Eddy Current Questions in English

(A) When a bar magnet falls vertically through the hollow region of a long vertical copper tube,the magnetic flux linked with the copper tube changes due to the non-uniform magnetic field of the magnet.
According to Faraday's law,this change in flux induces an electromotive force $(EMF)$ in the tube,which generates eddy currents in the body of the tube.
By Lenz's law,these eddy currents oppose the motion of the falling magnet,creating an upward retarding force.
As the velocity of the magnet increases,the rate of change of flux increases,which in turn increases the magnitude of the eddy currents and the retarding force.
Eventually,the retarding force becomes equal to the weight of the magnet $(mg)$.
At this point,the net force on the magnet becomes zero,and it attains a constant terminal velocity,falling with zero acceleration.

(A) When a bar magnet falls vertically through a long hollow metal cylinder,the magnetic flux linked with the metal tube changes,inducing eddy currents in the tube's body.
According to Lenz's law,these eddy currents create a magnetic field that opposes the motion of the falling magnet.
This creates a retarding force that increases with the velocity of the magnet.
As the magnet falls,the retarding force eventually equals the weight of the magnet $(mg)$.
At this point,the net force becomes zero,and the magnet reaches a constant terminal velocity.
Since the velocity is constant,the final acceleration of the magnet is zero.

(D) Eddy currents are currents induced in a conductor when it is subjected to a changing magnetic field.
These currents are utilized in various practical applications:
$1$. Induction furnaces: High-frequency eddy currents are used to produce intense heat for melting metals.
$2$. Electromagnetic brakes: Used in trains to provide braking by inducing eddy currents that oppose the motion of the wheels.
$3$. Speedometers: The magnetic field of a rotating magnet induces eddy currents in a metal drum,which creates a torque proportional to the speed,allowing for measurement.
Since all the given options utilize eddy currents,the correct answer is $D$.

(D) Eddy currents are loops of electrical current induced within conductors by a changing magnetic field in the conductor.
They are intentionally used in devices like moving coil galvanometers (for electromagnetic damping),electric brakes (to stop trains),and induction motors.
However,in a dynamo,eddy currents are generally considered undesirable as they cause energy loss in the form of heat.
Therefore,the dynamo is the exception among the given options.

(C) Eddy currents are induced in a metallic conductor when it is subjected to a changing magnetic flux.
According to the principles of electromagnetic induction,these currents flow in closed loops within the material in a plane perpendicular to the direction of the magnetic field lines.
As shown in the figure,the magnetic field lines pass through the metallic block,and the eddy currents circulate in a plane that is perpendicular $(90^o)$ to these field lines.
Therefore,the angle between the plane of the eddy currents and the plane of the magnetic lines of force is $90^o$.

(D) Eddy currents are loops of electrical current induced within conductors by a changing magnetic field.
$1$. Induction furnaces use eddy currents to produce high heat for melting metals.
$2$. Galvanometer damping uses eddy currents to bring the coil to rest quickly.
$3$. Speedometers in automobiles use eddy currents to measure the rotation speed of the wheels.
$4$. $X$-ray crystallography is a technique used for determining the atomic and molecular structure of a crystal,which relies on the diffraction of $X$-rays,not eddy currents.
Therefore,$X$-ray crystallography is not an application of eddy currents.

(A) The core of a transformer is subjected to a changing magnetic flux,which induces eddy currents in the core material.
These eddy currents cause significant energy loss in the form of heat ($I^2R$ loss).
By laminating the core,the path for these eddy currents is broken into smaller sections,which increases the resistance to their flow.
Consequently,the magnitude of the eddy currents is significantly reduced,thereby minimizing energy losses.

(A) Eddy currents are induced currents produced in a bulk piece of a conductor when the magnetic flux linked with it changes. According to Faraday's law of electromagnetic induction,a change in magnetic flux induces an electromotive force $(EMF)$. When a metal is placed in a varying magnetic field,the magnetic flux linked with the metal changes over time,which induces circulating currents within the bulk of the metal. These currents are known as eddy currents. Therefore,the correct condition is that the metal must be in a varying magnetic field.

(D) The core of a dynamo (or transformer) is laminated to reduce the production of eddy currents. When a magnetic flux changes through a solid metallic core,large induced currents called eddy currents are produced,which cause significant energy loss in the form of heat. By using thin,insulated metal sheets (lamination),the path for these eddy currents is broken,thereby increasing the resistance and significantly reducing the energy loss due to eddy currents.

(C) When magnetic flux linked with a coil changes,an induced $emf$ is produced in it,and the induced current flows through the material. These induced currents are set up in the conductor in the form of closed loops. These currents resemble eddies or whirlpools and are known as $Eddy$ currents. These currents oppose the cause of their origin; therefore,due to $Eddy$ currents,a significant amount of energy is wasted in the form of heat energy. If the core of the transformer is laminated,the path for these currents is restricted,and their effect is minimized.

(B) Eddy currents are induced currents that flow in the metallic core of a transformer due to the changing magnetic flux,leading to energy loss in the form of heat.
To minimize these currents,the core is constructed using thin,insulated sheets of metal stacked together,known as a laminated core.
Lamination increases the resistance of the path for eddy currents,thereby significantly reducing their magnitude and the associated power loss.

(B) The core of a transformer is laminated to minimize the energy losses due to eddy currents. When a time-varying magnetic flux passes through the metallic core,it induces eddy currents in the core,which lead to heating and energy dissipation. By using thin,insulated laminated sheets,the path for these eddy currents is restricted,significantly reducing the magnitude of the currents and thus minimizing energy loss.

(D) When the coil oscillates in the magnetic field,the magnetic flux linked with the nearby aluminium plate changes continuously.
According to Faraday's law of electromagnetic induction,this change in magnetic flux induces eddy currents in the aluminium plate.
According to Lenz's law,these eddy currents create a magnetic field that opposes the motion of the coil.
This phenomenon is known as electromagnetic damping,which causes the coil to come to rest quickly.

(B) As the bar magnet falls through the copper tube,the change in magnetic flux linked with the tube induces eddy currents in the copper walls.
According to Lenz's law,these eddy currents create a magnetic field that opposes the motion of the falling magnet.
As the magnet gains speed,the induced electromotive force $(EMF)$ and the resulting eddy currents increase,leading to a greater opposing magnetic force.
Eventually,the upward magnetic force balances the downward gravitational force $(mg)$.
At this point,the net force on the magnet becomes zero,and it reaches a terminal velocity,moving with an almost constant speed.

(D) When the metallic ring moves through the magnetic field,the magnetic flux linked with the ring changes. According to Faraday's law of electromagnetic induction,an electromotive force $(EMF)$ is induced in the ring. Since the ring is metallic,this induced $EMF$ causes eddy currents to flow within it. According to Lenz's law,these eddy currents produce a magnetic force that opposes the motion of the ring. This opposing force acts as a damping force,causing the kinetic energy of the pendulum to dissipate as heat due to the electrical resistance of the ring. Consequently,the amplitude of oscillation decreases rapidly,and the pendulum comes to rest very soon.

(B) When a bar magnet falls vertically through the hollow region of a long vertical copper tube,the changing magnetic flux linked with the tube induces eddy currents in the body of the tube.
According to Lenz's law,these eddy currents oppose the motion of the falling magnet,creating a retarding force.
As the velocity of the magnet increases,the rate of change of magnetic flux increases,which in turn increases the magnitude of the eddy currents and the resulting retarding force.
Eventually,the retarding force becomes equal to the weight of the magnet.
At this point,the net force on the magnet becomes zero,and it attains a constant final terminal velocity,falling with zero acceleration.

(A) According to Lenz's law,the direction of the induced eddy current is such that it opposes the change in the magnetic flux that produces it.
Given that the magnetic field is increasing from zero to maximum,the magnetic flux through the metal sheet is increasing.
To oppose this increase,the eddy current must create a magnetic field that points in the opposite direction to the external magnetic field.
The figure shows the eddy currents flowing in a counter-clockwise direction.
Using the right-hand thumb rule,a counter-clockwise current creates a magnetic field that points outwards,perpendicular to the plane of the paper.
Since the eddy current's magnetic field opposes the external field,the external magnetic field must be pointing inwards,perpendicular to the plane of the paper.

(D) When the metallic ring moves through the magnetic field,the magnetic flux linked with the ring changes.
According to Faraday's law of electromagnetic induction,an induced electromotive force $(EMF)$ is produced in the ring.
Since the ring is metallic,it has some resistance,which leads to the flow of induced eddy currents.
According to Lenz's law,these eddy currents oppose the motion that causes them.
This results in a damping force acting on the pendulum,which dissipates the mechanical energy of the pendulum as heat.
Consequently,the amplitude of oscillation decreases rapidly,and the pendulum comes to rest very soon.

(D) Large eddy currents are produced in a non-laminated iron core of a transformer by the induced $emf$,as the resistance of a bulk iron core is very small.
By using thin iron sheets as a core,the electrical resistance is increased.
Laminating the core substantially reduces the eddy currents.
Eddy currents heat up the core of the transformer,leading to energy loss in the form of heat.
Therefore,more eddy currents result in greater energy loss,which decreases the efficiency of the transformer.
Since both the Assertion and the Reason are false,the correct option is $D$.

(D) Eddy currents are loops of electrical current induced within conductors by a changing magnetic field. They are utilized in induction furnaces for heating,magnetic braking systems in trains,and speedometers. An electric heater,however,operates on the principle of the Joule heating effect $(H = I^2Rt)$,where heat is produced by the resistance of a conductor to the flow of electric current,not by eddy currents.

(A) The material used for the core of an electromagnet should have high resistivity.
This is because high resistivity reduces the magnitude of eddy currents induced in the core when the magnetic flux changes.
By minimizing eddy currents,the energy loss in the form of heat is significantly reduced,thereby increasing the efficiency of the electromagnet.

(N/A) We have studied electric currents induced in well-defined paths in conductors like circular loops. Even when bulk pieces of conductors are subjected to changing magnetic flux,induced currents are produced in them. However,their flow patterns resemble swirling eddies in water. This effect was discovered by physicist Foucault ($1819$-$1868$) and these currents are called eddy currents.
Consider the apparatus shown in the figure. $A$ copper plate is allowed to swing like a simple pendulum between the pole pieces of a strong magnet. It is found that the motion is damped and in a little while,the plate comes to a halt in the magnetic field.
We can explain this phenomenon on the basis of electromagnetic induction. The magnetic flux associated with the plate keeps on changing as the plate moves in and out of the region between magnetic poles. The flux change induces eddy currents in the plate.
The directions of eddy currents are opposite when the plate swings into the region between the poles and when it swings out of the region. Eddy currents are often undesirable since they heat up the core and dissipate electrical energy in the form of heat. To reduce these effects,the core is often laminated or slotted to break the paths of the eddy currents.
Eddy currents are used to advantage in certain applications like:
$(1)$ Magnetic braking in trains: Strong electromagnets are situated above the rails in some electrically powered trains. When the electromagnets are activated,the eddy currents induced in the rails oppose the motion of the train. As there are no mechanical linkages,the braking effect is smooth.
$(2)$ Electromagnetic damping: Certain galvanometers have a fixed core made of non-magnetic metallic material. When the coil oscillates,the eddy currents generated in the core oppose the motion and bring the coil to rest quickly.
$(3)$ Induction furnace: Induction furnaces can be used to produce high temperatures and can be utilized to prepare alloys by melting the constituent metals. $A$ high-frequency alternating current is passed through a coil which surrounds the metals to be melted. The eddy currents generated in the metals produce high temperatures sufficient to melt them.
$(4)$ Electric power meters: The shiny metal disc in the electric power meter (analogue type) rotates due to the eddy currents. Electric currents are induced in the disc by magnetic fields produced by sinusoidally varying currents in a coil.

(N/A) Eddy currents are loops of electrical current induced within conductors by a changing magnetic field. Despite often being considered a nuisance due to energy loss as heat,they have several practical applications:
$1$. Magnetic Braking: Used in heavy trains and roller coasters. When the magnetic field is applied to the rotating wheels,eddy currents create a magnetic force that opposes the motion,bringing the vehicle to a stop.
$2$. Induction Furnaces: Used to melt metals. $A$ high-frequency alternating current is passed through a coil surrounding the metal,inducing strong eddy currents that generate intense heat,melting the metal.
$3$. Electromagnetic Damping: Used in galvanometers to bring the coil to rest quickly. The eddy currents induced in the metallic frame of the coil create a torque that opposes the motion,providing damping.
$4$. Induction Cooktops: Used for cooking. An alternating magnetic field induces eddy currents in the base of the cooking vessel,which heats up the vessel and the food inside.
$5$. Speedometers: Used to measure the speed of vehicles. $A$ rotating magnet near a metallic drum induces eddy currents,creating a torque that deflects the speedometer needle proportional to the speed.

(N/A) When a small magnet is allowed to fall through a long vertical aluminium pipe,the magnetic flux linked with the pipe changes as the magnet moves. This change in magnetic flux induces eddy currents in the pipe walls.
According to Lenz's law,these eddy currents oppose the motion of the falling magnet. The magnetic field produced by the eddy currents exerts an upward force on the magnet,which opposes its downward acceleration due to gravity. As a result,the magnet falls with a reduced acceleration,and its terminal velocity becomes much smaller than the acceleration due to gravity $(g)$.
If the pipe is cut along its length,the path for the eddy currents is broken,and the magnet will fall with an acceleration close to $g$.

(N/A) 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.
These currents flow in closed loops within conductors,in planes perpendicular to the magnetic field.
They were discovered by the French physicist $Léon$ $Foucault$ in $1851$.

(N/A) Electromagnetic damping is a phenomenon where the motion of a conductor (like a magnet moving near a metal) is retarded by the eddy currents induced within the conductor.
To demonstrate this,take two hollow thin cylindrical pipes of equal internal diameters made of aluminium and $PVC$ respectively. Fix them vertically with clamps on retort stands.
Take a small cylindrical magnet having a diameter slightly smaller than the inner diameter of the pipes and drop it through each pipe such that the magnet does not touch the sides.
You will observe that the magnet dropped through the $PVC$ pipe takes the same time to fall as it would in free space,because $PVC$ is an insulator and no eddy currents are generated.
However,the magnet takes a much longer time to fall through the aluminium pipe. This is because as the magnet falls,the changing magnetic flux induces eddy currents in the conducting aluminium pipe. According to Lenz's Law,these eddy currents create a magnetic field that opposes the motion of the magnet.
This retarding force,caused by the interaction between the magnet and the eddy currents,is known as electromagnetic damping.

(N/A) As the magnet falls through the aluminium pipe,the magnetic flux linked with the pipe changes.
This change in magnetic flux induces eddy currents in the aluminium pipe according to Faraday's law of electromagnetic induction.
According to Lenz's law,these eddy currents create a magnetic field that opposes the motion of the falling magnet.
This opposing force acts upwards,reducing the net downward force on the magnet.
Consequently,the acceleration of the magnet becomes less than the acceleration due to gravity $(g)$.

(N/A) When a cylindrical bar magnet is dropped through the metallic pipe,the magnetic flux linked with the pipe changes as the magnet falls.
According to Faraday's law of electromagnetic induction,this change in magnetic flux induces eddy currents in the metallic pipe.
According to Lenz's law,these eddy currents create a magnetic field that opposes the motion of the falling magnet.
Consequently,the magnet experiences an upward retarding force,causing its acceleration to be less than the acceleration due to gravity $(g)$.
In contrast,when an unmagnetised iron bar is dropped,there is no change in magnetic flux,and no eddy currents are induced.
Therefore,the unmagnetised iron bar falls with an acceleration equal to $g$,taking less time to pass through the pipe compared to the bar magnet.

(A) Eddy currents are induced in a conductor whenever the magnetic flux linked with the conductor changes.
This change in magnetic flux can occur if a metal block is placed in a time-varying magnetic field or if a metal block moves through a non-uniform magnetic field.
Therefore,when a metal block is kept in a changing magnetic field,the magnetic flux through it changes,inducing eddy currents according to Faraday's law of induction.
Thus,option $A$ is correct.

(B) When a metallic ball falls through the Earth's magnetic field,the change in magnetic flux linked with it induces eddy currents within the metal.
According to Lenz's Law,these eddy currents create a magnetic force that opposes the motion of the ball.
As a result,the metallic ball experiences a retarding force,causing it to take more time to reach the ground compared to the insulating ball,which does not experience such electromagnetic damping.
Therefore,the insulating ball reaches the Earth's surface earlier than the metal ball.

(A) As the bar magnet falls through the copper tube,the changing magnetic flux through the tube induces eddy currents in the copper walls according to Faraday's law of induction.
According to Lenz's law,these eddy currents create a magnetic field that opposes the motion of the magnet.
This opposing force acts upwards,counteracting the gravitational force $(mg)$ acting downwards.
As the speed of the magnet increases,the induced eddy currents and the resulting opposing magnetic force also increase.
Eventually,the upward magnetic force becomes equal to the downward gravitational force,resulting in a net force of zero.
At this point,the magnet reaches a terminal velocity and moves down with an almost constant speed.

(C) When a bar magnet falls through a metallic pipe,the magnetic flux linked with the pipe changes continuously.
This change in magnetic flux induces Eddy currents in the metallic pipe according to Faraday's law of electromagnetic induction.
According to Lenz's law,these Eddy currents oppose the cause that produces them,which is the motion of the falling magnet.
This creates an upward magnetic force that opposes the gravitational force,resulting in a slower downward acceleration of the magnet compared to a non-magnetic bar.
Therefore,both Assertion $A$ and Reason $R$ are true,and $R$ is the correct explanation of $A$.

(A) . $A$ magnetic pole will exert an attractive or repulsive force on a magnetic sheet,so a force is required to hold it in place.
$B$. If the sheet is non-magnetic,there is no magnetic interaction,so no force is needed to hold it.
$C$. If the sheet is conducting,moving it through a magnetic field induces eddy currents. According to Lenz's Law,these currents create a force that opposes the motion. Therefore,an external force is required to move the sheet with uniform velocity.
$D$. $A$ non-conducting and non-polar sheet does not interact with the magnetic field of the pole,so no force is required to move it.

(B) When the magnet is revolved above the disc,the magnetic flux linked with the aluminium disc changes continuously.
According to Faraday's law of electromagnetic induction,this change in magnetic flux induces an electromotive force $(EMF)$ in the disc.
Since the disc is a conductor,this induced $EMF$ causes eddy currents to flow within the disc.
According to Lenz's law,these eddy currents will flow in such a direction as to oppose the cause producing them,which is the relative motion between the magnet and the disc.
To oppose this relative motion,the disc experiences a torque that causes it to rotate in the same direction as the motion of the magnet.
This phenomenon is known as Arago's disc experiment.

(D) $1$. According to Faraday's law of induction,eddy currents are loops of electrical current induced within conductors by a changing magnetic field.
$2$. When a solid metal core is used in a transformer,the changing magnetic flux induces large eddy currents,which lead to significant energy loss in the form of heat.
$3$. By laminating the core (using thin,insulated sheets of metal),the path for these eddy currents is restricted,significantly reducing their magnitude.
$4$. Therefore,the core is laminated to reduce eddy current losses and improve the overall efficiency of the device.

(D) In a transformer, energy is lost due to various factors, one of which is the formation of eddy currents in the iron core.
When the magnetic flux through the core changes, it induces circulating currents (eddy currents) within the bulk of the core material, leading to significant heat dissipation ($I^2R$ loss).
By laminating the core—using thin, insulated sheets of metal instead of a solid block—the path for these eddy currents is restricted.
This increase in resistance significantly reduces the magnitude of the eddy currents, thereby minimizing the energy loss in the form of heat.

(D) Eddy currents are induced loops of electrical current generated within conductors by a changing magnetic field in the conductor,according to Faraday's law of induction. When a thick metal plate is subjected to a varying magnetic field,the magnetic flux linked with the plate changes,which induces electromotive force $(EMF)$ and consequently produces circulating currents known as eddy currents.

(B) According to Lenz's Law,when a bar magnet falls through a copper ring,the changing magnetic flux through the ring induces an electromotive force $(EMF)$ and a corresponding eddy current in the ring.
This induced current creates a magnetic field that opposes the change in magnetic flux that produced it.
As the magnet enters the ring,the induced magnetic field exerts an upward repulsive force on the magnet.
As the magnet leaves the ring,the induced magnetic field exerts an upward attractive force on the magnet.
In both cases,the induced magnetic force acts in a direction opposite to the motion of the magnet (upwards),while gravity acts downwards.
Therefore,the net acceleration of the magnet is $a = g - a_{induced}$,which is less than the acceleration due to gravity $(g)$.

(B) As the bar magnet falls through the hollow metal pipe,the magnetic flux linked with the pipe changes continuously.
According to Faraday's law of electromagnetic induction,this change in magnetic flux induces an electromotive force $(emf)$ in the pipe.
Since the pipe is a conductor,this induced $emf$ causes eddy currents to flow within the body of the pipe.
According to Lenz's law,these eddy currents create a magnetic field that opposes the motion of the falling magnet.
This opposing force acts upwards,reducing the net downward force on the magnet.
Consequently,the net acceleration of the magnet is less than the acceleration due to gravity $(g)$.

(B) The working of magnetic braking of trains is based on $Eddy$ currents.
When a conductor moves through a magnetic field,the change in magnetic flux induces $Eddy$ currents within the conductor.
These currents create a magnetic field that opposes the motion of the conductor,according to $Lenz's$ law.
This opposing force acts as a braking mechanism,effectively slowing down or stopping the train.

(B) The shiny metal disc in the electric power meter rotates due to eddy currents.
When the alternating magnetic field produced by the current in the coils passes through the metal disc,it induces eddy currents within the disc.
These eddy currents interact with the magnetic field to produce a torque,which causes the disc to rotate.

(C) Eddy currents are induced currents that circulate within the core of a transformer when it is subjected to a changing magnetic field,leading to energy loss in the form of heat.
To minimize these losses,the core is constructed using thin,insulated sheets of metal stacked together,known as a laminated core.
This lamination increases the electrical resistance of the path for eddy currents,thereby significantly reducing their magnitude and the associated energy dissipation.

(C) metal detector consists of a coil that generates a time-varying magnetic field. When a metallic object is brought near the coil,the changing magnetic field induces eddy currents in the metal object. These eddy currents create their own magnetic field,which is detected by the device. This process is based on the principle of electromagnetic induction.