generating and using electricity

electric transformers

An electric transformer is a device that uses electromagnetic induction to change the voltage of electric current. A transformer may either increase or decrease voltage, but it only works with alternating current. You can see the components of an electric transformer in Figure 25.13. As you can see in Figure 25.13, a transformer consists of two wire coils wrapped around an iron core. When alternating primary current passes through coil P, it magnetizes the iron core. Because the current is alternating, the magnetic field of the iron core keeps reversing. This changing magnetic field induces alternating current in coil S, which is part of another circuit. In Figure 25.13, coil P and coil S have the same number of turns of wire. In this case, the voltages of the primary and secondary currents are the same. However, when the two coils have different numbers of turns, the voltage of the secondary current is different than the voltage of the primary current. Both cases are illustrated in Figure 25.14. When coil S has more turns of wire than coil P, the voltage in the secondary current is greater than the voltage in the primary current. This type of transformer is called a step-up transformer. When coil S has fewer turns of wire than coil P, the voltage in the secondary current is less than the voltage in the primary current. This type of transformer is called a step-down transformer. For an animation of a transformer, go to this URL: .

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electric generators

An electric generator is a device that changes kinetic energy to electrical energy through electromagnetic induction. A simple diagram of an electric generator is shown in Figure 25.11. In a generator, some form of energy is applied to turn a shaft. This causes a coil of wire to rotate between opposite poles of a magnet. Because the coil is rotating in a magnetic field, electric current is generated in the wire. If the diagram in Figure 25.11 looks familiar to you, thats because a generator is an electric motor in reverse. Look back at the electric motor in Figure 25.8. If you were to mechanically turn the shaft of the motor (instead of using electromagnetism to turn it), the motor would generate electricity just like an electric generator. You can learn how to make a very simple electric generator by watching the video at the URL below. Making your own generator will help you understand how a generator works. Generators may be set up to produce either alternating or direct current. Generators in cars and most power plants produce alternating current. A car generator produces electricity with some of the kinetic energy of the turning crankshaft. The electricity is used to run the cars lights, power windows, radio, and other electric devices. Some of the electricity is stored in the cars battery to provide electrical energy when the car isnt running. A power plant generator produces electricity with the kinetic energy of a turning turbine. The energy to turn the turbine may come from burning fuel, falling water, or some other energy source. You can see how falling water is used to generate electricity in Figure 25.12 and in the video at this URL:

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electrifying the home

Power plant generators produce high-voltage electric current. Many power plants also use step-up transformers to increase the voltage of the current even more (see Figure 25.15). By increasing the voltage, the amount of current is decreased, so less power is lost as the electricity travels through the power lines. However, the voltage in power lines is too high to be safe for home circuits. The voltage in power lines may be as great as 750,000 volts, whereas most home circuits are 240 or 120 volts. One or more step-down transformers decrease the voltage of current before it enters the home. Other step-down transformers within the home lower the voltage of some of the homes circuits. For an overview of electric power generation, transmission, and distribution in the U.S., go to this URL: http://w

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electromagnetic induction

The process of generating electric current with a changing magnetic field is called electromagnetic induction. It occurs whenever a magnetic field and an electric conductor, such as a coil of wire, move relative to one another. As long as the conductor is part of a closed circuit, current will flow through it whenever it crosses magnetic field lines. One way this can happen is pictured in Figure 25.9. It shows a magnet moving inside a wire coil. Another way is for the coil to move instead of the magnet. You can watch an animated version of Figure 25.9 at this URL: http://jsticca.wordpress.com/2009/09/01/the-magn

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from magnets to electricity

Just about a decade after Oersted discovered that electric current produces a magnetic field, an English scientist named Michael Faraday discovered that the reverse is also true. A magnetic field produces an electric current, as long as the magnetic field is changing. This is called Faradays law.

electric generators and transformers

Two important devices depend on electromagnetic induction: electric generators and electric transformers. Both devices play critical roles in producing and regulating the electric current we depend on in our daily lives.

the current produced by a magnet

The device in the circuit in Figure 25.9 is an ammeter. It measures the current that flows through the wire. The faster the magnet or coil moves, the greater the amount of current that is produced. If more turns were added to the coil, this would increase the strength of the magnetic field as well. If the magnet were moved back and forth repeatedly, the current would keep changing direction. In other words, alternating current would be produced. This is illustrated in Figure 25.10.

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instructional diagrams

No diagram descriptions associated with this lesson

questions

Current will flow through a wire in a closed circuit whenever the wire crosses magnetic field lines.

-->  a. true

b. false

Which of the following sentences correctly states Faradays law?

a. The flow of electric current creates a changing magnetic field.

b. Only an alternating current can produce a magnetic field.

c. A rotating electromagnet has a changing magnetic field.

-->  d. A changing magnetic field produces an electric current.

Electromagnetic induction occurs when an electromagnet creates a magnetic field.

a. true

-->  b. false

Moving a magnet back and forth inside a coil of wire produces a(n)

a. electromagnet.

-->  b. alternating current.

c. increase in voltage.

d. decrease in voltage.

If you were to mechanically turn the shaft of an electric motor, the motor would generate electricity.

-->  a. true

b. false

Uses of electromagnetic induction include

a. generating electricity.

b. changing the voltage of current.

c. changing electrical energy to kinetic energy.

-->  d. two of the above

An electric generator is the reverse of an

a. electric transformer.

-->  b. electric motor.

c. electromagnet.

d. none of the above

An electric generator contains a magnet and a rotating coil of wire.

-->  a. true

b. false

An electric generator can produce only direct current.

a. true

-->  b. false

An electric transformer with more turns of wire in coil S

a. decreases the voltage of current.

b. increases the amount of current.

c. is a step-down transformer.

-->  d. none of the above

A car generator uses the kinetic energy of a turning crankshaft.

-->  a. true

b. false

A hydroelectric power plant uses the kinetic energy of steam under pressure.

a. true

-->  b. false

An electric transformer works only with alternating current.

-->  a. true

b. false

The iron core of an electric transformer becomes an electromagnet when current passes through the P coil.

-->  a. true

b. false

The P and S coils of an electric transformer always have the same number of turns of wire.

a. true

-->  b. false

Faradays discovery is the reverse of Oersteds discovery about electric currents and magnetic fields.

-->  a. true

b. false

process of generating electric current with a changing magnetic field

a. electric generator

b. ammeter

c. step-up transformer

d. turbine

-->  e. electromagnetic induction

f. step-down transformer

g. Faradays law

Electromagnetic induction always requires movement of a magnet.

a. true

-->  b. false

device that uses electromagnetic induction to increase the voltage of electric current

a. electric generator

b. ammeter

-->  c. step-up transformer

d. turbine

e. electromagnetic induction

f. step-down transformer

g. Faradays law

device in a power plant that provides kinetic energy to the electric generator

a. electric generator

b. ammeter

c. step-up transformer

-->  d. turbine

e. electromagnetic induction

f. step-down transformer

g. Faradays law

The current produced by a magnetic field is always alternating current.

a. true

-->  b. false

idea that a changing magnetic field produces an electric current

a. electric generator

b. ammeter

c. step-up transformer

d. turbine

e. electromagnetic induction

f. step-down transformer

-->  g. Faradays law

The two coils of a transformer are connected to different electric circuits.

-->  a. true

b. false

Power plant generators produce low-voltage electric current.

a. true

-->  b. false

device that changes kinetic energy to electrical energy through electromagnetic induction

-->  a. electric generator

b. ammeter

c. step-up transformer

d. turbine

e. electromagnetic induction

f. step-down transformer

g. Faradays law

device that uses electromagnetic induction to decrease the voltage of electric current

a. electric generator

b. ammeter

c. step-up transformer

d. turbine

e. electromagnetic induction

-->  f. step-down transformer

g. Faradays law

device that measures the amount of current flowing through a wire

a. electric generator

-->  b. ammeter

c. step-up transformer

d. turbine

e. electromagnetic induction

f. step-down transformer

g. Faradays law

What happens if a magnetic field and an electric conductor move relative to one another?

a. The magnetic field becomes stronger.

b. The conductor becomes an electromagnet.

c. The conductor becomes an electric insulator.

-->  d. Electric current flows through the conductor.

Assume that a bar magnet is placed inside a coil of wire that is part of a closed circuit. Which situation produces an alternating current in the wire?

a. The magnet is moved back and forth inside the coil.

b. The coil is moved back and forth over the magnet.

c. The magnet is connected to the closed circuit.

-->  d. two of the above

You can create more current by moving the coil or magnet in question 2

a. slower.

-->  b. faster.

c. farther.

d. none of the above

Devices that use electromagnetic induction include

a. electric motors.

b. electric generators.

c. electric transformers.

-->  d. two of the above.

An electric generator uses kinetic energy to

-->  a. rotate a coil of wire.

b. create a magnetic field.

c. reverse the poles of an electromagnet.

d. change the voltage of electric current.

When alternating current flows through coil P of an electric transformer, it

a. changes to direct current.

-->  b. magnetizes an iron core.

c. repels a magnetic field.

d. charges a battery.

Increasing the voltage of current leaving a power plant causes the

a. amount of current to increase.

-->  b. amount of current to decrease.

c. direction of current to change.

d. two of the above

diagram questions

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