# Bouncing Magnet

#### Abstract

A very large, very strong rare earth magnet is dropped onto a copper block cooled in liquid nitrogen. The magnet is seen to bounce “off the field” a few centimetres above the copper block. This surprising behaviour cannot be explained using Faraday’s law and Lenz’s laws as they are presented in secondary school textbooks alone. Why does the magnet bounce? High speed video shows the details of the motion. The effect is even more spectacular and more interesting when dropping a magnet on a superconductor.

Yes

#### Principles Illustrated

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. See the videos below.

Bouncingmagnet.m4v

Download high speed video of bouncing motion with data analysis below (right-click and “save link as”, 4 MB): Magnetbouncehighspeedcamera.m4v

This effect is slight if the copper block is cooled by ice water, but increases noticeably with dry ice and dramatically 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

See the high speed video of the bouncing magnet taken at 240 frames per second that you can download below. 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 explanation 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.

#### NCEA & Science Curriculum

PHYS 2.4, PHYS 2.6, PHYS 3.6, University Physics

#### Instructions

Don’t try this yourself unless you are expert with liquid nitrogen and very strong magnets, and have suitable facilities.

#### Safety

This demonstration features the largest and most powerful neodymium magnet we have used (70 mm diameter, 25 mm thick, N42). This magnet is seriously dangerous if not used with extreme caution. We keep it locked up.

You really can get a serious injury from large neodymium magnets. They will fly toward another magnet or even a steel object and will be moving fast enough to shatter when they hit. The chunks that fly out of these collisions tend to have sharp edges. You don’t want to get your fingers between the magnet and some steel!

People with pacemakers or other medical electronics should never approach these very strong magnets!

Also, this demonstration requires a large quantity of liquid nitrogen and the copper block cooled to liquid nitrogen temperature is dangerous. You need to be an expert with liquid nitrogen.

Again, our recommendation is that teachers not attempt this demonstration on their own.

Individual teachers are responsible for safety in their own classes. Even familiar demonstrations should be practised and safety-checked by individual teachers before they are used in a classroom.

#### Notes

Applications and further reading: Eddy current drag is used to make brakes in certain applications. See for example the following exchange on physlink: eddy currents brakes. Also see a Wikipedia discussion of eddy currents.

#### Related Resources

We have lots of eddy current resources. See Eddy Current Drag and Eddy Current Tank in particular.

#### Credits

• The strong magnets were supplied by the NMR research laboratory founded by the late Sir Paul Callaghan at the Victoria University of Wellington campus.
• This demonstration resource was developed by a senior high school student under the direction of Victoria University academics.

This teaching resource was developed with support from