A copper plate is allowed to swing like a simple pendulum between the pole pieces of a strong magnet . Eddy currents are induced due to constant change in magnetic flux . This damps the motion of the plate . But why ?

  • $\begingroup$ Lenz law. $\endgroup$ – Steeven Jul 10 '17 at 9:48
  • $\begingroup$ That doesn't tell why the velocity decreases $\endgroup$ – S.R. Jul 10 '17 at 9:54
  • $\begingroup$ I am sorry . But can u elaborate, please $\endgroup$ – S.R. Jul 10 '17 at 9:56
  • $\begingroup$ Lenz law: "The direction of current induced in a conductor by a changing magnetic field due to Faraday's law of induction will be such that it will create a magnetic field that opposes the change that produced it." The appearing eddy currents will counteract whatever caused them. In this case it is the changing magnetic field that causes them, in turn causes by the swinging. Counteracting the magnetic field changes means counteracting the swinging. I am aware that this is not a full answer to your question, so I'll only leave this as a comment. $\endgroup$ – Steeven Jul 10 '17 at 10:11

In terms of energy transformation the energy of the swinging pendulum is converted into heat in the copper plate.

Consider what would happen if there was no opposition to the motion of the pendulum ie the induced current would be in such a direction as not to oppose the motion producing it.

In this case every time the copper plate was swinging down between the magnetic poles the induced current would be such as to produce a force of attraction.
This would me that the copper plate would have an extra accelerating force on top of that due to gravity.
Thus the speed and hence the kinetic energy of the copper plate would be larger.

So you still have a heating effect in the copper plate but on top of that you would also have an increase in the mechanical energy of the pendulum.
Overall the system would gain energy which is contrary to the law of conservation of energy.
So that induced current cannot help the motion producing it, rather it must oppose the motion producing it.

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