Eddy Currents MCQ Quiz - Objective Question with Answer for Eddy Currents - Download Free PDF

The correct answer is Option 1: Transformers.

Explanation:

Eddy currents: Induced circular currents in conductors when exposed to changing magnetic fields.

Common Applications: Speedometers, magnetic brakes, induction furnaces.

Transformers: Eddy currents in transformers cause energy losses and are not desired.

Transformers do not use eddy currents; rather, they are negatively affected by them, resulting in efficiency loss.

Speedometers, magnetic brakes, and induction furnaces intentionally utilize eddy currents for their operation.

​The correct option is: a changing magnetic field

Explanation:

Eddy Currents

Definition: Eddy currents are loops of electrical current induced within conductors by a changing magnetic field in the conductor, due to Faraday's law of induction. These currents flow in closed loops within the conductor, in planes perpendicular to the magnetic field.

This option is correct because eddy currents are produced in a conductor when it is subjected to a changing magnetic field. According to Faraday’s law of electromagnetic induction, a time-varying magnetic field can induce an electromotive force (EMF) in a conductor. This induced EMF can cause circulating currents within the conductor, known as eddy currents. The direction of these currents is given by Lenz’s law, which states that the induced currents will flow in such a way as to oppose the change in magnetic flux that caused them.

When a conductor moves through a magnetic field or when the magnetic field around a stationary conductor changes, the magnetic flux through the conductor changes. This change in magnetic flux induces an EMF in the conductor, leading to the formation of eddy currents. These currents can create their own magnetic fields, which can oppose the original magnetic field, leading to various effects such as heating and electromagnetic damping.

Applications and Effects:

• Electromagnetic Braking: Eddy currents are used in electromagnetic braking systems, where they provide a non-contact method of braking by inducing currents in a metallic disc or drum, which generates a resistive force opposing the motion.
• Induction Heating: Eddy currents are utilized in induction heating processes, where the currents generate heat within the material, making it useful for processes like melting, forging, and heat treatment of metals.
• Transformers: In transformers, eddy currents can lead to energy losses in the form of heat. Laminated cores are used to minimize these losses by reducing the path for eddy currents, thus improving efficiency.
• Metal Detectors: Eddy currents are used in metal detectors to locate metallic objects. The changing magnetic field induces eddy currents in the metal, which in turn produce a secondary magnetic field that can be detected.

CONCEPT:

Eddy current:

• When a metal moves in a magnetic field or when the magnetic field through a stationary metal is altered, an induced current is produced in the metal.
• This induced current flows in the metal in the form of closed loops resembling ‘eddies’ or whirlpool.
• Hence this current is called eddy current.
• The direction of the eddy current is given by Lenz’s law.

EXPLANATION:

• In diathermy, eddy currents are used for the localized heating of tissues in the human body. Therefore option 1 is correct.
• The speedometer is a device used to measure the instantaneous speed of the vehicle.
• Energy meters use the concept of an eddy current to record the consumption of electricity.
• The induction furnace is based on the heating effect of eddy currents.

CONCEPT:

Eddy current- Eddy current is defined as the induced electromotive force produced in the coil when there is a change of magnetic flux linked to the given coil or we can say that the eddy current is the loops of electrical current induced within the conductors by changing magnetic field in the conductor. It is called as eddy current.

EXPLANATION:

1. Induction furnace- An induction furnace, it is having a coil that is carrying an alternating current that surrounds the chamber of the metal. In this furnace, the eddy currents are induced in the metal and the circulation of these currents will be producing extremely high temperatures which make metal melt.
2. Magnetic braking in trains - In trains, the eddy current is used in the brakes. It is used to slow or stop a moving object by dissipating its kinetic energy.
3. Electromagnet- Electromagnetic is the substance in which a magnetic field is produced by supplying the electric current through the material. When the electric current is supplying through the material, there is a change of magnetic flux into the material which produces loops.
4. Electric heater- The electric heater works on the principle of Joule's heating effect. It states that the heat produced in the resistor is directly proportional to the resistance for a given current and the time in which current is flowing through the conductor.

​Hence, option 4) is the correct answer.

CONCEPT:

• On applying an alternating magnetic field to a magnetic material an emf is induced in the material itself according to Faraday’s Law of Electromagnetic induction.
• Since the magnetic material is a conducting material, this EMF circulates currents within the body of the material. These circulating currents are called Eddy Currents.
  • They will occur when the conductor experiences a changing magnetic field.

EXPLANATION:

• Eddy current is used in induction heating. So option 1 is wrong.
• The magnetic braking system in trains and the electromagnets both produce the eddy current. So options 2 and 3 are wrong.
• Induction heating is the process of heating in which heat generated in the object by eddy currents.
• The electric heater is based on the Joule heating effect. It doesn't involve eddy currents. So option 4 is correct.

Important Points

• An induction heater made an electromagnet and an electronic oscillator that passes a high-frequency alternating current (AC) through the electromagnet
• The rapidly alternating magnetic field penetrates the object, generating electric currents inside the conductor called eddy currents

CONCEPT:

• Eddy Current: When a changing magnetic flux is applied to a bulk piece of conducting material then circulating currents is called eddy currents are induced in the material.
  • Because the resistance of the bulk conductor is usually low, eddy currents often have large magnitudes and heat the conductor.

EXPLANATION:

• Eddy currents are the currents induced in solid metallic masses when magnetic flux threading through them changes.
• Eddy current is also known as“Focault current”.
• Eddy current also opposes the change in magnetic flux, so there is given by Lenz’s law. 

Eddy current:

• The changing flux due to a circulating current induces voltages in the material according to Faraday’s laws of electromagnetic induction.
• Since the material is conducting, these induced voltages circulate currents within the body of the material.
• These induced currents do no useful work and are known as eddy currents.
• The losses that occurred due to eddy current are known as eddy current loss.

Eddy current loss is given by:

\({P_e} = K{f^2}B_m^2{t^2}V\)

Where,

K - coefficient of eddy current. Its value depends upon the nature of magnetic material

Bm - Maximum value of flux density in Wb/m2

t - Thickness of lamination in meters

f - Frequency of reversal of the magnetic field in Hz

V - Volume of magnetic material in m3

Thus, we can say that the eddy current depends on the frequency, flux density, and thickness of the core.

CONCEPT:

Induction type wattmeters: 

• Induction type wattmeters work on the induction phenomenon and hence this instrument only measure AC power
• It has two laminated electromagnets, one of which is excited by load current or fraction of it and is connected in series with the circuit, known as series magnet
• The second one is excited by the current proportional to the applied voltage or fraction of it and is always connected across the supply, known as shunt magnet.

EXPLANATION:

• An aluminium disc is so mounted so that it cuts the fluxes produced by both the magnets, as a result of which, two e.m.fs are produced which induces two eddy currents in the disc
• Deflecting torque is produced due to the interaction of these eddy currents and the inducing flux. Therefore option 4 is correct.
• Copper shading bands are provided either on the central limb or on the outer limb of the shunt magnet and can be so adjusted as to make the resultant flux in the shunt magnet lag the applied voltage by 90°.

CONCEPT:

Eddy Current:

• When a changing magnetic flux is applied to a bulk piece of conducting material then circulating currents is called eddy currents are induced in the material.
• Because the resistance of the bulk conductor is usually low, eddy currents often have large magnitudes and heat the conductor.

EXPLANATION:

• Speedometers: In a speedometer, a magnet rotates with the speed at the vehicle.
• The magnet is placed inside an aluminum drum which is carefully pivoted and held in position by a hairspring.
• As the magnet rotates, eddy currents are set up in the drum which opposes the motion of the magnet.
• A torque is exerted on the drum in the opposite direction which deflects the drum through an angle depending on the speed of the vehicle

CONCEPT:

• Eddy Current: When a changing magnetic flux is applied to a bulk piece of conducting material then circulating currents is called eddy currents are induced in the material.
• Because the resistance of the bulk conductor is usually low, eddy currents often have large magnitudes and heat the conductor.
• Due to heating of the conductor there is a loss in energy. This loss is called eddy current loss.

EXPLANATION:

• The eddy current loss can be reduced by increasing the resistance of the core. So option 1 is not correct.
• The eddy current loss can be reduced by using a laminated core like in the above diagram. So option 2 is correct.

CONCEPT:

• Eddy Current: The loops of electrical current induced within conductors by changing magnetic fields in the conductor are called eddy currents.
  • Eddy currents are generated according to Faraday’s law of induction.
  • Eddy currents flow in the form whirlpool.
  • Within conductors, Eddy currents flow in closed loops, in the perpendicular plane of the magnetic field.
  • Eddy currents produce undesirable effects if the large eddy current flows in the transformer or core of a choke coil etc.
• By using a laminated core, the eddy currents produced in the core of a transformer are reduced.
• Eddy currents cause a loss of energy because they have the tendency to oppose.
• Eddy currents transform useful energy, into heat, which isn’t generally useful.
• Nowadays, electric brakes are provided on the trains in addition to those vacuum brakes.

EXPLANATION:

• Option 1: Eddy currents flow in a closed-loop, not in a straight line.
  • So this is not the correct answer.
• Option 2: Eddy currents lose the generated electrical energy by generating heat.
  • So this is not the correct answer.
• Option 3: By making laminated core in the transformer, eddy currents are reduced.
  • This makes to decreases the loss of energy.
• So this is not the correct answer.
• Option 4: Eddy currents transform useful energy, into heat, which isn’t generally useful.
  • So this is the correct statement.
• Hence the correct answer is option 4.

CONCEPT:

Eddy current:

• Eddy currents are loops of electrical current induced within conductors by a changing magnetic field in the conductor according to Faraday’s law of induction.

​

​EXPLANATION:

• Induction heating is the process of heating an electrically conducting object (usually a metal) by electromagnetic induction, through heat generated in the object by eddy currents.
• So we can say that the heat is generated in the induction furnace by eddy current. Hence, option 1 is correct.

 First, check \(\frac{v}{f}\) ratio of transformer.

\(\frac{{160}}{{40}} = \frac{{200}}{{50}} = 4\)

\(\frac{v}{f}\) Ratio of the transformer is constant.

So, eddy current loss in the transformer will be directly proportional to the square of the frequency

∴ P_e ∝ f²

\(\frac{{{P_{e2}}}}{{{P_{e1}}}} = {\left( {\frac{{{f_2}}}{{{f_1}}}} \right)^2}\)

\(\frac{{{P_{e2}}}}{{{P_{e1}}}} = {\left( {\frac{{50}}{{40}}} \right)^2}\)

\({P_{e2}} = \frac{{25}}{{16}} \times 240\)

P_e2 = 25 × 15 = 375 W

New eddy current loss will 375 W.

The correct answer is Change in magnetic flux.

CONCEPT:

Eddy current:

• When a metal moves in a magnetic field or when the magnetic field through a stationary metal is altered, an induced current is produced in the metal.
  • This induced current flows in the metal in the form of closed loops resembling ‘eddies’ or whirlpool.
  • Hence this current is called eddy current.
  • The direction of the eddy current is given by Lenz’s law.

EXPLANATION:

• When a changing magnetic flux is applied to a bulk piece of conducting material then circulating currents is called eddy currents are induced in the material. Therefore option is 2 correct.

CONCEPT:

Eddy current- Eddy current is defined as the induced electromotive force produced in the coil when there is a change of magnetic flux linked to the given coil or we can say that the eddy current is the loops of electrical current induced within the conductors by changing magnetic field in the conductor. It is called as eddy current.

EXPLANATION:

1. Induction furnace- An induction furnace, it is having a coil that is carrying an alternating current that surrounds the chamber of the metal. In this furnace, the eddy currents are induced in the metal and the circulation of these currents will be producing extremely high temperatures which make metal melt.
2. Magnetic braking in trains - In trains, the eddy current is used in the brakes. It is used to slow or stop a moving object by dissipating its kinetic energy.
3. Electromagnet- Electromagnetic is the substance in which a magnetic field is produced by supplying the electric current through the material. When the electric current is supplying through the material, there is a change of magnetic flux into the material which produces loops.
4. Electric heater- The electric heater works on the principle of Joule's heating effect. It states that the heat produced in the resistor is directly proportional to the resistance for a given current and the time in which current is flowing through the conductor.

​Hence, option 4) is the correct answer.