Unit 8J: Magnets and Electromagnets What do magnets do? 8J1
What is a magnet?
8J2
Magnetic material for breakfast
8J3
Test the magnet Can magnetism be stopped? Can magnets be made?
8J4
How to create and destroy a magnet
8J5
Make a paper clip float
8J6
Magnetic shielding What is a magnetic field?
8J7
How to build a magnetic field viewer
8J8
Magnetic field of a bar magnet
8J9
Magnetic field of two bar magnets
8J10 Magnetic field of a horseshoe magnet 8J11 How to build a compass I 8J12 How to build a compass II How can electricity make a magnet? 8J13 The electromagnet 8J14 Oersted effect 8J15 Magnetic field in parallel conductors 8J16 Magnetic field of a solenoid How can magnetism make a electricity 8J17 The Faraday effect: a Neodymium magnet in a copper tube 8J18 Make a speaker telephone 8J19 How to make a simple motor
8J Equipment list Specialised Equipment 0.5 mm varnished copper cable 0-8 amp rheostat 4.5V battery Ammeter (0-10 amps) Bar magnet Beaker 250 ml + cold water Box of pins or paperclips Bracket with holes Broken bar magnet Bunsen burner Cables + clips Cutter Cylindrical neodymium magnet Cylindrical non-magnetic object Iron filings Large plastic container Low voltage P/S Magnet: horseshoe Magnet: Neodymium Magnetic field viewer (expt 6.3) Magnets: collection OHP Petri dish Plotting compasses Retort stand set (wood) Rheostat Solenoid Solenoids with iron and wood core Spatula Speakers x2 Stand set Steel washers: large bore Sticky tack Sticky tape Thin plates of different material Tongs Transparency Tripod
FULL SITE LICENCE
Household/stationary/market Aluminium foil Blue tac ® Breakfast cereals Copper tubing Cotton thread Iron Nail Old CD Sandpaper Scissors Shoe box lid Coloured card
Place the two halves of a broken magnet together and note the poles using a plotting compass.
2
Now take the two separate sections of the magnet and observe which is the north and south pole in each case.
Show your results in the diagram below by indicating the compass direction and noting the polarity (N or S):
?
?
Supposing you broke the magnet in half again, indicate below which you think would be the north and south poles.
If you were to keep on dividing the magnet in half in this way you will eventually end up with a single Iron atom . Therefore what conclusions can you draw about the Iron atom from this experiment? ……………………………………………… ………………………………………………………………………………………………………………………………………………………………………………………………………………
8J2 Magnetic material for breakfast Breakfast cereals
1
Neodymium magnet
Crush some cereal flakes with your finger.
2
Touch the powdered flakes with a neodymium magnet and observe what happens.
What evidence does this experiment provide that cereals contain Iron?…………………………………………………………………………
Why is Iron important for our diet?…………………………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………………………………………………………………………………………………
Why use a Neodymium magnet in this experiment and not an Iron one? …………………………………………………………………………… ………………………………………………………………………………………………………………………………………………………………………………………………………………
Try to find out what Neodymium magnets are made from. …………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………………………………………………………………………………………………
8J3 Test the magnet Collection of different magnets
1
Steel paper clips
Your teacher has supplied you with various types of magnet.
2
Find out which magnet is strongest by counting the number of paper clips it can pick up.
Record your results in the table below: Type of magnet
Number of safety clips
Represent your results in a histogram. Which was the strongest magnet?………………………………………………………… How did you decide? ……………………………………………………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………………………………………………………………………………………………
Leave a large nail on top of a permanent magnet for 5 to 10 minutes.
2
Observe how many iron filings the magnetised nail can pick up.
Iron nail
I on Irron ilings FFilings
3
Remove the iron filings with a paper towel, heat the nail until red hot then cool rapidly in cold water. Repeat two or three times.
4
Now test the magnetism of the nail by repeating step two above.
The nail is made of iron which is a magnetic material and consists of billions of small magnetic crystal domains (regions) which are normally disorientated :
Before being placed on the magnet
Draw what happens to the orientation of these domains after the nail has been left on a permanent magnet and then after heating and cooling:
After leaving on the permanent magnet
After Heating and Cooling
Explain why heating and cooling a magnet destroys its magnetism?…………………………………………………………………………………… ………………………………………………………………………………………………………………………………………………………………………………………………………………
Without letting the magnet touch the paper clip suspend it in the air as indicated below.
1
Tie a paper clip to a piece of cotton held at one end to your desk with sellotape or blue tack.
Thin plates of different material
3
While the paper clip is suspended, insert thin plates of different material between the magnet and the clip.
What holds the paper clip in the air a pushing force or a pulling force? ………………………………………… Would this experiment work with a plastic paper clip? ………………………………………… Why not? …………………………………………………………………………………………………………………………………………………………………………………………… How does this experiment show you that magnetic forces can pass through non magnetic material? ……………………………………………………………………………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………………………………………………………………………… What happens to the paper clip if an iron plate is inserted between the clip and the magnet? ……………………………………………………………………………………………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………………………………………………………………………………………………
Separate two bar magnets with a stack of large bore steel washers and examine the magnetic field pattern, paying attention to the patterns inside the steel washers. Repeat, this time without the steel washers and compare with the results above. Repeat with a horseshoe magnet and its protector as indicated below.
Now sketch below the magnetic field lines as indicated by the iron filings:
What do you notice about the magnetic field in the centre of the stack of steel washers ?………………………………………… ……………………………………………………………………………………………………………………………………………………………………………………………………………… Why is this?……………………………………………………………………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………………………………………………………………………………………………
8J8 Magnetic field of a bar magnet Magnetic field viewer (expt 8J7)
Iron filings
Place the magnet on a piece of white paper and cover with the viewer (expt 8J7) ronn IIro nggss Fil iin Fil
1
Bar Magnet
Plotting compasses
OHP or white paper
2
Sprinkle iron filings over the magnet to observe the magnetic field patterns.
3
Investigate the direction of the magnetic field lines with a small plotting compass.(see below).
Sketch the magnetic field patterns produced by the bar magnet below:
Where is the magnetic field strongest? …………………………………………………………………………………………………………………………………… How can you tell using iron filings? ……………………………………………………………………………………………………………………………………………… Where is the magnetic field is weakest? …………………………………………………………………………………………………………………………………… Draw the direction of the magnetic field for each plotting compass position shown below:
Place a bar magnet in a petri dish then float the dish in a washing up bowl as shown below.
3
Take note of the direction and repeat two or three times until you are sure of magnetic north. Then cut out a card board arrow and stick it on your desk as shown below to indicate north.
2
Gently push the petri dish so that it doesn´t touch the side of the bowl and allow it to stabilize.
Why are compass housings usually made from plastic or aluminium and NEVER from steel? ………………………………………… ……………………………………………………………………………………………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………………………………………………………………………………………………
Suspend a bar magnet by a thin thread. Once it has stopped rotating note its direction.
Tack
Bar magnet
2
Repeat two or three times until you are sure of magnetic north. Then cut out a card board arrow and stick it on your desk as shown below.
In this experiment why is it important to use a wooden stand and not a steel one?………………………………………………………… ……………………………………………………………………………………………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………………………………………………………………………………………………
Make an electro magnet by turning a 1mm diameter core insulated cable around a steel coach bolt. Hold it in place with insulating tape.
2
Starting at zero amps increase the current (using the rheostat) in intervals of 0.5 amps up to about 8 amps, each time counting the number of pins the magnet can pick up.
!
Use a low voltage power supply. 6
8
4 a.c
10 12
2
d.c
0-8 amp rheostat
3
Each time discard the pins which are now magnetised in a separate container. Don´t use these pins again in the experiment.
Use the Rheostat to change the current in 0.5 amp intervals. Each time record the number of pins you could pick up in the table below: Current (amps)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
Nº of pins Now plot a graph showing your results What do the number of pins indicate?………………………………………………………………………………………………………………………………………… What happens to the magnetic field strength as the current becomes large?………………………………………………………………… ……………………………………………………………………………………………………………………………………………………………………………………………………………… Why does this happen?…………………………………………………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………………………………………………………………………………………………
Place 5 or 6 plotting compasses in a circle on an old CD disc and position the CD on a tripod stand as shown below.
Tripod
Old CD
Plotting compasses
Switch on the current and observe what happens to the compass needles.
Pass a cable through the centre of the CD disc and support it using two clamps.
Change the direction of the current and note the effect on the magnetic field direction.
+ -
What is the direction of current flow in a d.c circuit?
from:
+g -
or
-g+
Indicate in the diagram above in which direction the current is flowing as it passes through the compact disc (hint: the
+ and – terminals are marked on the battery).
Draw in the diagram below the direction of the compass needles when the current flows upwards through the disc and downwards through it:
Current upwards through CD
Current upwards through CD
Current downwards through CD
How did you change the direction of the current? ………………………………………………………………………………………………………………… What shape is the magnetic field produced by a conducting wire? …………………………………………………………………………………… When the current is flowing down through the CD are the magnetic field lines aligned in clockwise or anti clock wise sense?
8J15 Magnetic field in parallel conductors Aluminium foil
0-8 amp rheostat
Scissors
Cables + clips
Low voltage P/S
Ammeter (0 - 10 amps)
Using your conclusions from expt 8J14 – the Oersted effect draw the magnetic field lines and their directions around the conductors in the configurations shown below: Foils connected in series: Current entering plane of paper
Current leaving plane of paper
Foils connected in parallel
Indicate in the diagram whether the foils are attracted to each other or forced apart? Can you explain these results?……………………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………………………………………………………………………… (Hint : Consider the resulting magnetic field between the foils, in which do they reinforce each other’s effect and in which do they cancel each other’s effect?)
8J16 Magnetic field of a solenoid Solenoids with iron and wood core
1
Cables + clips
Low voltage P/S
Ammeter (0-10 amps)
Rheostat
Iron filings
Make two solenoids by winding 1mm diameter core insulated cable around a large steel nail and wooden dowel of equal diameters. Use the same cable and number of turns for each solenoid.
2
Compare the magnetic fields produced when a current is passed through each. Use the magnetic field viewer (expt 8J7), and sprinkle with iron filings.
!
Use a low voltage power supply.
6
Iron Filin gs
4 a.c
2
8 10 12
d.c
0 - 8 amp rheostat for 2
Sketch the field lines you observe in each solenoid below:
In which solenoid is the magnetic field strongest?………………………………………………………………………………………………………………… Why is this?……………………………………………………………………………………………………………………………………………………………………………………… What happens to the compass needle when you change the direction of the current? ………………………………………………… ………………………………………………………………………………………………………………………………………………………………………………………………………………
8J17 The Faraday effect: a Neodymium magnet in a copper Copper tubing
Cylindrical neodymium magnet
Cylindrical non-magnetic object
1
You will need a long section of copper tubing into which a neodymium magnet will just fit. Make sure that the magnet is free to move in the tube.
2
Try and find a similar nonmagnetic object of the same diameter and show that it falls quickly to the ground.
3
Insert the neodymium magnet and observe what happens.
How does the motion of the Neodymium magnet compare with the non magnetic object as they fall down the tube? ……………………………………………………………………………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………………………………………………………………………… This phenomenon can be explained by the Faraday effect. Try and find out why the magnet slows down? ……………………………………………………………………………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………………………………………………………………………… Why use a Neodymium magnet and not an Iron one?……………………………………………………………………………………………………………… Try to find out what Neodymium magnets are made from: …………………………………………………………………………………………………… ………………………………………………………………………………………………………………………………………………………………………………………………………………
Working in pairs, connect two speakers over a fairly large distance (or between two classrooms). When you speak into the speaker you will find that it acts as a microphone and your partner will be able to hear your voice.
The diagram opposite shows a cross section of a speaker. On the diagram label the following parts: The Coil, The magnet, The Diaphragm, The sound wave When you talk into the speaker what happens to the diaphragm? ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… What is the diaphragm attached to? ……………………………………………………………………………………………………… In a speaker the magnets are usually ring-shaped and the coil vibrates inside the magnet. What is generated in a coil when in vibrates in the presence of a magnetic field? ……………………………………………………………………………………………………………………………………………………………………………………………………………… The current produced by this speaker is then carried to the other speaker where the reverse happens: ……………………………………………………………………………………………………………………………………………………………………………………………………………… When a changing current flows in a coil positioned in a magnetic field a force acts on the coil. If the coil is attached to a diaphragm the diaphragm vibrates and produces sound. In this case the sound is not very loud because the current generated is weak. What type of device could be included in the circuit to increase the volume of the sound produced? ………………………………………………………………………………………………………………………………………………………………………………………………………………
