Chapter 37, Problem 49A

When a bar magnet is dropped through a vertical length of copper pipe, it falls noticeably more slowly than it does when it is dropped through a vertical length of plastic pipe. If the copper pipe is long enough, the dropped magnet will reach a terminal falling speed. Propose an explanation.

To determine

To explain: if a bar magnet is dropped more slowly through a vertical length of copper pipe than it does when it is dropped through a vertical pipe

Explanation of Solution

Introduction:  A cooper material that create its own magnetic field when a current is passed through it, just like the eddy currents created. The magnetic field created by eddy currents will resist the magnetic field produced by the magnet, as a gravity pulls the magnet downwards through the pipe and results in slowing it down.

Explanation:  If a bar magnet falls vertically through the hollow region of long vertical copper pipe then the magnetic flux linked with the copper pipe changes and eddy currents are generated in the body of the pipe by the Lenz’s law and the eddy currents experiences a retarding force.

On increasing the velocity of magnet, the retarding force increases and is equal to the magnet weight. Then the magnet attains a constant velocity and hence has zero acceleration.

Conclusion:  The retarding force increases with increasing the velocity of magnet and finally equal to the magnet weight.