It is well known that as a bar magnet falls freely through a coil, an electromotive force is induced in the coil by Lenz's Law.

However, it appears that the peak e.m.f induced (ignoring the direction) is higher after the passing through a coil, than before it passes through the coil. Why so?

My explanation was that after passing through the coil, gravity causes it to accelerate after the coil in the same direction in which the magnetic flux linkage through the coil was changing. By Faraday's Law, e.m.f induced will be greater.

However, under this hypothesis, gravity would also have the same effect on the bar magnet before it passes through the coil.

Is there any theory that can explain this asymmetry well?

  • $\begingroup$ It appear to me simply because of the velocity of the bar magnet. As it comes down its velocity increases. It will have more velocity after passing through the coil, than before. Hence more rate of change of flux. $\endgroup$
    – user22180
    Jul 2, 2014 at 15:19

1 Answer 1


The emf is proportional to $\frac{d\Phi}{dt}$, and this is proportional to the velocity ($\frac{d\Phi}{dt}\propto v$), but not to the acceleration. Since the velocity increases, the $\frac{d\Phi}{dt}$ increases, causing a a-symmetrical peak.


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