Chapter 25: Electromagnetic Induction

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.

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.

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.

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.

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.

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.

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.

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.

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.

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."

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.

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.

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.

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.

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.

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.

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.

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.

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.

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."

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.

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.

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.

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.

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.

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.

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.

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."

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.

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.

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.

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.

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.