Introduction

Using the eddy currents from copper you can fly a magnet.

 

Material

• Copper disks (or aluminum), the thicker the better, at least 1 cm thick and 10 cm diameter. (We used disks but the shape is relatively unimportant.)
• neodymium magnet, any size, but disks 1 cm in diameter and 0.5 cm thick are great.
• Ferrite magnet a.k.a. refrigerator magnet, bigger is better (see test below)
• non magnetic spacers to hold the copper disks 1 or 2 cm apart.

Assembly

Use scraps to space the copper disks 1 to 2 cm apart.

(If you have two disks with different thicknesses put the thicker one on the bottom.

Place the neodymium magnet between the copper disks.

Test

Hold the ferrite magnet over the top copper disk.

Lower the ferrite magnet until it touches the copper, does the neodymium magnet lift up? If not turn over the ferrite magnet.

If the neodymium lifts up and stick to the bottom of the copper the ferrite is large enough, otherwise if neither orientation of the ferrite will lift the neodymium, get a larger ferrite magnet.

To Do and Notice

Use the ferrite magnet to lift up the neodymium magnet and fly it between the two copper disks.

Without the copper disks this is impossible to do, with the coper disks it is easier.

Notice that the closer the copper disks are together the easier it is to fly the neodymium.

What's Going On?

When the upper magnet is oriented to attract the lower magnet and moved close enough that the upward magnetic attraction is greater than the downward force of gravity, the neodymium magnet will accelerate upward.

When a magnet moves near a conducting metal like copper it creates eddy currents in the conductor. These eddy currents are electromagnets that oppose the motion of the magnet. The faster the motion of the magnet the greater the opposing force from the eddy currents. These eddy currents slow down the motion of the magnet.

The eddy currents from thick copper can slow down the motion of the magnet enough that human reaction time can move the upper ferrite magnet up and down and manage to hold the neodymium magnet flying in between the two copper disks.

Going Further

Compare copper and aluminum disks.

Notice that thicker disks produce greater affect than thinner ones.

Notice that copper creates larger forces than aluminum of the same size.

References

This exploration is based on an exhibit created by Shawn Lani and Paul Doherty

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Scientific Explorations with Paul Doherty

© 2001

27 Feb 2001