Look at Eddy Current Drag as an introduction to the physics illustrated by this experiment.

 

The falling magnet generates eddy currents in the copper block that act as an electromagnet repelling the magnet and slowing its fall. When the copper block is cooled below room temperature the magnet is seen to fall more slowly. This means the repelling magnetic interaction is stronger. The induced eddy currents must therefore be larger.

This effect is slight if the copper block is cooled by ice water, but increases noticeably with dry ice and dramaticaly with liquid nitrogen.

 

Why is it so? Note that the size of the eddy currents depends on the generated voltage and the resistance of the copper. As copper is cooled, its resistivity drops and the eddy currents are larger, given the same voltage. Although the magnet falls slower onto the cooled block and thus induces a smaller voltage, the currents actually increase because the resistivity of the copper has decreased more.

 

But when the block is cooled to liquid nitrogen temperature, something that seems at first impossible occurs. The magnet 'bounces off the field' a few centimetres above the block. This is not so easy to explain. Bouncing means the falling magnet slows, stops, and then moves upwards. We can understand the slowing and the stopping: the repulsion force on the magnet is larger than its weight. But, according to Faraday's law, the moment the magnet stops, the induced voltage disappears so there is no magnetic interaction.A magnet at rest subject only to its weight must fall. But it moves upward. Faraday’s law and Lenz’s law alone cannot explain why the magnet bounces.

 

Why does the magnet rebound? We infer that the current does not drop to zero instantly when the magnet reaches the bottom of its travel. Instead the current persists for a while.

This may seem surprising, but the copper block has an inductance just as a coil of copper wire does in the standard inductor - resistor (LR) circuit. When the voltage applied to an LR circuit is switched off, the current drops exponentially with time constant L/R. At liquid nitrogen temperature the resistance of the block is small enough for the time constant to be significant (perhaps a second or so). This persisting current continues to exert an upward force on the stopped magnet. So it rebounds.

 

We certainly very surprised by to see the rebound. In hindsight, the effect itself is not so surprising, but its magnitude is: the magnet, nearly 1 kg in mass, really does bounce.

 

High speed video and motion analysis

Click here to see a high speed video of the bouncing magnet taken at 240 frames per second. The motion was tracked using Tracker software (which we highly recommend) with the centre of mass of the magnet determined just by eye. We have developed a theoretical model giving an approximate explaination of the motion - the oscillations, including the sharp reversal on the first bounce, look close to correct. We are refining the model, and the results will be posted soon.

 

Contact us if you would like a high resolution version of the high speed video, data, or more information on the theoretical model.