Name CHAPTER 18

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Magnetism

SECTION

3 Electric Currents from Magnetism

KEY IDEAS As you read this section, keep these questions in mind:

• What is electromagnetic induction? • How are electricity and magnetism related? • What are the basic parts of a transformer?

What Is Electromagnetic Induction? Remember that electric motors use magnetism to convert electric energy into mechanical energy. In contrast, electrical power plants use magnetism to convert mechanical energy into electricity. They use technology discovered by British physicist Michael Faraday in 1831. Faraday produced a current by pushing a magnet through a coil of wire. The movement of the magnet into and out of the coil causes electrons in the wire to move. The moving electrons produce a current. This process is called electromagnetic induction. Look at the loop of wire moving between two magnets in the figure below. As the wire moves into and out of the magnetic field, the magnetic field induces a current in the loop. Rotating the loop or changing the strength of the magnetic field also induces a current in the loop. In each case, a changing magnetic field passes through the loop. In general, a current is induced in a circuit if the magnetic field passing through the circuit changes. This is known as Faraday’s law.

READING TOOLBOX Compare After you read this section, make a table showing the features of step-up and step-down transformers.

READING CHECK 1. Describe What is electromagnetic induction?

Magnetic field

S N

Current

When the loop moves in the magnetic field, a current is induced in the wire.

Direction of loop's motion

EHHDBG@K 2. Infer If you held the wire still and moved the magnets forward and backward, would there be a current in the wire?

You can use magnetic field lines to figure out whether the magnetic field through a circuit is changing. If the number of magnetic field lines passing through the circuit changes, the magnetic field through the circuit changes. Copyright © by Holt, Rinehart and Winston. All rights reserved.

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Electric Currents from Magnetism continued

What Happens to Energy During Induction?

READING CHECK 3. Explain Why doesn’t electromagnetic induction break the law of conservation of energy?

READING CHECK

Electromagnetic induction does not break the law of conservation of energy. It does not create energy from nothing. Actually, induction requires energy. Pushing a loop through a magnetic field requires work. The stronger the magnetic field, the more force you need to push the loop. The energy needed for that task comes from an outside source, such as your muscles.

What Do Generators Do? Remember that electric motors convert electrical energy into mechanical energy. Generators have the opposite effect. They convert mechanical energy into electrical energy. For example, look at the simple generator shown in the figure below. Notice that it is similar in structure to an electric motor.

4. Define What is a generator?

Slip rings N S

EHHDBG@K 5. Compare How is a generator similar to an electric motor?

A generator converts the mechanical energy used to turn the wire into electrical energy.

Brush Brush

READING CHECK 6. Identify What is an alternating current?

You must use energy to turn the wire in the magnetic field. As the coil turns in the magnetic field, a current is produced in the wire. This current can be used to light a light bulb, for example, Each time the loop rotates halfway around, the current in the wire changes direction. A current that changes direction at regular intervals is called an alternating current (AC).

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GENERATORS AND AC CURRENT

The diagram below shows the amount of current an AC generator produces as the loop rotates. Before the loop begins to rotate, it is perpendicular to the magnetic field. There is no current in the loop. When the loop has rotated 90°, it is parallel to the magnetic field. Then, the current is at its maximum. When the loop rotates another 90°, to 180°, it is again perpendicular to the magnetic field. Therefore, the current is zero. At 270°, the loop is parallel to the magnetic field. The current is at its maximum. However, because the sides of the loop are in opposite locations, the current in the loop is in the opposite direction. As the loop continues to rotate, the current continues to change direction. READING CHECK

Direction of wire motion

Graph of current versus angle of rotation

zero current Current

Magnetic field Direction of wire motion

Amount of current

maximum current

Current

Position of loop

0º

0º

90º

90º

180º

180º

270º

270º

360º

360º

7. Explain Why does the direction of the current change as the loop rotates?

Rotation angle

Rotation angle

Magnetic field

Direction of wire motion

Direction of wire motion

Magnetic field

Magnetic field Direction of wire motion

maximum current (opposite direction)

Current

Current

zero current 0º

90º

180º

270º

360º

Rotation angle

Rotation angle 0º

90º

180º

270º

360º

EHHDBG@K 8. Identify At which three angles of rotation is the current zero?

zero current Current

Magnetic field

Rotation angle 0º

90º

180º

270º

360º

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Electric Currents from Magnetism continued AC CURRENT IN THE HOME

8g^i^XVaI]^c`^c\ 9. Explain Why is the current from a power plant an alternating current? (Hint: What type of current do generators produce?)

Remember that batteries produce direct current. That is, the current from a battery does not change with time. In contrast, most electrical outlets supply alternating current. This is why many battery-powered devices, such as radios, require special adaptors to use power from an outlet. The electrical energy supplied through electrical outlets comes from generators at power plants. The mechanical energy needed to turn these generators can come from many sources. Some sources, such as wind, turn the generators directly. Other sources, such as coal and oil, are burned to release heat energy. The heat energy is used to boil water and produce steam. The steam turns special fans called turbines. The turbines are connected to the generator. As the steam turns the turbines, the generator produces electricity.

What Is the Electromagnetic Force?

READING CHECK 10. Identify What are three forms of energy that are produced by the electromagnetic force?

Recall that moving electric charges produce magnetic fields and that changing magnetic fields cause electric charges to move. Those two facts show that electricity and magnetism are two aspects of a single force, called the electromagnetic force. The electromagnetic force produces electromagnetic energy. Visible light is a form of electromagnetic energy. So are other forms of radiation, such as radio waves and X rays. All are electromagnetic waves, or EM waves. The figure below shows the nature of EM waves. As you can see, they consist of vibrating electric and magnetic fields that are perpendicular to each other. All EM waves have the two fields in common, whatever their frequency. Vibrating magnetic field

EHHDBG@K 11. Describe How do the vibration directions of the electric and magnetic fields in an electromagnetic wave compare?

Vibrating electric field

Direction of the electromagnetic wave

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GENERATING AND REGENERATING

An EM wave’s electric and magnetic fields are both perpendicular to the direction in which the wave travels. Therefore, EM waves are transverse waves. As an EM wave travels, the changing electric field generates the magnetic field. At the same time, the changing magnetic field generates the electric field. Because each field generates the other, EM waves have a special property: they can travel through a vacuum. Without that property, light from the sun would not be able to reach Earth.

What Are Transformers? You have probably seen metal cylinders on power line poles. These cylinders contain devices called transformers that increase or decrease the voltage of alternating current. The simplest type of transformer consists of two coils of wire wrapped around opposite sides of a closed iron loop. In the transformer shown below, one wire is attached to a source of alternating current, such as a power outlet. This wire is called the primary coil. The other wire is attached to an appliance, such as a lamp. This is the secondary coil. Current in the primary coil creates a changing magnetic field. This, in turn, magnetizes the transformer’s iron core. The changing magnetic field of the iron core then induces a current in the secondary coil. That current’s direction changes every time the direction of the current in the primary coil changes.

Primary coil

READING CHECK 12. Explain Why can EM waves travel through a vacuum?

READING CHECK 13. Define What is a transformer?

Secondary coil

EHHDBG@K 14. Identify Label the core of the transformer in the figure.

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Electric Currents from Magnetism continued STEP-UP AND STEP-DOWN TRANSFORMERS

8g^i^XVaI]^c`^c\ 15. Apply Concepts A transformer has 10 turns in its primary coil and 15 turns in its secondary coil. Is it a step-up or step-down transformer?

The voltage induced in a transformer’s secondary coil depends on the number of loops, or turns, in the coil. If the secondary coil has more turns than the primary coil, the voltage in the secondary coil is higher. Then, the transformer is called a step-up transformer, shown at the top of the figure below. If the secondary coil has fewer turns than the primary coil, the voltage in the secondary coil is lower. Then, the transformer is a step-down transformer, shown at the bottom of the figure below. Primary coil

Secondary coil

In a step-up transformer, the voltage in the secondary coil is higher than the voltage in the primary coil.

Primary coil

Secondary coil

EHHDBG@K 16. Identify Circle the side of each transformer that has the higher voltage.

READING CHECK 17. Describe How does the current in the secondary coil of a step-up transformer compare to the current in the primary coil?

In a step-down transformer, the voltage in the secondary coil is lower than the voltage in the primary coil.

TRANSFORMERS AND ENERGY CONSERVATION

You might think that step-up transformers provide something for nothing. They do not, because voltage is only one aspect of electrical power. The secondary coil’s power output cannot exceed the primary coil’s power input. Therefore, if the voltage in a coil increases, the current in the coil must decrease.

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Electric Currents from Magnetism continued

TRANSFORMERS ON POWER LINES

Real transformers are not perfectly efficient. Because of resistance in the coils, they lose some of the energy put into them as heat. The power loss increases as the current increases. Power companies want to decrease loss to maximize the amount of energy they transmit. Therefore, they use high voltages and low currents in their long-distance power lines. However, it is safer for appliances in the home to use lower voltage. Therefore, there are transformers on power lines. Step-up and step-down transformers help to transmit electrical energy from power plants to homes and businesses. Step-up transformers near power plants increase the voltage to about 120,000 V. Then, near homes, step-down transformers reduce the voltage to the 120 V that is safe to use in the home.

READING CHECK 18. Identify What causes energy loss in transformers?

8g^i^XVa I]^c`^c\ 19. Compare How does the current in power lines compare to the current in wires in the home? Explain your answer.

Step-down transformers like the ones shown here are used to reduce the voltage across power lines when they enter people’s homes.

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Section 3 Review SECTION VOCABULARY alternating current an electric current that changes direction at regular intervals (abbreviation, AC) electromagnetic induction the process of creating a current in a circuit by changing a magnetic field

generator a machine that converts mechanical energy into electrical energy transformer a device that increases or decreases the voltage of alternating current

1. Compare Describe how the current produced by a battery is different from the

current produced by a generator.

2. Identify What is Faraday’s law?

3. Describe How can you use magnetic field lines to determine whether a magnetic

field will induce a current in a circuit?

4. Explain How can the chemical energy in oil be used to produce electrical energy?

5. Describe The primary coil of a transformer has 1,000 turns of wire. The secondary

coil has 500 turns of wire. Identify what type of transformer this is, and describe how the voltage in the two coils will be different.

6. Compare How is a generator different from an electric motor?

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