Question

Does dropping a magnet down a copper tube produce a current in the tube?Explain.

Answer 1

Yes, and it is one of the coolest physics demos ever! Here'swhat happens:

As the magnet approaches the copper tube, the magnetic flux throughthe tube changes, which causes an induced current in thetube. This current then causes a magnetic field opposing thechange in flux brought on by the magnet, which means that a forcepushes up on the magnet as the magnet falls through the tube,slowing down the magnet.

If the copper tube had no electrical resistance, the current insideit would continue to build and build, until the force was greatenough to stop the magnet from falling at all; rather, it would belevitating inside the tube. However, copper is not asuperconductor at room temperature, so the current isresisted. The result is that the magnet falls through thetube at a slow rate, continually changing flux, which createscurrent, which dies away, but is built up again at the same time asthe magnet continues to fall through the tube. The processends, of course, when the magnet reaches the end of its slowedjourney and drops out the bottom of the tube.