Chapter 25 Physics- Electromagnetic Induction

What happens when a wire is held between the poles of a magnet?

The wire becomes magnetized.

A voltage is generated in the wire.

Nothing happens.

nothing happens

What happens when a wire is moved between the poles of a magnet?

Nothing happens.

The wire becomes magnetized.

A voltage is generated in the wire.

a voltage is generated in the wire

What happens to the induced voltage if you increase the number of loops in the wire?

The induced voltage decreases.

The induced voltage stays the same.

The induced voltage increases.

The induced voltage increases.

How do you induce a voltage in a conducting loop of wire?

Change the magnetic field in the loop.

Hold the loop outside a magnetic field.

Hold the loop in a magnetic field.

Change the magnetic field in the loop.

For electromagnetic induction to occur in a circuit there must be a _________.

change in magnetic field intensity

meter to measure current

coil of wire

battery or some other voltage source

change in magnetic field intensity

When a magnet is quickly pushed into a coil of wire connected to a circuit _________.

work is needed to overcome a resistance to the push

the magnet is drawn farther into the coil

there is no resistance to the push

resistance via Ohm's law is experienced

work is needed to overcome a resistance to the push

A magnet is rapidly plunged into a coil, and then equally rapidly extracted. How would you describe the voltages induced in each case?

There will be no voltage because there is no battery connected.

They will be equal.

They will be of similar magnitude and opposite sign.

They will be different.

They will be of similar magnitude and opposite sign.

What was the important discovery that physicists Michael Faraday and Joseph Henry made at about the same time?

Electromagnetic waves

Electromagnetic induction

Electromagnetic deduction

Electromagnetic relativity

Electromagnetic induction

Increasing the number of loops of a coil exposed to a changing magnetic field will _________________.

produce no change in the induced voltage

increase the induced voltage

change voltage

increase the induced current in the coil

increase the induced voltage

What are the three ways in which voltage can be induced in a loop of wire?

Move a wire near the loop, move the loop near a wire, change current in a nearby wire

Move a magnet near the loop, move the loop near a magnet, move a capacitor near the loop

Move a magnet near the loop, move the loop near a magnet, change current in a nearby loop

Move a charge near the loop, move the loop near a charge, change current in a nearby loop

Move a magnet near the loop, move the loop near a magnet, change current in a nearby loop

How does the frequency of induced voltage relate to how frequently one pole of a magnet is plunged in and out of a coil of wire?

The frequency of the induced voltage is half the frequency of the magnet.

The frequency of the induced voltage is twice the frequency of the magnet.

The induced voltage is dc regardless of the frequency of the magnet.

The frequencies are the same.

The frequencies are the same.

A primary difference between a motor and a generator is _________.

that a motor operates a generator

the design of the moving parts

that a generator operates a motor

the reversed roles of input and output

the reversed roles of input and output

A generator normally produces ac because the magnetic field experienced by interior coils _________.

remains steady

periodically alternates

experiences force

produces electromagnetic induction

periodically alternates

A common characteristic of motors and generators is that they both ___________.

use electricity

produce electricity

transform energy from one form into another

store electromagnetic energy

transform energy from one form into another

What are the basic differences and similarities between a generator and an electric motor?

In both, moving electrons experience a force. The motor transforms electrical energy to mechanical energy. The generator transforms mechanical energy to electrical energy

Who discovered electromagnetic induction, and who put it to practical use?

Discovered by Faraday and Henry; put to use by Tesla and Westinghouse

What are the two most common ways that energy is supplied directly to the turbines?

Wind and nuclear fuel

Batteries and alternators

Steam and falling water

Motors and generators

steam and falling water

Why is the bird not harmed by the voltage difference across its feet?

There is no voltage difference across its feet because the voltage of 120,000 V is between the transmission line and one of the parallel neighboring lines.

What is the purpose of transmitting power at high voltages over long distances?

High voltages can be used to transmit power at low current, thus decreasing losses due to heating in the wire.

Why will magnetic braking not occur for a sheet of plastic that swings into a magnetic field?

Because there are too many free electrons in a plastic sheet.

Because there are no free electrons in a plastic sheet.

Because there are too many free positive ions in a plastic sheet.

Because there are no free positive ions in a plastic sheet.

Because there are no free electrons in a plastic sheet.

What is Lenz's law?

Lenz's law says that an induced effect opposes the inducing cause.

When the magnetic field changes in a coil of wire, voltage in each loop of the coil is induced. What is the name of the voltage induced in a loop if the source of the magnetic field is the coil itself?

Self-resistance

Self-induction

Self-capacitance

Lenz's law

self-induction

What ultimately transfers from one location to another in electromagnetic induction is ___________.

light

energy

current

force

energy

If there are two times the number of loops in the primary coil of a transformer than in the secondary, what will the output voltage be with respect to the input voltage?

The output voltage will be lower.

The output voltage will be half.

The output voltage will be double.

The output voltage will be higher.

The output voltage will be half.

Is it correct to say that a transformer boosts electric energy? Defend your answer.

Yes. The transformer condenses energy into a shorter time, thereby boosting it without violating the law of conservation of energy.

No. Conservation of energy does not allow energy output to exceed energy input.

Yes. The energy is proportional to the voltage that can be increased.

No. The power is the same for the input and the output, but the energy can be different.

No. Conservation of energy does not allow energy output to exceed energy input.

Which of these does a transformer change: voltage, current, energy, power?

Current, power, and energy

Voltage and current

Energy and power

Voltage, current, and power

voltage and current

Specifically, what quantity is stepped up by a step-up transformer?

Current

Power

Voltage

Energy

voltage

Why does a transformer require ac?

A voltage is induced only when the magnetic field is changing.

Flux capacitors require ac to power transformations.

No dc current can flow through the primary coil of wire.

Transformers are designed to use ac because that is the common power source for houses in the U.S.

A voltage is induced only when the magnetic field is changing.

When there is a change in the magnetic field in a closed loop

of wire

A) a voltage is induced in the wire.

B) a current is created in the loop of wire.

C) electromagnetic induction occurs.

D) all of these

E) none of these

D) all of these

When a bar magnet is thrust into a coil of copper wire, the

coil tends to

A) attract the magnet as it enters.

B) repel the magnet as it enters.

C) both of these

D) neither of these

B) repel the magnet as it enters.

* If a magnet is pushed into a coil, voltage is induced across

the coil. If the same magnet is pushed into a coil with twice

the number of loops,

A) one half as much voltage is induced.

B) the same voltage is induced.

C) twice as much voltage is induced.

D) four times as much voltage is induced.

E) none of these

C) twice as much voltage is induced.

An electric motor is very similar to

A) an electric generator.

B) an automobile battery.

C) a radio receiver.

D) None of the above choices are correct.

A) an electric generator.

A device that transforms electrical energy to mechanical

energy is a

A) generator.

B) motor.

C) transformer.

D) magnet.

E) none of these

B) motor

The principal advantage of ac power over dc power is that

A. ac circuits multiply power more easily.

B. ac voltage oscillates while dc voltage does not.

C. ac circuits are safer.

D. more energy is dissipated during transmission.

E. ac voltage can be transformed via conventional

transformers.

E. ac voltage can be transformed via conventional

transformers.

A transformer produces

A. energy.

B. power.

C. neither, but transforms mechanical energy to electric

energy.

D. none of the above

D. none of the above

The output power of an ideal transformer is

A. smaller than the input power.

B. equal to the input power.

C. greater than the input power.

D. may be any of the above

B. equal to the input power.

A transformer actually transforms

A. generators into motors.

B. non-safe forms of energy to safe forms of energy.

C. magnetic field lines.

D. voltage.

E. all of the above

D. voltage

Power is transmitted at high voltages because the

corresponding current in the wires is

A. low so that overheating of the wires is minimized.

B. also high to deliver appreciable power to distant places.

C. either of these

D. neither of these

A. low so that overheating of the wires is minimized.

The amount of Voltage induced

depends on:

the velocity -change of

magnetic flux over change in time- and

the number of coils

faraday's law

The induced voltage in a coil is proportional to the product of the number of loops and the rate at which the magnetic field changes within those loops.

When a magnet is plunged into a coil of twice as many loops

as another:

twice as much voltage is induced. If the magnet

is plunged into a coil with three times as many loops, then

three times as much voltage is induced.

More ___ means more

voltage, which means we do

more work to induce it

loops

electromagnetic induction is all around us in

the road, hybrid cars, security systems, ATM's, and tape recorders

More loops means more

voltage, which means we do

more work to induce it

Guitar pickups are tiny coils

with magnets inside them. The

magnets magnetize the steel

strings. When the strings

vibrate, voltage is induced in

the coils

simple generator

Voltage is induced in the loop when it

is rotated in the magnetic field.

motor effect

when a current is passed along a wire in a magnetic field, and the wire is not parallel to the lines of the magnetic field, a force is exerted on the wire by the magnetic field

generator effect

When a wire with no initial current is

moved downward, the electrons in the wire experience a

deflecting force perpendicular to their motion. There is a

conducting path in this direction that the electrons follow,

thereby constituting a current.

The metal detectors that people walk through in security areas operate via

Newton's laws.

Coulomb's law.

Ohm's law.

Faraday's law.

Faraday's Law. Voltage is induced in the coils of metal detectors when metals pass through them, in accord with Faraday's law. The other laws listed may be evident in the metal detectors, but are not primary factors. This is a pure and simple Faraday application.

Voltage can be induced in a wire by

All of the above

moving the wire near a magnet.

changing the current in a nearby wire.

moving a magnet near the wire.

all of the above. Change is the key word in electromagnetic induction. To induce voltage, a change of some kind must occur relative to a conducting wire and a magnet.

Voltage is induced in a wire coil connected to a voltmeter when a magnet is moved to and fro in the coil. If the coil has twice as many loops, the voltage induced is

the same.

four times as much.

half.

twice.

twice. Any movement of a magnet in a coil of wire will induce a voltage in that coil. In accord with Faraday's law, the amount of voltage is directly proportional to the number of loops. So if the number of loops is doubled, the voltage will also be doubled.

Thrust a magnet into a coil of wire and you'll induce

Neither

Both

voltage.

current.

both. Choice (a) is certainly true. But a task in exams is to check that other choices may also be correct, especially in a question with a choice of "both." The induced voltage in the wire coil also produces current, so (c) is the correct answer. Read all choices before answering. Interestingly, if the coil were of non-conducting rubber, then induced voltage would not be accompanied by current, wherein choice (a) would be correct.

The essential physics concept in an electric generator is

Ohm's law.

Coulomb's law.

Faraday's law.

Newton's second law.

Faraday's law. Electric motors and electric generators employ electromagnetic induction for their operation. Of the names listed, only Faraday is associated with such induction. If Joseph Henry were listed, he too would be a correct answer. The other choices are peripheral to Faraday's law and cannot be considered correct responses.

A transformer transforms voltage while

producing energy.

producing power.

transferring energy from one coil to another.

transferring current from one coil to another

transferring energy from one coil to another. First we ask what is voltage? It's energy per charge. When a transformer transforms voltage it does so by transferring a fixed amount of energy from one coil to another. A transformer transfers energy and power. It does not produce energy and power. And it does not transfer current, but induces it. Hence correct choice (c).

A step-up transformer steps up voltage by ten times. If voltage input is 120 volts, voltage output is

120 V.

60 V.

1200 V.

12000 V.

1200 V. Quite simply, 120 volts stepped up by 10 equals 1200 V (120 V ( 10 = 1200 V). Other choices do not apply this simple arithmetic.

Compared with power input, the power output of an ideal transformer is

greater.

Any of these

less.

the same.

the same. The key word in this statement is "ideal," which in the case of a transformer means no energy losses. In practice, about 1% of energy turns to heat, a value is so low that we approximate the ideal IV(input) = IV(output) transformer relationship.

When a sheet of copper moves between the poles of a permanent magnet

kinetic energy of the sheet is increased.

eddy currents in the sheet are induced.

no change in motion occurs.

electric and magnetic fields are induced in the sheet.

eddy currents in the sheet are induced. This is magnetic braking, caused by eddy currents that are induced in the moving copper sheet. The sheet is slowed by the interaction between the permanent magnet and the resulting induced magnetic fields of the eddy currents. Electric fields are not induced in the sheet.

The linkage of induced electric and magnetic fields produces

higher voltages produced by Faraday induction.

All of the above

electromagnetic waves.

strengthening of both fields.

electromagnetic waves. The linking of E and M fields was recognized by Maxwell who predicted the existence of E&M waves. When the frequency of such waves fall in the frequency range of visible light, the corresponding wave IS light—a tiny part of the overall E&M spectrum.

Is electromagnetic induction (EMI) an energy source?

No. EMI is not a source of energy but a method of transforming mechanical energy into electric energy. Work must be done to produce energy by EMI.

A simple security system consists of a miniature electric circuit at the top of a door and a nearby small magnet fastened to the doorframe. How is an electric current generated when the door opens or closes?

The magnetic field of the magnet extends to a coil in the system. The field in the coil changes when the door moves, inducing voltage and current. No batteries needed. A simple application of Faraday's law.

When is energy input needed for energy output by electromagnetic induction?

Always. Loops of wire in any magnetic field need an energy source to provide rotation.

Is electricity a primary source of energy? Is electric power a source of energy?

Definitely not! It's important to realize that neither of these is a source of energy. Electricity of any kind is a carrier of energy that requires a primary source such as solar, wind, water, or fossil fuel.

What exactly is transferred by a transformer? What is transformed by a transformer?

Energy is transferred from one set of coils to another. Voltage is transformed from one value to another from one set of coils to another.

To light a household lamp you connect it to a wall outlet by its cord, which is composed of two parallel wires insulated from each other. What does a lamp cord have in common with overhead power lines? How does insulation for each differ?

Although voltages differ, both are wired in parallel for ac and deliver energy from one location to another. Whereas safety requires insulation between the millimeters-apart wires in a lamp cord, no insulation is needed for the meters-apart overhead power lines—except where towers hold up the wires and large plastic insulators separate the wires from the metal.