Understanding Lenz’ Law as it applies to a magnet falling in a tube

Hi All! This is my first question/post here, so I will try to make it work!

1. The problem statement, all variables and given/known data

Ok, so the question is simple-ish.


A bar magnet falls vertically from rest through a coil of wire. The potential difference (pd) across the coil is recorded by a datalogger.

[Click here for picture]

Explain, with reference to Faraday’s and Lenz’ laws, the shape of the graph.

2. Relevant equations

Faradays Law (ε=-NΔΘ/ΔT)

3. The attempt at a solution

Now, I understand the concept that as the magnet is falling towards the coil, the coil is cutting through the magnetic flux at a faster and faster rate (and from the equation, must result in a negative EMF that goes more negative).

I also understand that when the magnet is at the middle of the coil, the rate of flux cutting is the same on both sides of the magnet (and thus EMF is instantaneously zero) and, afterwards, the emf is positive as the rate of flux linkage is actually negative (as the magnet is moving away from the coil).

In terms of Lenz’ law, I understand that initially, a current will be induced in the coil so as to counteract the motion of the magnet (which is causing the flux linkage). This force will act upwards against the magnet as the magnet falls into the coil initially (and thus the EMF will be somewhat lower than the value without Lenz’ law).

What I can’t seem to understand is how to rationalise the direction of the Lenz’ Law "force" on the magnet after it has passed through the centre of the coil. I presume that since the rate of flux linkage is decreasing, and this is caused by the downward motion of the magnet, the force from the current will act upward to counteract this downward motion. However I do not get how this therefore means that the peak magnitude of the EMF in the second "portion" of the magnet’s fall is higher than the peak magnitude of the EMF in the first "portion" of the magnet’s fall (as the graph demonstrates). Any help here would be very greatly appreciated!

Kind Regards,

Bob12321 🙂


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