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Gallery 2

Falling Magnets

Magnetic repulsion and eddy current braking create curving patterns.

How it works

Watch the magnets dance in a pattern as you spin a large disc and set them in motion.

Things to try or ask around the exhibit

If we stop the disk while it's spinning, what will the magnets inside the disk do?

Background

There are a few forces at work in this disk.

When you spin the disk quickly, the magnets fly out to the edge of this disk. The disk's rim forces the magnets to take a curved path. Any force that keeps an object going in a curved path is a centripetal force (a centre seeking force).

When you stop spinning the disk, gravity’s force works on the magnets to pull them downwards. But they don’t fall down quickly as you might expect.

When a magnet is moved near a conductor such as the metal that makes up the circular disk, small rings of electrical current form in the metal. Those rings of electricity are called eddy currents.

Magnetic fields surround eddy currents and repel any magnets that come near them. This causes the magnets to fall slowly.

When the magnets are close enough to each other, they begin to repel each other, so appear to float on top of each other. A magnetic field comes about because the atoms inside a magnet contain unpaired electrons that are all oriented the same way. This orientation is known as an electron’s spin. When the spins of many unpaired electrons align with each other, their effects add together to create a magnetic area called a magnetic domain. If the alignment is permanent, the material is ferromagnetic.

All magnets have a north pole and a south pole. The magnetic field is directed from the north pole to the south pole of the magnet. Another magnet (or a compass!) that is placed in this field will feel a torque that will push the magnet to align itself with the field’s direction. This results in magnetic attraction!

Magnetic attraction can be used to do some unusual work. Farmers give cows magnets to swallow so that any metal debris in their feed will stick to the magnet and not injure the cow’s stomach lining. This is not recommended for humans!

Finding the science in your world

Wet clothes in a washing machine are held in a centripetal force by the walls of the machine, while the water in the clothes is not. The water escapes through the holes in the walls.