# Chapter 25: Electromagnetic Induction

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• 1.

### 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
Explanation
When there is a change in the magnetic field in a closed loop of wire, all of these statements are true. A voltage is induced in the wire, which leads to the creation of a current in the loop of wire. This phenomenon is known as electromagnetic induction, where a changing magnetic field induces an electric current in a conductor. Therefore, all of the given options are correct.

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• 2.

### Thrust a magnet into a coil of wire and the coil

• A.

Becomes an electromagnet.

• B.

Has a current in it.

• C.

Both of these

• D.

Neither of these

C. Both of these
Explanation
When a magnet is thrust into a coil of wire, it induces a current in the wire due to electromagnetic induction. This current flowing through the wire creates a magnetic field, making the coil behave like an electromagnet. Therefore, both of these statements are correct.

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• 3.

### 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.
Explanation
As the bar magnet is thrust into the coil of copper wire, the changing magnetic field induces an electric current in the coil according to Faraday's law of electromagnetic induction. According to Lenz's law, the induced current creates a magnetic field that opposes the change in the original magnetic field. Therefore, the coil of copper wire repels the magnet as it enters, in an attempt to counteract the change in the magnetic field.

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• 4.

### Electromagnetic induction occurs in a coil when there is a change in

• A.

Electric field intensity in the coil.

• B.

Magnetic field intensity in the coil.

• C.

Voltage in the coil.

• D.

The coil's polarity.

• E.

Electromagnetic polarity.

B. Magnetic field intensity in the coil.
Explanation
Electromagnetic induction occurs in a coil when there is a change in magnetic field intensity in the coil. This is because a change in magnetic field induces an electric current in a nearby coil according to Faraday's law of electromagnetic induction. When the magnetic field intensity in the coil changes, it creates a flux that cuts across the coil, inducing an electromotive force (EMF) and hence an electric current. Therefore, the correct answer is magnetic field intensity in the coil.

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• 5.

### Voltage can be induced in a wire by

• A.

Moving the wire near a magnet.

• B.

Moving a magnet near the wire.

• C.

Changing the current in a nearby wire.

• D.

Choices A, B, and C are all true.

• E.

None of the above choices are true.

D. Choices A, B, and C are all true.
Explanation
Voltage can be induced in a wire by moving the wire near a magnet, as the changing magnetic field induces an electric field in the wire. Similarly, moving a magnet near the wire also induces voltage in the wire due to the changing magnetic field. Additionally, changing the current in a nearby wire can induce voltage in the wire through electromagnetic induction. Therefore, all three choices A, B, and C are true.

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• 6.

### A wire moving at right angles to a magnetic field has NO induced voltage

• A.

If it is made of copper.

• B.

If it is moving slowly.

• C.

If it is moving very fast.

• D.

If the wire is covered with insulation.

• E.

None of the above choices are correct.

E. None of the above choices are correct.
Explanation
The correct answer is "None of the above choices are correct." This is because when a wire moves at right angles to a magnetic field, an induced voltage is always generated regardless of the material it is made of, its speed, or whether it is insulated or not. The induced voltage is determined by the rate at which the magnetic field changes, the strength of the magnetic field, and the length of the wire.

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• 7.

### The magnetic field strength inside a current-carrying coil will be greater if the coil encloses a

• A.

Vacuum.

• B.

Wooden rod.

• C.

Glass rod.

• D.

Rod of iron.

• E.

None of these

D. Rod of iron.
Explanation
The magnetic field strength inside a current-carrying coil will be greater if the coil encloses a rod of iron. This is because iron is a ferromagnetic material, which means it can easily magnetize and amplify the magnetic field produced by the current. In contrast, a vacuum, wooden rod, or glass rod do not have this magnetic property, so they would not enhance the magnetic field strength inside the coil.

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• 8.

### 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.
Explanation
When a magnet is pushed into a coil, the magnetic field lines cut across the coil's loops, inducing a voltage. The voltage induced is directly proportional to the rate at which the magnetic field lines are cut. If the same magnet is pushed into a coil with twice the number of loops, the magnetic field lines will cut across the loops at twice the rate compared to the coil with fewer loops. As a result, twice as much voltage will be induced in the coil with twice the number of loops.

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• 9.

### An electric motor is very similar to

• A.

An electric generator.

• B.

An automobile battery.

• C.

• D.

None of the above choices are correct.

A. An electric generator.
Explanation
An electric motor and an electric generator are similar because they both involve the conversion of electrical energy into mechanical energy. In both cases, there is a rotating component that interacts with a magnetic field to produce motion. An automobile battery and a radio receiver are not similar to an electric motor in terms of their function and operation. Therefore, the correct answer is an electric generator.

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• 10.

### 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.
Explanation
A device that transforms electrical energy to mechanical energy is a motor. A motor converts electrical energy into mechanical energy, typically through the interaction of magnetic fields. It uses the principle of electromagnetic induction to generate rotational motion. Unlike a generator, which converts mechanical energy into electrical energy, a motor works in the opposite direction. A transformer, on the other hand, is a device that transfers electrical energy between two or more circuits through electromagnetic induction, but it does not directly convert electrical energy to mechanical energy. A magnet, although it can interact with magnetic fields, does not have the capability to transform electrical energy to mechanical energy.

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• 11.

### A device that transforms mechanical energy into electrical energy is a

• A.

Generator.

• B.

Motor.

• C.

Transformer.

• D.

Magnet.

• E.

None of these

A. Generator.
Explanation
A device that transforms mechanical energy into electrical energy is a generator. A generator works by converting the mechanical energy from a rotating shaft into electrical energy through the process of electromagnetic induction. This is achieved by using a magnetic field and conductive coils to generate an electric current. In contrast, a motor converts electrical energy into mechanical energy, a transformer transfers electrical energy between different circuits, and a magnet is a material that produces a magnetic field. Therefore, the correct answer is generator.

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• 12.

### If the voltage produced by a generator alternates, it does so because

• A.

Unlike a battery, it produces alternating current.

• B.

The changing magnetic field that produces it alternates.

• C.

Alterations in the mechanical energy input.

• D.

In effect it is an ac motor in reverse.

• E.

The current it produces alternates.

B. The changing magnetic field that produces it alternates.
Explanation
The correct answer is "the changing magnetic field that produces it alternates." This is because a generator works based on the principle of electromagnetic induction, where a magnetic field is used to induce a current in a conductor. As the magnetic field changes, it causes the electrons in the conductor to move back and forth, resulting in an alternating current. Unlike a battery, which produces direct current, a generator produces alternating current due to the alternating magnetic field.

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• 13.

### A principle difference between an MHD generator and a conventional generator is that the MHD generator

• A.

Has no moving parts.

• B.

Operates more efficiently at high temperatures.

• C.

Choices A and B are both true.

• D.

Choices A and B are both false.

C. Choices A and B are both true.
Explanation
An MHD generator is different from a conventional generator because it has no moving parts and operates more efficiently at high temperatures. This means that it does not require any mechanical components to generate electricity, which reduces the chances of mechanical failure and increases its reliability. Additionally, the MHD generator's efficiency is enhanced at high temperatures, allowing it to convert a greater amount of thermal energy into electrical energy. Therefore, both statements A and B are true, making the answer "Choices A and B are both true."

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• 14.

### A transformer actually transforms

• A.

Voltage.

• B.

Magnetic field lines.

• C.

Generators into motors.

• D.

Non-safe forms of energy to safe forms of energy.

• E.

All of the above choices are correct.

A. Voltage.
Explanation
A transformer is a device that is used to transfer electrical energy between two or more circuits through electromagnetic induction. It works by changing the voltage of an alternating current. Therefore, the correct answer is voltage.

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• 15.

### The primary of a transformer is the coil connected to

• A.

• B.

The Internet.

• C.

The power line.

• D.

None of these

C. The power line.
Explanation
The primary coil of a transformer is the one that is connected to the power line. This coil is responsible for receiving the alternating current from the power source and transmitting it to the secondary coil. The primary coil is designed to have a higher number of turns compared to the secondary coil, allowing it to step up or step down the voltage as required. Therefore, the correct answer is "the power line."

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• 16.

### Transformers use ac so there will be the required

• A.

Transfer of energy from coil to coil.

• B.

Voltage for transformation.

• C.

Change in magnetic field for operation.

• D.

Change in input current.

• E.

Magnetic field intensities.

C. Change in magnetic field for operation.
Explanation
Transformers operate by inducing a change in magnetic field. When an alternating current (AC) flows through the primary coil, it creates a changing magnetic field. This changing magnetic field then induces a voltage in the secondary coil, allowing for the transfer of energy from one coil to another. Therefore, the change in magnetic field is essential for the operation of transformers.

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• 17.

### Compared to the primary voltage, the secondary voltage may be

• A.

Larger.

• B.

Smaller.

• C.

The same.

• D.

The same or smaller, but not larger.

• E.

Larger, smaller, or the same.

E. Larger, smaller, or the same.
Explanation
The secondary voltage can be larger, smaller, or the same as the primary voltage. This is because the secondary voltage is determined by the turns ratio of the transformer, which can be adjusted to step up or step down the voltage. Therefore, depending on the design and configuration of the transformer, the secondary voltage can vary in relation to the primary voltage.

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• 18.

### The output power of an ideal transformer is

• A.

Greater than the input power.

• B.

Equal to the input power.

• C.

Smaller than the input power.

• D.

May be any of these

B. Equal to the input power.
Explanation
An ideal transformer is designed to have no losses, meaning that all the power supplied to the input is transferred to the output. This is achieved by the principle of energy conservation, where the input power is converted into magnetic fields in the transformer's core and then back into electrical power at the output. Therefore, the output power of an ideal transformer is equal to the input power.

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• 19.

### Disconnect a small-voltage battery from a coil of many loops of wire and a large voltage is produced by

• A.

The sudden collapse in the magnetic field.

• B.

Latent energy in the battery.

• C.

The resistance of the battery to a change in polarity.

• D.

• E.

The electric field between the battery terminals.

A. The sudden collapse in the magnetic field.
Explanation
When a small-voltage battery is disconnected from a coil of many loops of wire, a large voltage is produced due to the sudden collapse in the magnetic field. This phenomenon is known as electromagnetic induction. When the current flowing through the coil is abruptly interrupted, the magnetic field collapses rapidly, inducing a high voltage in the coil. This is explained by Faraday's law of electromagnetic induction, which states that a change in the magnetic field induces an electromotive force (EMF) in a nearby conductor. Therefore, the sudden collapse in the magnetic field is the correct explanation for the large voltage produced.

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• 20.

### Power is transmitted at high voltages because the corresponding current in the wires is

• A.

Also high to deliver appreciable power to distant places.

• B.

Low so that overheating of the wires is minimized.

• C.

Choices A and B are both true.

• D.

Choices A and B are both false.

B. Low so that overheating of the wires is minimized.
Explanation
Power is transmitted at high voltages because when voltage is increased, the corresponding current in the wires decreases according to the equation P = IV. This helps to minimize the overheating of the wires, as lower current means less resistance and less heat generated. Additionally, transmitting power at high voltages allows for more efficient long-distance transmission, as lower current reduces energy losses due to resistance in the wires. Therefore, the correct answer is "low so that overheating of the wires is minimized."

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• 21.

### Rapid change of a magnetic field induces

• A.

A magnetic field of greater magnitude.

• B.

A magnetic field of the same magnitude.

• C.

An electric field.

C. An electric field.
Explanation
When there is a rapid change in a magnetic field, it induces an electric field. This phenomenon is known as electromagnetic induction and is described by Faraday's law of electromagnetic induction. According to this law, a changing magnetic field creates an electric field that circulates around the changing magnetic field. This electric field is perpendicular to both the direction of the magnetic field and the direction of the change in the magnetic field. Therefore, the correct answer is that a rapid change of a magnetic field induces an electric field.

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• 22.

### The metal detectors that people walk through at airports operate via

• A.

Ohm's law.

• B.

• C.

Coulomb's law.

• D.

Newton's laws.

• E.

Civil laws.

Explanation
The metal detectors at airports operate based on Faraday's law. Faraday's law states that a changing magnetic field induces an electric current in a conductor. The metal detectors emit a magnetic field, and when a person walks through it, any metal objects on their body will disrupt the magnetic field, causing a change. This change in the magnetic field induces an electric current in the metal object, which is detected by the metal detector. Therefore, Faraday's law is the correct explanation for how metal detectors at airports work.

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• 23.

### A step-up transformer increases

• A.

Power.

• B.

Energy.

• C.

Both power and energy.

• D.

Neither power or energy.

D. Neither power or energy.
Explanation
A step-up transformer does not increase power or energy. It is designed to increase voltage while decreasing current in an electrical circuit. This allows for efficient long-distance transmission of electricity, but it does not change the total power or energy in the system. Power is the rate at which energy is transferred or converted, and a transformer only changes the voltage and current, not the overall power or energy. Therefore, the correct answer is neither power nor energy.

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• 24.

### A step-up transformer has a ratio of one to ten. Neglecting slight losses, if 100 W of power go into the primary coil, the power coming from the secondary coil is

• A.

1 W.

• B.

10 W.

• C.

100 W.

• D.

1000 W.

• E.

None of these

C. 100 W.
Explanation
In a step-up transformer, the number of turns in the secondary coil is greater than the number of turns in the primary coil. This results in an increase in voltage and a decrease in current. However, the power remains the same in an ideal transformer. Since the power going into the primary coil is 100 W, the power coming from the secondary coil will also be 100 W.

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• 25.

### A certain transformer doubles input voltage. If the primary coil has 10 A of current, then the current in the secondary coil is

• A.

2 A.

• B.

5 A.

• C.

10 A.

• D.

25 A.

• E.

None of these

B. 5 A.
Explanation
A transformer works by changing the voltage and current levels in a circuit. In this case, the transformer doubles the input voltage. According to the principle of conservation of energy, the product of voltage and current should remain constant. Therefore, if the voltage doubles, the current in the secondary coil must be half of the current in the primary coil. Since the primary coil has 10 A of current, the current in the secondary coil would be 5 A.

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• 26.

### Metal detectors, like the ones used at airports, are activated by

• A.

Electric fields.

• B.

Magnetic fields.

• C.

Electromagnetic induction.

• D.

Alternating current.

C. Electromagnetic induction.
Explanation
Metal detectors, like the ones used at airports, are activated by electromagnetic induction. This is because metal detectors work by creating a magnetic field and then detecting any disturbances in that field caused by metal objects. When a metal object passes through the magnetic field, it induces a current in the metal, which in turn creates its own magnetic field. This change in the magnetic field is detected by the metal detector, alerting the user to the presence of metal.

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• 27.

### The principal advantage of ac power over dc power is that

• A.

More energy is dissipated during transmission.

• B.

Ac voltage oscillates while dc voltage does not.

• C.

Ac voltage can be transformed via conventional transformers.

• D.

Ac circuits multiply power more easily.

• E.

Ac circuits are safer.

C. Ac voltage can be transformed via conventional transformers.
Explanation
AC power has the advantage over DC power because AC voltage can be easily transformed using conventional transformers. Transformers are devices that can change the voltage level of an AC power signal. This allows for efficient transmission and distribution of electricity over long distances. In contrast, DC voltage cannot be easily transformed using transformers, which limits its use in power transmission systems.

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• 28.

### The major advantage of MHD generators over conventional generators is that MHD generators

• A.

Do not use electromagnetic induction.

• B.

Do not require magnets.

• C.

Require no power input.

• D.

All of these

• E.

None of these

E. None of these
Explanation
The major advantage of MHD generators over conventional generators is not that they do not use electromagnetic induction, do not require magnets, or require no power input. Therefore, the correct answer is none of these. The actual advantage of MHD generators lies in their ability to directly convert the kinetic energy of a high-velocity fluid, such as hot gases or plasma, into electrical energy without the need for any moving parts. This makes MHD generators more efficient, reliable, and suitable for certain applications, such as space propulsion and power generation in fusion reactors.

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• 29.

### Neon signs require about 12,000 volts to operate. If the circuit uses a 120-volt power source, the ratio of primary to secondary turns on the transformer should be

• A.

1:100.

• B.

100:1.

• C.

Neither of these

A. 1:100.
Explanation
Neon signs require a high voltage of around 12,000 volts to operate. To achieve this voltage from a 120-volt power source, a transformer is used. The ratio of primary to secondary turns on the transformer determines the voltage output. In this case, the correct answer is 1:100, which means that for every 1 turn in the primary coil, there are 100 turns in the secondary coil. This high ratio allows the transformer to step up the voltage from 120 volts to the required 12,000 volts for the neon sign to function properly.

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• 30.

### The voltage across the input terminals of a transformer is 110 V. The primary has 50 loops and the secondary has 25 loops. The voltage the transformer puts out is

• A.

25 V.

• B.

55 V.

• C.

110 V.

• D.

220 V.

• E.

None of these

B. 55 V.
Explanation
The voltage across the input terminals of a transformer is determined by the ratio of the number of loops in the primary and secondary coils. In this case, the primary has 50 loops and the secondary has 25 loops. Since the primary has more loops than the secondary, the output voltage will be lower than the input voltage. The voltage the transformer puts out is 55 V, which is less than the input voltage of 110 V.

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• 31.

### Four amperes of current exist in the primary coil of a transformer. The voltage across the primary coil is 110 V. What is the power output of the secondary coil?

• A.

27.5 W

• B.

110 W

• C.

440 W

• D.

880 W

• E.

Not enough information to say

C. 440 W
Explanation
The power output of the secondary coil can be calculated using the formula P = IV, where P is power, I is current, and V is voltage. In this case, the current in the primary coil is 4 A and the voltage across the primary coil is 110 V. Therefore, the power output of the secondary coil is 440 W.

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• 32.

### Disregarding the effects of friction, a cyclist will coast farther when a lamp powered by a wheel generator is

• A.

Of low wattage.

• B.

Of high wattage.

• C.

Burned out.

• D.

None of these

C. Burned out.
Explanation
When the lamp powered by a wheel generator is burned out, it means that there is no resistance or load on the wheel generator. Without any load, the cyclist can coast farther because there is no energy being consumed by the lamp. This allows the cyclist to utilize all the energy generated by the wheel generator for forward motion, resulting in a longer distance covered.

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• 33.

### If the primary of a transformer were connected to a dc power source, the transformer would have a voltage output

• A.

At a higher efficiency than with an ac source.

• B.

The same.

• C.

That is also dc.

• D.

Only while being connected or disconnected.

• E.

None of these

D. Only while being connected or disconnected.
Explanation
If the primary of a transformer is connected to a DC power source, the transformer would only produce a voltage output while it is being connected or disconnected. This is because a transformer operates based on the principle of electromagnetic induction, which requires a changing magnetic field to induce a voltage in the secondary coil. Since a DC power source provides a constant voltage, there is no changing magnetic field and therefore no voltage output.

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