Question

There was a demonstration of the falling magnet in a simple copper tube. Explain, in detail, the physics behind why the magnet slows its descent through the tube (it does not touch the sides). In your discussion be sure to explain why the magnet doesn't come to a stop in the tube and why it does not speed up.

Answer 1

According to Faraday, experiment, when the source of magnetic field is moved towards near or away from a stationary coil, induced emf is produced in the coil. In this question, we have the MAGNET as a source of magnetic field while copper tube as a stationary coil.

Explaination: When the magnet falls, the magnetic field through the copper ring changes i.e it increases . This increase in magnetic field increases the magnetic flux associated with the copper tube. Since, change in magnetic flux leads to the production of induced emf (as stated by Lenz law) expressed as

e = -dk/dt

where e is the induced emf and dk/dt is the rate of change of magnetic flux.

The direction of this induced emf would be such that it opposes the change in magnetic field through the copper tube i.e it opposes the falling magnet.

It is because of this opposing force that causes the magnet to slow down and have the value of acceleration(say a) which is lesser than that due to gravity (g). Since acceleration is lesser, it speeds down rather than speeding up.

To show why the magnet does not stop, we can understand this problem in a more detailed way;

On Changing magnetic field ( by moving a magnet) an electric field is set up in the metal, which generates a magnetic field of its own with opposite orientation as that of the magnet.This is what gives rise to induced emf.The induced emf induced in the metal creates resistance that slowed down the magnet. As the magnet slows down, it stops generating as much current, which decreases the resistance acting on the magnet’s movement. Since, the magnet is falling vertically downwards It is the gravity that takes the charge and  speeds the magnet back up again. Hence, the magnet does not stop.

It slows down under the effect of induced emf produced in copper tube . As soon as this opposing force diminishes, gravity takes the charge not allowing it to stop and fall freely.