Question

A bar magnet is released from rest along the axis of a very long, vertical aluminum tube. After sometime the magnet will :

• A. move down with an acceleration equal to g
• B. move down with an acceleration grater than g
• C. will move with almost constant speed
• D. oscillate inside the tube

Answer 1

The Correct Option is C

Step 1: Understanding Lenz's Law.
According to Lenz's Law, the induced current in the copper tube due to the motion of the bar magnet will produce a magnetic field that opposes the motion of the magnet. This means the magnet will experience a resistive force as it falls down the tube.
Step 2: Analyzing the forces.
Initially, when the bar magnet is released, it experiences an acceleration due to gravity (\( g \)). However, as the magnet moves, the changing magnetic flux through the conducting tube induces an electromotive force (EMF), which generates a current in the tube. This current creates a magnetic force that opposes the motion of the magnet, according to Lenz's Law.
Step 3: Reaching terminal velocity.
The opposing magnetic force increases as the magnet's velocity increases, and eventually, it balances the downward gravitational force. At this point, the net force on the magnet becomes zero, and the magnet moves with a constant speed, also known as terminal velocity.