Question

1. When a magnet moves relative to a conducting, non-magnetic material (such as copper), some force slows down the magnet, and this is attributed to "eddy current." Describe in your own words what this "eddy current" is, how it is caused, and how this eddy current affects the magnet which caused it.
2. In the demonstrations of eddy current, these demonstrations involve non-conservation of mechanical energy. You see: (1) large magnet slowing down to a stop at the end of a short drop, before hitting the copper plate (loss of kinetic energy), (2) a magnet moving downward at a constant velocity (loss of potential energy), or (3) a pendulum slowing down to a stop (loss of total mechanical energy). These mechanical energies are likely being lost to heat; explain the mechanism by which this loss happens. [Hint: What happens when a current flows in a material with non-zero resistance?]

Answer 1

According to Faraday's law when the magnetic flux linking a circuit changes, an electromotive force is induced in the circuit proportional to the rate of change of the flux linkage. So, when a magnet moves relative to a conducting material (such as copper) flux linkage to the non-magnetic conducting material changes and according to Faraday's law an electromotive force is induced in the non-magnetic conducting material. This electromotive force creates current inside the non-magnetic conducting material. This current is eddy current ,a localized electric current induced in a conductor by a varying magnetic field.Now according to Lenz's law  the direction of an induced current is always such as to oppose the change in the circuit or the magnetic field that produces it. So, induced current produced in the non-magnetic conducting material will oppose the change in the circuit or the magnetic field that produces it and in this case it is the motion of the magnet that is creating the change in flux.So, the eddy current in non-magnetic conducting material will oppose the motion of the magnet relative to the non-magnetic conducting material.

(1) large magnets slows down to a stop at the end of a short drop, before hitting the copper plate. This is because in the case of a short drop the magnet is accelerating with gravitational acceleration g . flux linkage in the copper plate will change because of the acceleration. But as the magnet comes closer to the copper plate the flux linkage to the copper plate will increase at the rate of g .this change in flux linkage will create an electromotive force inside the copper plate which will create eddy current. This eddy current in the copper plate will oppose the flux that is causing it in this case it is the movement of the large magnet. So, large magnet will slow down to a stop at the end of a short drop and the eddy current in the copper plate will be lost as heat . In this way kinetic energy in the large magnet will be converted to heat energy in the copper plate

(2) When a magnet moves downward at a constant speed the flux linkage to the conducting material also changes at constant speed which will create electromotive force which will cause localized current inside the conductor. This current which is called eddy current will oppose the constant speed of the magnet and the eddy current will be converted to  heat and will be dissipated. In this way potential energy is converted into heat energy.

(3) A pendulum when moving it has kinetic energy and when it stops at the end of its motion it has potential energy. If a magnet is around the path of the pendulum ,when the pendulum enters the magnetic field there is a change in flux linkage which will create eddy current inside the pendulum according to the Faraday's law and the direction of current will be in the direction according to the Lenz's law which will oppose the movement of the pendulum. Because movement of the pendulum is causing the change in flux. And the eddy current inside the pendulum will be converted to heat energy.After entering the magnetic field the pendulum will face no opposition from the magnetic field as flux inside this region is constant and there is no change in flux linkage to the pendulum.When the pendulum exits the magnetic field the pendulum will move away from the magnetic field so there is a change in the magnetic field which will in turn produce eddy current inside pendulum which will oppose the movement of the pendulum and again this eddy current will be dissipated as heat. In this way both potential energy and kinetic energy will be changed to heat energy by the means of the eddy current inside the pendulum.