Question

As a lecture demonstration a short cylindrical bar magnet is dropped down a vertical aluminum pipe of slightly larger diameter, about 2 meters long. It takes several seconds to emerge at the bottom, whereas an otherwise identical piece of unmagnetized iron makes the trip in a fraction of a second. Explain why the magnet falls more slowly.

Short answer

The attraction force act between the magnet and ring of pipe. Therefore, the magnet falls slowly.

Step-by-step solution

Determine the currents in the falling magnet and ring of pipe.

Let’s consider the current is flowing into the falling magnet in the counter clockwise direction; then, according to the flaming’s rule, the magnetic field due to the magnet is in the upward direction.

Now assume the ring of pipe below the falling magnet, and the magnetic flux into the ring increases as the magnate approaches the pipe. Therefore, the induced current into the pipe ring is clockwise.

Determine the reason of falling the magnet slowly.

When the magnet falls above the pipe, the current in the magnet is in the counter clockwise direction, the induced current into the pipe is in a clockwise direction, and the opposite current repels each other.

When the magnet falls below the pipe, the current's directions become in the same direction, which means the attraction force act between them, and the current attracts each other. Therefore, the magnet falls more slowly.