## Lenz's and Faraday's Laws affecting falling magnets

What you need

1) a bar magnet - the stronger the better

2) a large block of solid copper - ideally on order of 5cm*10cm*10cm - two or more is even better. Again, bigger is better for this experiment

3) Liquid nitrogen or dry ice (liquid nitrogen is better).

4) something soft - a box of packing peanuts or a folded blanket

5) an insulated container that can hold the liquid nitrogen and the copper block

Liquid nitrogen and large blocks of solid copper might be hard for you to get a hold of, but your teacher might be able to arrange for them for an in class demonstration or a science fair project

What to do.

1. If you have two blocks of copper, place one in the container, surrounded by liquid nitrogen (or dry ice) so that it has time to come to the temperature of the liquid nitrogen. If you only have one block, do the next few parts first, then give the block time to cool before you continue. Be careful not to touch the cold metal - you could easily give your self a serious burn! Prepare the block on the floor so that you don’t have to move it while the liquid nitrogen is there.
2. Place the soft target on the ground in front of you and drop the bar magnet on to it. Notice how it falls.
3. Now replace the soft target with the copper block and repeat step 2. Notice the difference in how the magnet falls?
4. Now, when you pick up the magnet, check that it is not attracted to or repelled by the copper - the copper is not a magnetic material, so there is no “regular” interaction.
5. Now, once the block has reached the temperature of the liquid nitrogen, repeat step three - What is the difference in how it falls now?
6. To further investigate this, look at the nitrogen next to the copper block’s edges. If you leave everything alone for a minute, it will be boiling gently as it slowly heats up due to the heat in the room around it. Now, wave your empty hands over the block, notice anything happen? No? Good.
7. Now take the magnet and wave it over the block - what happens to the liquid nitrogen now?
What is happening.

When you drop the magnet, gravity takes over and accelerates it towards the ground. If there is just the padding under it, it continues to accelerate at 9.8 m/s/s until it hits the padding.

However, if there is the copper block under it, the changing magnetic field experienced by the block causes (due to Faraday’s Law) currents to flow in the block - just like the moving magnets in many generators cause currents to flow through the wires surrounding them. From Lenz’s law, we know that the direction of these loops is always such that they act against what ever change is happening to cause them to exist in the first place. In this case, they are caused by the falling magnet, so the current loops automatically are oriented so that they repel the magnet - the faster it is falling, the stronger the loops get! So, when the magnet gets close to the copper block and is moving quite fast, the effect of the loops becomes very strong and it slows the magnet down substantially, bringing it to a gentle(r) landing.

Now, when these loops form, the strength of their magnetic effects is determined by the current they carry. The current is determined by the electromotive force and the resistivity of the copper. The electromotive force is fixed by the speed and position of the falling magnet and the resistivity is caused by the ions that make up the copper metal lattice vibrating and disturbing the flow of the electrons that make up the current.

By cooling down the metal with the liquid nitrogen, we reduce the vibration of the ion (because, that is all temperature really is at a microscopic scale), reducing the resistance of the copper.

This allows much greater currents to flow, causing much stronger magnetic effects as the magnet nears the copper, drastically increasing the “slow-down” effect. In fact, if you try to shake your hand (holding the magnet) over the two blocks, you might feel some resistance to your motion from the room temperature one, but trying to move the magnet over the cold block feels like you are pushing it through molasses!

Also, when you shake the magnet over the cold block, you will have noticed that the boiling of the nitrogen increases rapidly! This is because, the kinetic energy that is taken from your hand by the resistance is turned into electrical energy in the currents that flow that are then turned into heat by the resistance in the copper (it is interesting and almost counterintuitive that the larger currents in low resistance situations loose energy at a higher rate than the lower currents flowing in high resistance!).

The heat energy produced within the copper flows out into the liquid nitrogen, causing an increase in its boiling rate.