A bar magnet is released from rest along the axis of a very long vertical copper tube. After some time, the magnet will:
Oscillate inside the tube
Move down with an acceleration greater than
Move down with almost constant speed
Move down with an acceleration equal to
Correct answer:C
Standard Method
Given: A bar magnet is released from rest along the axis of a very long vertical copper tube.
Find: What happens to the motion of the magnet after some time.
According to Lenz's law, the induced current in the copper tube due to the motion of the bar magnet produces a magnetic field that opposes the motion of the magnet. Therefore, the magnet experiences a resistive magnetic force as it falls.
Initially, the magnet accelerates downward due to gravity . As it moves, the changing magnetic flux through the conducting tube induces an emf and hence a current in the tube. This induced current creates a magnetic force opposite to the motion.
As the speed increases, the opposing magnetic force also increases. Eventually, this magnetic force balances the downward gravitational force. Then the net force becomes zero, so the magnet moves with terminal velocity, that is, with almost constant speed.
Therefore, the correct option is C.
Assuming the magnet continues to fall with acceleration throughout is incorrect because the conducting copper tube produces induced currents that oppose the motion. Use Lenz's law and account for the magnetic retarding force.
Thinking the magnet speeds up more than free fall is wrong because the induced magnetic effect resists motion rather than assisting it. The magnetic force acts upward while the magnet moves downward.
Assuming the magnet oscillates inside the tube is incorrect because the situation involves damping due to induced currents. The motion settles toward terminal speed instead of repeated up-and-down motion.
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