1. Physical situation

A bar magnet is released inside a long vertical copper tube along its axis.

As the magnet falls, the magnetic flux through the conducting tube changes with time.

2. Induced currents in the copper tube

Because the tube is a conductor, the changing flux induces eddy currents in the tube.

By Lenz's law, the induced currents produce a magnetic field that opposes the change causing them.

• If the magnet is moving downward, the induced magnetic effect opposes its downward motion.
• Therefore, an upward magnetic retarding force acts on the magnet.

3. Effect on motion

Initially, the magnet starts from rest, so the induced current is small.

• At first, it accelerates downward due to gravity.
• As speed increases, the rate of change of flux increases.
• Hence induced eddy currents increase.
• So the upward magnetic braking force also increases.

4. Terminal speed

After some time, the upward magnetic force becomes equal to the weight of the magnet:

$F_{mag} = mg$

Then net force becomes zero:

$F_{net} = mg - F_{mag} = 0$

So acceleration becomes zero, and the magnet continues to move with constant speed (terminal velocity).

Thus it will move down with almost constant speed.

5. Checking options

• A: move down with almost constant speed — Correct.
• B: move down with an acceleration equal to $g$ — Wrong, because eddy currents produce an opposing force.
• C: move down with an acceleration greater than $g$ — Wrong, impossible here since magnetic force opposes motion.
• D: oscillate inside the tube — Wrong, no restoring mechanism for sustained oscillation; instead there is damping.

Final Answer

$\boxed{\text{A}}$

The magnet eventually falls with an almost constant terminal speed due to magnetic braking by eddy currents.