Eddy Currents at LN2 Temperature

What it shows:

A rectangular block of copper (measuring 6"×6"×2"), offers VERY little resistance to eddy currents generated by dragging a magnet across its surface. Thus the Lorentz force between the eddy currents and magnetic field is quite strong and you can feel a sizable drag force. Dropping a magnet onto the surface likewise produces a sizable Lorentz force, as evidenced by the damped motion of the magnet's fall. The effects are quite dramatic at liquid nitrogen temperature.

How it works:

Copper has a positive temperature coefficient (≈ 3.9×10-3 per ˚C), which means that its resistance drops with temperature. The block of copper is immersed in liquid nitrogen (77˚K = -196˚C), decreasing its resistance (from room temperature) by almost a factor of 2. This enhances the Lorentz force as can be seen by dropping a magnet onto the block from a height of several inches. The motion of the magnet is as if it were falling through a very viscous fluid. If the magnet is dropped from higher up, it reaches a higher velocity before encountering the copper block and this results in a significantly greater interaction — the magnet literally bounces off the block (without ever coming in contact with it)

Setting it up:

It takes some time (how much?) to bring the massive block of copper down to -196˚C, so plan accordingly. Storing the copper in a container with dry ice (-78.5 ˚C) overnight helps reduce the time and amount of liquid nitrogen needed. Having pre-cooled overnight, 4 liters of LN2 will cool the copper block in less than ½ hour. Set up the experiment on the lecture bench in such a way that the magnet doesn't bounce off the copper and onto the floor ... it will shatter!