Question

A bar magnet is released from rest along the axis of a very long, vertical aluminum tube. After sometime the magnet:
A. will stop in the tube
B. will move with an acceleration 'g'
C. will move with almost constant speed
D. will oscillate

Answer 1

Hint: Aluminum is a paramagnetic material and on interacting with a magnetic field, eddy currents will be generated which will then oppose the motion of the magnet. This shall bring equilibrium in the motion of the magnet, now check the options.

Complete step-by-step answer:

Induced Electromotive force- Whenever there is a generation of potential difference in a conductor due to changes in the magnetic flux through it.

Eddy currents- When there is relative motion between a conductor and some magnetic field, looped currents are generated which are called eddy currents. Eddy currents can produce significant drag, called magnetic damping, on the motion involved. Eddy currents and magnetic damping occur only in conductors.

Lenz’s law- It states that the induced electric current will flow in a direction such that it tries to oppose the cause of induction. Lenz’s law is an example of law of conservation of energy. And it helps in determining the direction of the induced current.

Initially, when the magnet is dropped it falls down with an acceleration g. When the bar magnet is dropped in the aluminum tube, aluminum being paramagnetic, its magnetic poles align themselves in the direction of the magnetic field crossing it. This constant interaction of fluctuating fields then, generates the eddy currents in the tube. These eddy currents produced oppose the motion of the bar magnet. This can be observed from Lenz's law. So, an opposing force constantly acts on the magnet until it stops accelerating. When there is no acceleration, the magnets move with a constant velocity.
So, the correct option is (C).

Note: A similar behavior is observed in case of a diamagnetic rod like copper. Same thing happens with a diamagnetic rod so you can check it out.