260S: Short Answer Teaching Notes | Key Terms | Answers | Key Skills
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These questions are about the potential difference induced across the wings of an aeroplane flying through the Earth’s magnetic field. The charge on an electron = – 1.6 10–19 C An airliner is flying due east from North America to Europe. The Earth’s magnetic field acts at 70 to the horizontal, and has a strength of 1.7 10–4 T.
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North
Earth’s B-field
–
70°
+
As the aircraft flies through the field, the north-pointing tip of the wing becomes positively charged and the south-pointing tip becomes negatively charged. 1. Explain why the wing tips become charged. Assume that the wings act as continuous electrical conductors.
2. On the diagram above, show the direction of the component of the Earth’s magnetic field which is responsible for this horizontal movement of charge along the wings. 3. Calculate the magnitude of this component.
4. The aircraft’s speed is 270 m s–1. Calculate the horizontal component of the force exerted by the Earth’s magnetic field on an electron in the wing.
5. The wing span of the aircraft is 60 m. Calculate the potential difference induced between the tips of the wings.
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The shape of the Earth’s magnetic field is as if there were a bar magnet at the centre of the Earth, aligned approximately along its rotational axis. 6. Explain why there is no significant voltage induced between the wing tips when the aircraft flies from west to east over the equator.
The Birmingham maglev Question 270C: Comprehension Teaching Notes | Key Terms | Answers | Key Skills
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This passage is about maglev, a guided transport system that was built to carry passengers 620 m between Birmingham Airport and Birmingham International railway station. Passengers are carried in a single carriage, which runs on a special raised track. Lifted above the rails by the force of magnetic attraction, maglev is propelled along a rail by the forces created by electromagnetic induction using a linear induction motor (LIM). A complex and innovative control system has enabled principles of electromagnetism to be put to practical use in the maglev vehicle. The suspension system, propulsion system and control system form maglev’s ‘engine’, which is located underneath the carriage, so a separate locomotive is not needed. Read the following passage carefully and then answer all the questions. Suspension system Maglev has eight electromagnets below the suspension rails of the carriage so the carriage is lifted by magnetic attraction.
suspension rail electromagnet
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They are arranged as a pair at each corner, presenting magnetic poles to the underside of the suspension rails. Adjacent poles are of opposite polarity to maximise the levitation. This diagram below shows the arrangement of the electromagnet and the suspension rail. steel suspension rail
air gap 15 mm
suspension electromagnet
When the carriage stops at a station it rests on pads so that it feels solid when passengers step on and off the platform. This also simplifies the task of the magnets and control systems which otherwise, if all the passengers were on the same side of the carriage, would have to cope with an extremely uneven distribution of load. The track The track is elevated above ground level. Because there is no contact between the moving carriage and the track (except for the electricity pick-ups) the ride is smooth and quiet. It also means that there is little wear, and maintenance is minimised. The track consists of five rails. Two are steel rails for the suspension system, two are insulated aluminium conductor rails from which sliding contacts pick up maglev’s electricity supply at 600 V d.c., and the central T-section rail is the LIM reaction rail, in which eddy currents are induced by alternating currents through the coils of the linear induction motor. linear induction motor vehicle carriage chassis
suspension rail suspension electromagnet conduction rails
LIM reaction rail
The LIM reaction rail is built in sections to allow for thermal expansion. The overlapped joins provide an unbroken return path for the magnetic flux and continuous paths for the currents induced by the 52
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motor. Control systems One of the features which the operators of Birmingham International Airport specified at the design stage was that the vehicle should be physically stable to give the passengers a smooth quiet ride. Another feature was safe easy access for passengers between the carriage and the station platform. To provide this, the gap between the carriage and the platform is small, but the control system must ensure that it is never zero! Several control systems monitor the carriage position to ensure that it is at the right height and to prevent it rolling from side to side, back to front, or twisting sideways. The two magnets at each corner of the carriage are controlled separately to adjust the height of the carriage, by altering the size of the air gap between the magnet and the suspension rail. Pitch and roll are prevented by controlling the height of the carriage at each corner. Controlling all four pairs of magnets relative to each other is much more complex than controlling one in isolation. It is a bit like trying to adjust a chair so that all four legs are flat on an uneven floor! The control system has to cope with acceleration and deceleration of the carriage, and with the wind, which provides a force which varies in size and direction. People moving about in the carriage will change the weight distribution, so that forces on each magnet pair will change. The force of attraction on each pair of magnets depends on the size of the air gap. As the air gap increases, the force of attraction will decrease. To return the carriage to the required air gap, the force must be increased. Similarly, if the air gap decreases, the force will increase, decreasing the gap still further. In this case, the force must be decreased to prevent the magnets sticking to the rail. (Adapted from: Dale S 1994 The maglev train Physics Review (May). By permission of Philip Allan Publishers Ltd.) Now answer the questions below Questions 1 to 3 are about the suspension system. 1. On the copy of figure 4 below, the current direction and one of the poles have been labelled. Label any other poles of the electromagnet N or S as appropriate. steel suspension rail
S
suspension electromagnet
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2. On the same diagram, draw lines to represent the magnetic field responsible for the suspension system. 3. Explain what would happen to a moving maglev train in the event of a power failure.
Questions 4 to 7 are about the track. 4. Explain why steel is chosen rather than aluminium for the suspension rail.
5. Explain why aluminium is preferred to steel for the sliding contacts.
6. Suggest a suitable material for the LIM reaction rail. Justify your choice.
7. Draw a labelled sketch to show how two sections of the LIM reaction rail might be joined so as to allow for thermal expansion while providing continuous paths for magnetic flux and eddy currents.
Questions 8 to 11 are about the control systems. 8. Give two reasons why it is desirable to keep the height of the carriage above the track constant.
9. The passage states that ‘the force of attraction on each pair of magnets depends on the size of the air gap’. Why must the air gap not be too small? 54
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10. A disturbance to the system causes the air gap to decrease slightly. Draw a diagram to summarise how the attractive force and the size of the air gap affect one another in the absence of a height control system (last section of the reading). This diagram should consist of labelled boxes linked by arrowed lines.
11. Draw a similar diagram to represent how the height control system modifies this chain of events.
Explaining with induction Question 130X: Explanation–Exposition Teaching Notes | Key Terms | Answers | Key Skills
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Each device, described by one of the three sheets below, uses the principles of electromagnetic induction to work. Use your knowledge of electromagnetic induction to draw up a briefing sheet for one of the devices. Make full use of the Advancing Physics CD-ROM as you do so. Three devices
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Measurement of turbine speed Principle: Changes in magnetic flux induce emf. This principle is used to measure the speed of turbines in a power station. Speed sensing is important at many stages during the production of electricity. If the speed of the turbine is too high then the amount of fuel being burnt / volume of water being used (etc) can be reduced to slow the turbine down.
A simple method to measure the speed of the turbine is to use a toothed ferrite wheel rotating near a coil of wire adjacent to a permanent magnet
ferrite wheel coil permanent magnet
The rotating teeth of the wheel change the magnetic flux through the coil. Therefore as the wheel rotates the induced emf in the coil of wire will change. The flux lines when a tooth is not opposite the coil will be as shown right.
permanent magnet
permanent magnet
The flux lines when a tooth is opposite the coil will be as shown left.
Thus as the teeth move round the flux through the coil is changing. This changing flux generates an emf in the coil which fluctuates with a frequency equal to the number of teeth which pass the coil each second. If the rotating wheel is driven by the rotating turbine then the speed of the turbine can be measured.
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Vibration detectors Principle: Relative motion is required between a magnet and a coil to produce an emf In power stations large turbines are used to produce the power. Since these turbines are turning it is important that they are aligned correctly so there is no vibration. One method of measuring the vibration is to use a magnet which moves relative to the coil. This relative motion produces an emf. The vibration detectors are placed round the casing of the turbine shaft. As the turbine vibrates so does the casing. The device consists of a permanent magnet and a coil. The magnet is attached to the casing and the coil is independent. When the casing vibrates the magnet moves relative to the coil, producing an emf. This current is detected by an a.c. voltmeter. The greater the amplitude of vibration the greater the emf detected. magnet coil V casing
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The current tracer Principle: Changing magnetic flux induces an emf.
In machinery that is already working it is difficult to break a circuit so that a current can be measured. It is also difficult to find those wires which are carrying current and those which have developed a fault and are not. A device has been designed that uses changing flux to detect current flow. The device is basically in the form of a transformer. The core can be dismantled so that the wire in position can be enclosed without breaking it.
coil of N turns
ferrite core
current in wire The magnetic field due to the current in the unknown wire being tested causes a flux in the ferrite core. If this is a changing current then the emf at the coil will be: V = N d dt where is the flux through each turn of the coil. Since the flux produced by the wire depends on the current in the wire we can measure this current.
An outline structure Whichever device you choose, try the following structure: 1. Identify the current-turns that produce the flux. 2. Show where the flux is linked. 3. Say how the changes in flux linkage happen. 4. Relate this change in flux linkage to the induced emf. 5. Say how the device uses the induced emf. 6. Comment on the design compromises likely to be made in any realisation of the device.
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A bicycle speedometer Question 140X: Explanation–Exposition Quick Help
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This question is about a proposed design for a bicycle speedometer. Here is how it works in principle: A plane coil rotates about a vertical axis in a uniform horizontal magnetic field. The voltage induced in the rotating coil is measured between X and Y. The coil passes through a position parallel to the field to a position perpendicular to the field, one-quarter of a revolution later.
B-field
B-field
X
X
Y
Y B
A
1. Explain why a voltage is induced across XY as the coil rotates.
This is how the induced voltage varies with the position of the coil: Position of coil relative to field
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2. Explain why the induced voltage is greatest when the plane of the coil is parallel to the magnetic field.
3. Explain why the voltage alternates.
The geophone Question 160X: Explanation–Exposition Quick Help
Teaching Notes | Key Terms | Hints | Answers
This device is a transducer that uses electromagnetic induction to measure movements of the Earth’s surface.
spring
aluminium former output
coil
magnet
spike
ground
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Thinking through the design 1. Can you suggest how an output is generated? Draw a sequence of diagrams showing changing flux linkage to help explain your answer.
2. Explain clearly what is meant by the sensitivity of the device. Suggest ways in which you can increase the sensitivity of this geophone.
3. Explain clearly what is meant by the resolution of the device? Suggest ways in which you can increase the resolution.
4. How can you alter the device so that it can detect oscillations of higher frequency, that is, reduce the response time?
Electronic ignition Question 190X: Explanation–Exposition Teaching Notes | Key Terms | Hints | Answers
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Use your knowledge of the behaviour of magnetic flux, and of the relationship between changes in magnetic flux linked, to expand on this dialogue below, derived from the technical pages of an auto 61
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magazine. A question ‘However, on looking under the distributor cap I find no points, but some type of sensor connected to a ballasted coil. Would you please supply a diagram of the circuitry and some basic fault diagnosis for this type of ignition system?’ An answer ‘…is fitted with an electronic ignition system. These have been very reliable and need little servicing. The drawing here is a schematic diagram only. What you will see with the distributor cap off looks quite different. magnetic pulse generator ignition coil
ignition switch
to distributor HT lead
reluctor
transistor
magnet
battery
pickup coil
The circuit shows a magnetic pulse generator, used to trigger the transistor in the circuit. The transistor switches the ignition primary circuit, so these components take the place of the distributor contacts.…these ignition systems are best left alone, with maintenance restricted to keeping the coil tower and outside of the distributor cap clean.’ Explanations 1. Explain how an emf is induced in the pickup coil.
2. Why must there be a diode in the circuit?
The transistor is a current-operated switch, here triggered by the pulses of current driven by the induced emf. The transistor allows a current driven by the 12 V of the car battery to pass through the left-hand coil of the pair labelled ignition coil. 3. Explain how this 12 V pulse becomes a high-tension (HT) pulse (high p.d.), suitable for providing 62
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a spark in the cylinder.
The induction motor Question 200X: Explanation–Exposition Quick Help
Teaching Notes | Key Terms | Answers
A motor driven by a rotating flux These questions are about how an induction motor works. Rotating flux iron stator
3 pairs of coils carrying current in different phases
flux lines
1. An induction motor has a stator with coils arranged to produce a rotating magnetic flux. The figure shows the flux at one instant of time. The flux is rotating anticlockwise. On a copy of the figure below, sketch the flux when it has turned through about 60.
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Lenz’s law and rotating flux A solid cylindrical copper rotor is put in the rotating flux. It rotates, being ‘dragged around’ by the rotating flux.
copper rotor spins
2. Think of the rotor held still. There are eddy currents induced in the rotor by the rotating flux. Lenz’s law says that their effect must be to reduce the effect causing them. The effect causing them is the relative rotation of flux and rotor. Explain why Lenz’s law implies that the rotor should turn. 3. Suppose you spin the rotor at exactly the speed of the rotating flux. What can you say about eddy currents in the rotor? Direction of rate of change of flux 4. The next diagram shows just the rotor, and the flux at one moment of time, rotating anticlockwise. Explain why the maximum rate of change of flux in the copper is in the direction shown in the diagram.
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flux rotor maximum rate of change of flux in this direction
5. The diagram below shows eddy currents in the rotor, produced by the rate of change of flux. Explain why the eddy currents circulate in a loop as shown. rotor rate of change of flux
eddy currents
Forces between poles 6. The diagram below shows the poles produced in the rotor by the eddy currents at this instant. It also shows the poles produced on the stator by the original rotating flux. With the poles as shown, which way will the rotor turn? Why? stator S rotor
N
S
eddy current
N pole from rotating flux
7. In drawing the last diagram, you used Lenz’s law to decide that the direction of the eddy currents must produce N and S poles on the rotor as shown, and not the other way round. How does Lenz’s law predict this?
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Improving the motor… 8. A rotor made of solid steel is tried instead of the rotor made of solid copper. State one reason why it might do better, and one reason why it might do worse than the copper. 9. Look up in the Advancing Physics A2 student’s book, chapter 15, what a ‘squirrel cage’ rotor looks like. Explain why it might do better than either a solid copper or a solid steel rotor.
A variable-speed linkage Question 220X: Explanation–Exposition Quick Help
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This device provides a variable-speed output from a constant-speed motor. It does so by providing a variable grip between the left- and right-hand shafts. The following questions allow to you to explore how the device works.
small gap (air / liquid) rotor
input
output
to constant speed a.c. motor
armature
magnetic field coil
Thinking through the design When the field coils have current passed through them, a series of poles appear on the outer face of the rotor. 1. Draw a possible flux pattern, assuming that there is just one pair of coils.
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2. The rotor is not rotating at the same rate as the armature. Use this to explain how eddy currents are induced in the armature.
3. What will happen to the magnitude of these eddy currents as the flux generated by the field coils is increased. Explain your answer.
4. How will this change in the eddy currents change the grip that the spinning armature exerts on the rotor?
5. Why does the rotor spin, i.e. how does the armature drag it round?
ICT driven by precision motors Question 280X: Explanation–Exposition Teaching Notes | Key Terms | Answers
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The laptop on which I write this is full of chips – and motors. There are motors to spin up the disk drive, keep the whole thing cool using a fan, spin the CD-ROM player, position the laser in the CD-ROM, and probably a few more well hidden. Motors for a purpose 1. List all the motors in a computer near you. 67
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2. Identify the function of three, suitably different, motors and give some of the design factors that each must satisfy.
Now choose one motor. 3. State which motor you have chosen. Say how the design requirements you have outlined might be implemented in terms of the design of the electric and magnetic circuits.
People and electromagnetism: The discoverers Reading 10T: Text to Read Teaching Notes | Key Terms
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The attractive force of amber on small non-conducting objects has been known since ancient times. In 1600 William Gilbert published his book On Magnetism. People like Stephen Gray, Charles Dufay and especially Benjamin Franklin, during the eighteenth century, made major advances in the understanding of static electricity. Finally, during the nineteenth century the connections between electricity and magnetism were uncovered.
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In the candle-lit world of the late eighteenth century, the Italian physiologist Luigi Galvani (above) at the University of Bologna noticed that animal muscle tissue contracted when touched with metals (he worked with the muscles in frogs’ legs). Galvani concluded that animal tissue made electricity. From the modern point of view he wasn’t entirely wrong, since nerve cells do generate potential differences which act as signals to the muscles.
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But Alessandro Volta (above) at the University of Pavia showed that Galvani was wrong about the origin of the potential difference in his experiments. It was due to the contact of two different metals. From this insight Volta invented the ‘voltaic pile’, the predecessor of the modern electric battery. He described it in a letter of 20 March 1800 to the Royal Society in London. The unit of potential difference, the volt, is named after him. The argument between Galvani and Volta was fierce and public, made more bitter by the fact that Galvani was opposed to Napoleon Bonaparte, whilst Volta supported him. But between them they had found a new way to make electric currents. Experiments with electrostatic charges had previously aroused public interest throughout Europe, with shows of ‘electric fire’ at fairs and markets. The ‘electric kiss’, in which a girl secretly connected to an electrostatic source gave an unexpected shock to a male volunteer from the crowd, was a popular exhibit. The potential differences were large (hence sparks and slight shocks) but the charges and currents were small, so of little practical use or danger. Even when charge was stored in capacitors, it would flow for only a short time when discharged. The voltaic cell changed things completely, providing a source of continuous, large currents (‘galvanic electricity’) at low potential differences. Soon currents from voltaic piles were being used to produce chemical changes, opening up the new science of electrochemistry. Every laboratory had to have a voltaic pile, and new and better kinds were invented. The technology of making better batteries is, 200 years later, still a matter of active development, for example for electric cars. Volta’s electric cell was sufficiently sensational for Napoleon to invite him to Paris to demonstrate it. The large electric currents it made available soon led to the discovery of a connection between electricity and magnetism.
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Hans Christian Oersted (above) was a professor of physics at Copenhagen, famous for his public lectures on electricity. In 1820, during such a lecture, he rediscovered an effect seen earlier by the Italian Romagnosi, that an electric current deflects a compass needle. The discovery was not as accidental as it is sometimes said to be. Oersted was convinced that there must be deep connections between the forces of nature: currents produced heat and light – why not also magnetism? So Oersted found something he was looking for. It’s easy to forget that these new experiments on electric current needed something nowadays taken for granted: a supply of insulated conducting wire. No supplier stocked such an item (‘No call for it, sir!’). One early source of wire for experiments was the gold braid used on naval uniforms. Wires in coils were insulated in various ways, using varnish, cloth and twine.
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The American Joseph Henry (above) had a crucial idea, which made insulation especially important. It was that the more turns in the coil the better. So he used silk thread unravelled from his wife’s wedding dress to insulate several hundred metres of thin copper wire wrapped round and round an iron core, to build an electromagnet able to lift a hundred times its own weight. His wife’s feelings are not recorded. Soon he had an electromagnet able to lift 1 tonne. And this magnet could be turned on and off. Henry saw how such a thing could be used in telegraphy, as well as in lifting heavy weights. He also had hold of an important principle. The amount of flux increased with the number of current-turns producing it. Turns matter as much as current.
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André Marie Ampère (above) effectively founded the science of electromagnetism. Showing early talent, Ampère had an unhappy personal life, and leapt from one intellectual question to another, in fields as diverse as mathematics, psychology, philosophy and chemistry. But the news of Oersted’s discovery of magnetism from electricity so inspired him that, for seven years, Ampère devoted all his attention to the interactions between electricity and magnetism. He demonstrated the force which acts between two parallel, current-carrying wires, and correctly predicted that a coil of wire carrying a current would behave just like a bar magnet. His ideas also led to the development of a moving coil galvanometer. In 1827 Ampère published precise mathematical descriptions of electromagnetism. The first to distinguish clearly between current and potential difference, Ampère has been honoured by having the unit of current named after him.
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Wilhelm Weber (above) was a German who made important advances in the measurement of electricity and magnetism by devising sensitive instruments, and defining electric and magnetic units. This work, much of it done collaboratively with an older physicist, Karl Gauss (1777–1855), was very important to the development of the science of electromagnetism. Initially interested in geomagnetism, they incidentally invented the telegraph. In 1832, to test whether Ohm’s law applied over long distances, they constructed a conductor a full kilometre in length. This itself was a great technical feat at that time, as neither pure copper nor even insulators had yet been made. Sending voltage pulses down the line, they soon realised that the device could be used for signalling.
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Faraday made the really key discovery, in 1831, that an emf can be induced by a changing magnetic flux. Later he also devised the first electric motor. Although this work was purely experimental, it made possible the development of a modern electrical supply industry. There were still a raft of practical difficulties to overcome, solved with great ingenuity by several key inventors and engineers.
Michael Faraday’s vision Reading 20T: Text to Read Quick Help
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Michael Faraday was a very unusual person. Here you can read about: how his religion helped him to discover electromagnetic induction how he tried and failed to discover ‘electro-gravitational induction’ how a mistake he made led him to an important new theory.
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Faraday knew what he was looking for In his notebook for 29 August 1831 Faraday describes the experiment which showed how electromagnetic induction works. It is surprisingly complicated. An iron ring. Several coils of wire wound laboriously over the ring. One coil connected to a battery. A second coil led to a wire near a compass needle. And even then, the only effect was that there was a current in the second coil if the current in the first coil was started or stopped. It’s impossible to believe that Faraday hit on all this by chance, by just happening to put the right apparatus together. So how did he come to think of this set-up? Nobody knows. But we do know that he had been looking for effects of magnetic fields on electrical conductors for at least 10 years, since he heard of Oersted’s demonstration of the magnetic effect of a current. He was sure that if currents made magnetic fields, magnetic fields should make currents. He wrote: …it appeared very extraordinary, that as every electric current was accompanied by a corresponding intensity of magnetic action at right angles to the current, good conductors of electricity, when placed in the sphere of this action, should not have any current induced in them… So it’s pretty clear that he knew what he was looking for, and was sure it was there to be found. Faraday the Sandemanian To understand Faraday better, you need to know more about his religious beliefs. He belonged to a minute sect called the Sandemanians, which broke away from the Scottish Presbyterian Church. Their central belief was that the sole duty of people is to live according to a simple, honest and careful reading of the Word of God. General rules for right living had to be worked out from particular passages of Scripture. Is doing science is just a matter of making an honest reading of the ‘Book of Nature’? The idea merits thoughtful discussion. The Sandemanians also believed in the unity of the Universe, as an expression of the powers of one God. And Faraday identified ‘powers’ of God with forces of Nature. No wonder he was sure that all these forces of nature were connected. Faraday won’t give up even in face of the facts Faraday was devoted to facts; to simple clear empirical evidence. But this is not the whole truth. He was also sure that all the forces of Nature must somehow be connected. In the case of electricity and magnetism he was stunningly successful, discovering electromagnetic induction. Following the same belief, he was also sure that there must be a connection between the other great known force, gravity, and electricity and magnetism: The long and constant persuasion that all the forces of nature are mutually dependent, having one common origin, or rather being different manifestations of one fundamental power, has made me often think upon the possibility of establishing, by experiment, a connexion between gravity and electricity… Experimental Researches: paragraph 2702 His approach was experimental. Faraday describes a long series of experiments, involving dropping coils under gravity, and making masses oscillate inside coils. Occasionally there seemed to be positive results, but always when tracked down they turn out to be ‘fallacious’. In the end he found nothing. 73
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So what should Faraday have done? The orthodox story about the nature of science says that he should have given up his belief in a connection. A theory is not supported by the facts? So much the worse for the theory. Did give up he? Not on your life. Instead, he ends his account of his many failed experiments with the following stunning remark: Here end my trials for the present. The results are negative. They do not shake my strong feeling of the existence of a relation between gravity and electricity, though they give no proof that such a relation exists. Experimental Researches: paragraph 2717 Faraday knew where he wanted to look, and indeed people are still looking in the same corner today, now under the name of Grand Unified Theories. Full success eludes them too, so far. Faraday gets the right answer from a mistake Nobody is in favour of sloppy thinking, and Faraday was no exception. But it remains the case that one of the most important new ideas ever introduced to science was introduced by Faraday on the basis of a rather simple mistake. Faraday was the first to dare to propose that magnetic fields should be thought of as real things. He thought that the magnetic field was a real something or other present in the space around a magnet or coil. Here is his remarkable reason for thinking so: It appears to me, that the outer forces at the poles can only have relation to one another by curved lines of force through the surrounding space; and I cannot conceive curved lines of force without the conditions of physical existence in that intermediate space. Experimental Researches: paragraph 3258 The idea of curvature being special to magnetic fields is just a mistake. Despite his experimental gifts and imaginative power, Faraday was no mathematician, and understood nothing of vector superposition. He thought that gravitational and electrical ‘lines of force’ were necessarily straight, not knowing that the resultant field of a number of sources can be represented as curved lines. But, however wrong its grounds, Faraday’s vision was essentially right. He rejected a mechanistic Universe made purely of particles in motion in empty nothingness. His essentially religious belief in the ‘powers’ of God being made manifest in Nature led him to look for such powers as real constituents of the world. Nowadays we regard fields as a basic part of the description of Nature. In modern quantum theory they are behind everything, including ‘real’ matter. Furthermore, this insistent imaginative vision led Faraday to one of his greatest speculative ideas: that light might somehow be electromagnetic. This really was ‘making the field real’. The view which I am so bold as to put forth considers, therefore, radiation as a high species of vibration in the lines of force which are known to connect particles and also masses of matter together ... The occurrence of a change at one end of a line of force easily suggests a consequent change at the other. The propagation of light, and therefore probably of all radiant action, occupies time; and, that a vibration of the line of force should account for the phenomena of radiation, it is necessary that such vibration should occupy time also. Experimental Researches: Thoughts on ray-vibrations 1846 It took James Clerk Maxwell to turn this into the electromagnetic wave theory, but the seeds lay in Faraday’s imagination. The argument to get there was wrong, but the answer was right enough to sow the seeds of much of our present understanding. References 74
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Faraday M 1965 Experimental Researches in Electricity (New York: Dover) (three volumes bound as two) Cantor G 1991 Michael Faraday, Sandemanian and Scientist (London: Macmillan) Footnote The Sandemanians were a minute religious sect, numbering only a few hundred in Faraday’s time, which had broken away initially under the leadership of a Scot, John Glas, from the Scottish Presbyterian Church. Then, and under the later leadership of Robert Sandeman, the sect opposed all worldly involvement, holding that the sole duty of people is to live according to a plain and literal understanding of the Bible. Because that understanding was held to be simple and direct, with no room for disagreement, the sect insisted on complete agreement of all its members. Those who could not accept the common view were excluded, ‘put away’.
Transformers: Designed for a purpose Reading 30T: Text to Read Teaching Notes | Key Terms
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The design of a transformer reflects its use. Transformers are used at a wide range of frequencies, from 50 Hz for power transmission to several megahertz in radios. One design feature that changes over this range of applications is how the transformer core is made. Magnetic core transformers From just a few hertz to about 1 MHz magnetic core transformers are used. The magnetic core has a higher permeability than air. This means that the core can be smaller and still have the same permeance. This reduction in size leads to considerable savings in running costs and in the materials needed to construct the transformer. The core is usually made up of silicon-steel laminations, each one about one-tenth of a millimetre thick. A similar effect is achieved by winding the core out of ribbons of the magnetic material. This lamination reduces eddy current losses. Such currents, induced by the changing magnetic fields, would cause heating. Of course the fact that some transformers must be cooled in oil implies that these losses are not completely eliminated. Amorphous cores Towards the upper end of this frequency range losses due to eddy currents start to increase. The solution is to use an amorphous metal core. These cores are made from a quickly cooled alloy of iron, silicon and boron. This has a very much smaller conductivity than the laminated core, reducing eddy current heating. Such cores actually have a lower permeability than laminated cores, so transformers need to be larger. This extra production cost is easily reclaimed in reduced running costs. Air core transformers At even higher frequencies the benefit gained from the higher permeability of a magnetic core is offset by quite unacceptable eddy current losses. Also at these higher frequencies the core cannot 75
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magnetise and demagnetise quickly enough. This is when air core transformers are used. Have I understood this reading? 1. When would an air core transformer be preferred and why? 2. Why would an amorphous core help reduce eddy current losses? 3. Why does a magnetic core allow the transformer to be smaller?
People and electromagnetism: The inventors and engineers Reading 60T: Text to Read Quick Help
Teaching Notes | Key Terms
The basic science of electromagnetism was in place by 1831. Within a year of Faraday’s discovery of electromagnetic induction, an instrument-maker in Paris made a hand-turned generator in which a horseshoe magnet rotated. By 1834, small rotating-coil generators were being manufactured commercially in London. Yet there were still many practical obstacles to overcome before electricity would become a major source of energy. Not least, these early generators produced alternating current, something that was regarded as a disadvantage in a world that knew only batteries.
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A farmer’s son, Werner von Siemens, learned his physics and mathematics as an engineer in the Prussian army. His parents died when he was still a young man; at his mother’s deathbed, he promised to support his 13 brothers and sisters. He made his first fortune by selling the technology of silver-plating to English silverware factories, and went on to further inventions. Repairing a dial telegraph for the Prussian army, Siemens realised how important communications systems were. He designed a new and smaller apparatus for sending and receiving messages. With a partner, he set up the beginning of what today is a global corporation. A weak point in telegraph lines of the time was the insulation of the copper wires. Siemens developed a process for mass-producing insulated wire by coating it in gutta-percha (a substance obtained from the latex of trees of the sapodilla family in 76
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Malaysia). Other people went on to invent better ways of insulating wire, a technology which later proved essential for power cables. In 1866 Siemens invented a dynamo which proved to be a practical way of generating electricity. By 1878, the first streets were lit with carbon arc lamps and a year later the first electric locomotive was introduced in Berlin.
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Thomas Edison was a prolific American inventor, who patented some 1000 ideas. Expelled from school because he was thought dim, Edison became a railroad newsboy. Soon he was printing his own newspaper on the Grand Trunk Railway. He worked as a telegraph operator during the American Civil War. As the telephone began to take over from telegraphy, Edison realised that telegraphy was but one of many applications of electricity. He turned his attention to electrical matters, devising ever better forms of d.c. generator to supply household electricity and inventing the electricity meter to charge for its use. In the 1880s, Edison in America and Joseph Swan in England finally solved the technical problems of producing light with a heated filament. Gradually people shifted from gas towards the new technology of lighting. By 1900, the advantages of filament lighting for domestic use were generally recognised: it is clean, safe and reliable. But electricity-generating companies could not make a profit with lighting alone: lighting is needed only in darkness, and the demand periods, both daily and seasonally, were too short.
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Croatian-born, Tesla was very clever as a child and grew up with a liking for experimentation. As a boy, with the help of an umbrella, he tried to fly like a bird, jumping off a barn roof – and spent 6 weeks in bed recovering. At school he was particularly good at mathematics, and read every book in the school library. He went to a university in Austria to study engineering, and then worked as a telephone engineer, first in Budapest and then in Paris. While walking in a park in Budapest he had a vision of a rotating magnetic field, and realised that this was the design for an a.c. induction motor. In 1884 he went to the USA and introduced himself to Edison. Committed to d.c. electrical systems, Edison rejected Tesla’s scheme for making an a.c. motor. Tesla then set up his own company, Tesla Electric, and the a.c. motors that he manufactured sold well. In Britain, at about this time, electric 77
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motors showed an advantage over steam engines when they were used to power London’s first tube trains. Tesla soon patented ideas covering the generation, distribution and use of alternating electricity, and started to manufacture big a.c. generators. By now a.c. showed its advantage: using transformers, electricity could be transmitted at high voltage, reducing energy lost by heating up the cables. Edison resorted to dirty tricks and ruthlessly tried to drive Tesla, now his competitor, out of business.
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At the age of 15 George Westinghouse ran away from home, to fight for the North in the American Civil War. His father was a manufacturer of farm machinery, and Westinghouse later returned to work in his father’s workshops. In 1865 he applied for the first of his more than 400 patents, for a steam railway locomotive. He went into business as the Westinghouse Air Brake Company, manufacturing improved brakes for trains. Later he founded the Westinghouse Electrical Company and in 1885, after hearing about Tesla’s lecture to the American Institute of Electrical Engineers, bought up Tesla’s patents for $1 million. It soon became obvious they were worth a great deal more. The Chicago World’s Fair in 1893 was powered with Tesla’s a.c. generators. This resulted in Westinghouse winning a contract to build a generating station at Niagara Falls, producing sufficient electricity for the town of Buffalo, 22 miles (35 km) away. Its success demonstrated conclusively that with an a.c. system, electricity could be generated in remote locations and brought into cities, where electricity demand was growing. Industry gradually turned its back on steam power, with its noisy and dangerous systems of shafts, pulleys and belts for each machine, and embraced cleaner and quieter electric motors. By 1888 Tesla realised that Westinghouse had exploited him. His attention turned to high-frequency a.c., including something which came to be called the ‘Tesla coil’, and many other inventions. In later years, his imagination became too fantastic, and he gradually undermined his reputation. He died without friends or family.
A wide variety of motors Reading 80T: Text to Read Teaching Notes | Key Terms
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All motors operate by a turning force or torque being generated. This may often be thought of as the 78
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force on a current-carrying conductor. In other motors it might be best be discussed as due to the attraction and / or repulsion between magnetic poles. This usually gives rise to a force acting along a line joining the two poles as in the case of the relay, but by suitable timing, these forces can lead to rotation. d.c. machines There are many machines that can now be classified under this heading, but we will just include the moving coil motor that most students will be familiar with from GCSE. These motors come with permanent magnets or field coils but all use a commutator to provide a current path to the rotor coil. Despite the apparent disadvantages of brushes, these motors are still very popular and find many applications ranging from small motors for models and toys to starter motors on cars and the motors on electric trains. This image shows a small d.c. motor with magnets and rotor.
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The brush / commutator arrangement is very reliable and cheap. There are many applications where the radio frequency interference can be tolerated or eliminated in other ways. The commutator does limit speed and wear and tear do take their toll. A close-up showing the brushes.
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The rotor winding on d.c. motors is commonly known as the ‘armature’. In most windings these are mounted on a laminated, magnetically soft material that increases the flux in the armature coils. Developments in magnets have also made them a good choice. Devices we now accept as commonplace such as the ‘Walkman’ type of radio-cassette became possible because of the development of small motors using small ceramic magnets. Permanent magnet d.c. (PMDC) motors are used for several motors and drives used in cars – windscreen wiper, fan motor and window winding mechanisms. PMDC motors are now being used instead of motors with field coils because of the high strength of modern magnets. Field coils are a common alternative to permanent magnets. Although the armature and field coils can be connected totally independently they are also found with the coils in series or parallel (shunt wound) arrangements. A rotor from a car windscreen-wiper motor. 79
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Taking control Control of d.c. motors can be very simple provided we remember that if the armature is stalled the current will be a maximum and may damage the armature windings. With little or no load the rate of rotation varies linearly with the current in the armature, but this changes under load conditions because of the effect of the induced emf in the armature windings. Greater load tends to reduce the induced emf that increases the current drawn from the supply. With no load a motor will accelerate until the induced emf is equal to the applied voltage. In many applications control is a matter of switching speed – from slow to fast for example. This is easily achieved by using a third brush – windscreen motors have used this technique since the early days of motoring. A car windscreen-wiper motor showing the third brush used to vary the speed.
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Motors with field coils offer more flexibility. A simple ‘two-speed’ control can be made by including resistance. Many schools will have ‘fractional horse-power’ motors, enabling students to investigate the behaviour of series and shunt wound motors. d.c. motors find applications both large and small, from computer fans to washing machines. A washing machine motor.
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A computer fan motor.
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To control the speed of a d.c. motor we need a variable-voltage supply. If, say, a 12 V supply is connected to the motor it will take some time for the motor to reach its maximum running speed. When the supply is turned off it also takes time for the armature to slow down. While the maximum speed can be controlled by varying the applied voltage, another design uses pulses of voltage. The motor accelerates while the pulse is applied and slows down when the pulse is off. The result is that the motor rotates at something less than its maximum speed. The actual speed can be controlled by varying the length of time that the pulse is on relative to the time it is off. This technique is known as pulse width modulation (pwm). Pulse width modulation is a useful way of providing fine control over the acceleration and deceleration of d.c. motors. d.c. motors can also be controlled from an a.c. supply using ‘thyristors’ that can be used to produce pulses of current. The length and size of the pulses controls the speed of the motor. Brushless motors Brushless d.c. (BLDC) motors are referred to by many aliases: brushless permanent magnet, permanent magnet a.c. motors, permanent magnet synchronous motors, etc. The confusion arises because a brushless d.c. motor does not directly operate off a d.c. voltage source. However, the basic principle of operation is similar to a d.c. motor. A brushless d.c. motor has a rotor with permanent magnets and a stator with windings. It is essentially a d.c. motor turned inside out. The brushes and commutator have been eliminated and the windings are connected to control electronics. The control electronics replace the function of the commutator and energise the relevant winding in turn. The windings are energised in a pattern that rotates around the stator. The energised stator winding leads the rotor magnet, and switches just as the rotor aligns with the stator. One advantage of BLDC motors is that they have no brushes and are faster, more efficient, not as noisy and more reliable than motors with brushes. However, BLDC motors do require electronic control. They are often used in disk drives and CD drives as the motor that actually makes the disk spin. A 12 pole brushless motor from a disk drive.
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A 9 pole brushless CD drive motor.
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The pictures show only the stator. The permanent magnet rotors surround the stators. This flat style of motor is often called a 'pancake motor'. a.c. motors Most a.c. machines are ‘synchronous’ or ‘induction’ machines. We use the word ‘machines’ to underline that motors and generators are basically the same; the difference is the source of the input energy. Such machines can be very large. Generators in power stations can be 500 to 600 MW. ‘Synchronous’ motors get their name because their rate of rotation depends on the frequency of the a.c. supply and they also maintain a steady speed at all loads. The rate of rotation, n, in revolutions per second is given by the equation n = f / p where f is the frequency and p is the number of ‘pairs’ of poles. One problem with synchronous motors is that they are not ‘self-starting’ and need another motor to start them which runs them up to synchronise with the supply. Some a.c. motors have ‘salient pole’ rotors. Salient means ‘protruding’ and these motors are used in smaller machines at moderate speeds. It is worth noting that when used as a generator, salient pole machines produce a waveform that is not sinusoidal. Other a.c. motors use cylindrical rotors and are better suited when higher speeds are needed. They are mechanically stronger and the flux in the rotor is stronger. The rotor windings are embedded within the laminated core. The stator windings are also embedded in a laminated frame, which distributes the flux more evenly within the rotor. When used as generators cylindrical stators produce a sinusoidal waveform. Induction motors Induction motors are ‘brushless’ motors. Induction motors are extensively used in a whole range of sizes and powers because of their simplicity, reliability and low cost. The most efficient and powerful are three-phase motors. The three-phase windings create a rotating magnetic flux which induces current in the rotor. These currents give rise to their own flux which, consistent with Lenz’s law, causes a turning force to act on the rotor so that it attempts to follow the primary rotating flux. It never quite catches up. This gives rise to what is termed ‘slip’ - more of this below. The rotor could be a solid cylinder of any conductor but that would be very inefficient. Instead the rotor is made from a laminated, magnetically soft material with aluminium or copper conductors running along the length of the cylinder, capped at each end by rings or plates so that complete current loops are created. This is known as the ‘squirrel cage’ rotor. The emf induced in the rotor is due to the fact that the flux from the stator coils is moving relative to the rotor. If the rotor moved at the same speed as this moving flux then there would not be an induced emf and therefore no current in the rotor and no torque. These motors cannot run at a synchronous speed but at a slower rate that allows a big enough emf and therefore a big enough current to cause a torque that will turn the load. That means that the speed depends on the load. Such a three-phase ‘cylindrical’ machine can be opened up to make a linear motor. Induction motors can be made to operate from a single-phase supply which makes them a practical proposition for devices in the home. There are two methods that are in common use, both of which create a rotating flux although not as well as three-phase machines. 82
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The shaded pole motor uses a copper or aluminium ‘shading’ ring.
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Shaded pole motors were commonly used in record decks in hi-fi systems and in some hair dryers. They are used in many other applications where a high torque is not important. Capacitor start motors use a capacitor to introduce a phase difference.
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The current in the coil with a capacitor in series will be approximately 90 out of phase with the current in the other coil. Full step stepper motor Stepper motors are designed, as their name implies, to rotate through a series of small steps. They can be controlled so that the size of the step (or angle turned through) and the speed of rotation are accurately set. Stepper motors are used in applications ranging from the drive that positions the read / write head in disk drives and moves paper through a printer to large ones used in magnetic resonance imaging (MRI) body scanners.
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Linear motors: From Laithwaite to levitating trains and rocket launchers Reading 90T: Text to Read 83
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The idea of unwrapping an induction motor and achieving linear motion was first proposed by Eric Laithwaite and was the topic of his 1966/7 Royal Institution Christmas Lecture. Here he is pictured working on an induction accelerator.
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The idea is relatively simple, although putting it into practice has many problems. The following set of diagrams, taken from Laithwaite’s book The Engineer in Wonderland, show how we can picture wrapping up a linear motor to get an induction motor.
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Take a row of coils
Roll them up and put an iron core in the middle
Add conducting end plates
Join the end plates with conducting bars
Since that time linear motors have been used in train propulsion and have even been proposed as a rocket launch mechanism. The following biography is taken from the BBC Local Heroes website. Eric Laithwaite went by the wonderful title of Professor of Heavy Engineering at Imperial College, in London. He pioneered a new form of transport, and was one of the first ‘television scientists’ – certainly Britain’s best known engineer in the ‘70s. But he was a controversial figure whose open-mindedness eventually led to his downfall. Born in Atherton, Lancashire, Laithwaite went to Kirkham Grammar school and Manchester University, where he worked before moving to Imperial College in 1964. An imposing figure, when asked what a Professor of Heavy Engineering was, he replied ‘One over sixteen stone’. Eric Laithwaite became famous in the early sixties for perfecting a new sort of electric motor, the linear motor. In a conventional motor, coils and magnets are arranged round a shaft to produce rotation. Some of the things you use electric motors for – like fans – need a motor that goes round, but many other applications – like trains – involve moving something in a straight line from place to place. So if you have a train, or conveyor belt, you start with a rotating motor, and have to convert the rotation to movement in a line. A train uses wheels on a track to do this, and there is a lot of friction 85
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and the wheels wear out. Wouldn’t it be great if you could somehow unwrap a normal motor and produce linear motion – in a line – directly, without any rotating parts? Indeed, without friction because induction works at a distance? The workings of a linear motor are explained on another page, but essentially it consists of a row of electromagnets. You arrange them so that each magnet is activated in turn, and the travelling magnetic field pulls the magnetic train along. The first linear motors needed a complex switching mechanism to turn on each coil in turn as the train or whatever came past. Laithwaite realised this would never be practical, and instead devised what he called the ‘magnetic river’. Normal mains electricity is a.c., alternating current, which means it goes from positive to negative 50 times a second. So if you connect it to the coils they are turned on and off – but all at the same time. But there is another sort of electric supply used in industry called ‘three-phase’, which uses three alternating current supplies slightly out of step with each other. Laithwaite was able to connect the coils so that not all of them went ‘north’ or ‘south’ at the same time, but slightly out of step so that north poles were swept along the motor like a magnetic river. Laithwaite’s train put the motor in the loco rather than the track. The first motors strongly attracted the track, so he used a hovercraft to keep them apart. The government chipped in and Tracked Hovercraft Ltd was formed to build a full-scale hovering train that ran in Cambridgeshire. But a lot of energy was spent keeping track and train apart. To get round the problem, he devised ‘magnetic levitation’. Rather than using iron or steel for the track - which is strongly attracted to the magnets – he used aluminium. Although not naturally magnetic, when held over the electromagnets, the coils ‘induce’ a current in the aluminium. The current in turn acts like an electromagnet – but with the same pole as the one in the original coil. Because like poles repel, the train and track are forced apart and the train hovers. But Britain wasn’t really interested. The late sixties were not good times for railways, and the funding was suddenly cut. Laithwaite was devastated, and the technology moved to Japan, Germany and the USA. Ironically, one of the few applications in Britain for this new form of transport was the rig used to crash cars at the Motor Industry Research Association. Linear motors only got as far as they did because Laithwaite was such an enthusiastic and self-confident advocate of engineering. He was brilliant on television, and was asked to present the first televised Christmas Lectures for Children from the Royal Institution. Then in 1974 it all went horribly wrong. Being famously open-minded, people brought him unusual problems. Someone drew his attention to the gyroscope – how could you have a ‘precessing’ gyroscope on a lightweight tower without the tower wanting to move or fall over? As he investigated, he discovered even more unusual properties. In particular, he found himself able to swing over his head a spinning 50 lb disc that he had been unable to lift above his waist when it was stationary. Did gyroscopes defy gravity? He was invited to give the Faraday lecture at the Royal Institution, and presented his gyroscope work as, he thought, a puzzle for his fellow scientists. But the mention of ‘defying gravity’ and rewriting the laws of physics was too much. This was an embarrassment, and he was effectively given the cold shoulder – his reputation never recovered and this was the only Faraday lecture not to be published. Eventually the equations were worked out, and although gyroscopes are complicated, they don’t defy the laws of physics. It is a puzzling episode. If Laithwaite had kept this to himself and quietly 86
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investigated it, nothing would have happened. But that wasn’t in his nature. You can draw your own conclusion: should he have kept quiet? Or should the scientific establishment have made room for his ‘heretical’ views? This biography is from the BBC 'Local Heroes' website (www.bbc.co.uk/history/programmes/local_heroes), and is reproduced with permission of Screenhouse Productions Limited.
The Eurostar train Reading 110T: Text to Read Teaching Notes | Key Terms
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The following article, 'The Eurostar Train - An Engineering Challenge', is by Roger Kemp from Electronics Education, Spring 1996. The development of the Eurostar train presented several electrical engineering challenges. Open the PDF file Source
Things to think about when reading 1. What were the major design challenges of the Eurostar train? 2. How does electromagnetic braking work? 3. Why motors with different numbers of phases? This article first appeared in the journal Electronics Education, and is reproduced with permission of the Institution of Electrical Engineers.
Away from it all Reading 120T: Text to Read Teaching Notes | Key Terms
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Glen Esk Glenmark Cottages A stone cottage hides at the end of a two-mile track in the scenic Glen Esk area of Scotland, a few miles north of Dundee. It is three miles from its nearest neighbour, a shooting lodge, and nine from 87
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the village post office, museum, tea and craft shop. There’s no television as there is no electricity, but a portable radio will keep you in touch with the world, while gas provides the lighting, cooking, heating and hot water. An isolated heaven or remote hell? If an isolated cottage is your idea of heaven, then this country is the place to find it. David Wickers and Bryn Frank search out Britain’s best hideaways. Holidays in remote places do not always demand a trawl through long-haul brochures and a flight to the far side of the globe. Despite our limited, densely packed acreage, Britain still offers some remarkable great escapes. One family’s heaven may be another’s nightmare. Some will panic at the thought of being miles from the nearest shop, let alone the nearest leisure centre or disco. They will grieve in the loneliness of a world where rush hour amounts to three backpackers sighted on a distant footpath and mourn a silence disturbed only by wind, birdsong and maybe the heaving of the ocean. The owner of one lonely property showed us a letter from a guest from urban Essex who wrote to complain that ‘the cottage was all right, but I hated the way the sheep kept staring at me and the lavatorial habits of the cattle were awful’. You are much more likely to find experiences of the heavenly kind recorded in the visitors’ books. How about: ‘The children were so happy, chasing each other through the bracken, they even refused a day trip to the beach’, or ‘The deer come down to breakfast every day: they are very partial to Shredded Wheat’. The general standard of self-catering accommodation has risen impressively over the past few years, with dishwashers, videos, stereos and microwaves fast becoming the norm. Yet there are many much-sought-after holiday homes where no television is good news and where gas mantles take the place of light bulbs and open fires are a matter of necessity, not show. You need to talk to owners and their agents and be alert to clues in the brochure copy, along the lines of the following, taken from official descriptions: ‘To reach the house you pass through three five-bar gates’…‘turn off the first unmade road on to another unmade track, then follow this for two miles past two lochs’…‘ignore the “no-vehicles-past-this-point” sign’…‘you will find binoculars in the desk drawer: with luck you should spot wild goats, red squirrels, badgers and foxes’…‘take extra care if your car has low clearance’.
But what will it be useful for? Reading 130T: Text to Read Teaching Notes | Key Terms
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Electricity supplied to homes was a catalyst for social change in Britain between the Wars. Time-consuming domestic tasks, which kept (usually) women busy at home, or required a large domestic staff, simply didn’t exist anymore. No need to get up early to lay a fire if the electric fire can be switched on. Convenient and easily controlled cooking, cleaning and heating became widely available. The two posters show some of the feeling of those times.
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Questions for thought 1. To what extent were the hopes people had for the electric future fully realised? 2. What aspects of our modern lives would be impossible without electricity, and which simply inconvenient? 3. What would you consider to be the most disruptive outcomes of this electrical revolution for society?
Relativity drives trains Reading 140T: Text to Read Teaching Notes | Key Terms
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Magnetic and electromagnetic effects can produce very big forces. After all, electric motors drive trains. Astoundingly, these ‘big’ forces are actually ‘small’ relativistic effects. Read about this here if a different ‘take’ on electromagnetism might intrigue you. Only relative motion matters You’ll have been shown that induced emfs only depend on relative motion of flux and conductors. For example, the two ‘different’ experiments shown below give exactly the same result.
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Only relative motion matters
start here:
coil
move the coil
move the magnet
exactly the same induced emf
It must be like this. If not then you could get induced emfs from coils and magnets both whizzing with the Earth (and you) round the Sun at 30 km s–1. If coil and magnet are relatively at rest, there is no emf. This suggests that ideas about relativity ought somehow to be involved. Forces between moving charges A beam of moving ions carries an electric current. Two currents running parallel to one another in the same direction make magnetic fields which cause them to be attracted. Think about two charges in the lab, not moving. But they are moving, with the lab and you and the Earth at 30 km s–1 round the Sun. They are carrying currents round the Sun. Why don’t we see any magnetic attraction between them? We see only their electrical repulsion.
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electric current? + No magnetic attraction. Only electrical repulsion +
you, the lab and the Earth
But what if the charges are moving past you in the lab? That’s usually how it is with ion beams. Then there is a magnetic attraction. In ion beams it is called the ‘pinch effect’. The attraction can narrow the beam, and the ‘pinch’ gives trouble in nuclear fusion devices carrying huge currents in ionised gas. electric current in ion beams + Magnetic attraction as well as electrical repulsion +
The reason lies in relativity. The electrical force between charges moving relative to you is slightly changed. The small change is the magnetic attraction. Forces between charges in wires Conducting wires contain charges (or they wouldn’t conduct). But wires don’t attract or repel one another if they aren’t charged or don’t carry current. Why not? Because the wires have exactly balanced numbers of positive and negative charges. All the forces of attraction and repulsion cancel out.
wire 1
++++++++++++++++++++ ++++++++++++++++++++ huge repulsion if no negative charges to balance positive charges
wire 2
++++++++++++++++++++ ++++++++++++++++++++
The repulsion between wires, if there were only charges of one sign, is huge. If you were standing at arm’s length from somebody and each of you had one per cent fewer electrons than protons, the force between you would be big enough to lift a ‘weight’ equal to that of the entire Earth!
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wire 1
+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+ +–+–+–+–+–+–+– +–+–+–+–+–+–+–+–+–+–+–+–+ huge repulsion
wire 2
huge attraction
drift speed of electrons
+–+–+–+–+–+–+– +–+–+–+–+–+–+–+–+–+–+–+–+ +– +–+–+–+–+–+– +–+–+–+–+–+–+–+–+–+–+–+–+
slight difference between huge attractions and repusions gives small attractive magnetic force
Revision Checklist I can show my understanding of effects, ideas and relationships by describing and explaining cases involving: transformers: where an induced emf is produced by changing the magnetic flux linking one coil and another A–Z references: transformer, electromagnetic induction, Lenz's law Summary diagrams: How a transformer works, Faraday's law of induction generators: where an induced emf is produced by conductors and flux moving relative to one another, either by moving flux or moving a conductor A–Z references: generator Summary diagrams: Transformer into generator, Large high-power generator, Motors and generators electric motors: where motion is produced when a force acts on a current-carrying conductor placed in a magnetic field, including the induction motor in which the current is induced in the conductor A–Z references: electric motor Summary diagrams: Motors and generators, Alternating fields can make rotating fields, A rotating field motor, Flux and forces linked electric and magnetic circuits: flux produced by current turns, qualitative need for large conductance and permeance and effects of dimensions of circuit and of iron and air gap 92 Advancing Physics
conductance and permeance, and effects of dimensions of circuit and of iron and air gap A–Z references: magnetic field, magnetic flux Summary diagrams: Flux and flux density, Electric circuits and magnetic flux, Electric and magnetic circuits electromagnetic forces: qualitatively as arising from tendency of flux lines to shorten or from interaction of induced poles; quantitative ideas restricted to force on a straight current-carrying wire in a uniform field A–Z references: magnetic field, magnetic flux, force on a current-carrying conductor Summary diagrams: Flux and forces, Force on a current-carrying conductor
I can use the following words and phrases accurately when describing effects and observations: magnetic B-field, magnetic flux, flux linkage A–Z references: magnetic field, magnetic flux Summary diagrams: Electric circuits and magnetic flux, Flux and flux density, Changing the flux linked to a coil induced emf (electromotive force), eddy currents A–Z references: electromagnetic induction, Lenz's law Summary diagrams: Faraday's law of induction
I can sketch and interpret: diagrams showing lines of flux in magnetic circuits Summary diagrams: Electric circuits and magnetic flux, Flux and flux density, Flux and forces graphs showing variations of current, induced emf and flux with time Summary diagrams: Graphs of changing flux and emf
I can make calculations and estimates involving: magnetic flux BA induced emf d(N) dt A–Z references: magnetic field, magnetic flux, electromagnetic induction, Lenz's law Summary diagrams: Flux and flux density, Faraday's law of induction forces acting on current-carrying conductors F = I LB where current, force and uniform magnetic field are at right angles 93 Advancing Physics
field are at right angles A–Z references: force on a current-carrying conductor Summary diagrams: Flux and forces, Force on a current-carrying conductor voltages, currents and turns in an ideal transformer: voltage ratio V1 N1 V2 N 2 power I1V1 I 2V2 A–Z references: transformer Summary diagrams: How a transformer works
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