Eddy Currents in Tubes

Eddy Currents in Tubes

Description:

Eddy currents are currents induced in conducting surfaces rather than wires. Their effect can be shown by dropping powerful magnets through aluminum or copper tubes. It takes much longer for the magnets to reach the end of the tube than if they were in free fall.

Discussion:

This demonstration is good for showing the interaction between electricity and magnetism. Circulating currents, called eddy currents are set up in metal objects moving with respect to a magnetic field. In turn, these circulating currents produce magnetic fields which, oppose the original magnetic field. The result is an upward force, opposing the free fall motion the observer expects, and slowing the movement of the magnets through the tube.

In this situation, Faraday’s Law of Induction tells us that the changing B fields of a falling magnet will generate circular eddy currents in the tube. Lenz’s Law says that the induced currents must act to oppose the change that created them. So they must give rise to a retarding force on the moving magnet, which slows its fall. If the reverse were true, we would have a very simple magnetic canon!! Lenz’s Law is simply the idea of energy conservation. You can’t generate electrical energy without doing work---in this case, the work done by gravity.

Consider a magnet dropped N pole first. Just below the magnet the downward B field is increasing, so the change in B is downward. The induced eddy currents produce an opposing upward B field that exerts an upward force on the N pole of the magnet. By the right hand rule, the eddy currents must be CCW when viewed from the top. Just above the falling magnet, the downward B field is decreasing, so the change in B is upward, the induced B’ field is downward, giving another upward force and the force on the S pole of the magnet. The eddy currents are CW. (In some classes, you may wish to give only the general explanation in the preceding paragraph.)

Hints for this demo

Be careful with dropping the magnets, they are very brittle.

This demonstration can be purchased, but it works fine to use a piece of scrap copper tubing and neodymium magnets. Even a short piece of copper pipe from a plumber works well if using neodymium magnets.

How to present this demo

If possible, drop an object outside the tube simultaneously, to show how much the motion of the magnet in the tube is retarded by the opposing magnetic field. You can also drop a non–magnetized slug through the tube for comparison. Now drop the magnet with the other pole first to show that you get the same retardation.

Equipment needed for this demo

Copper or aluminum tube

Strong magnets

Other ideas

A demonstration that can be easily made is included on the next page. As the cow magnet is thrust into the coil, one LED lights. As the cow magnet is pulled out, the other LED lights. Use LEDs of different colors so the students can easily see it.