Warning: The magnet required for this experiment is extremely strong. Take care not to put your fingers between the magnet and any other magnetic surface.
You will need
Roll of aluminium foil
20 cent coin
2.5cm diameter neodymium magnet (from most hobby and electronics stores. Make sure it is slightly smaller in size than the inside of the roll of aluminium foil)
What to do
In a carpeted area, hold the aluminium foil in front of you, perpendicular to the ground (so you can see the floor if you look through it).
Drop the coin through the middle of the tube and watch it fall. Notice how fast it fell.
Take the magnet and touch the aluminium foil with it. Does it stick?
Drop the magnet through the tube and watch it fall – watching it fall looking down the tube from above gives the best view! Was it slower than the coin?
What’s happening?
Electromagnetism is a force responsible for a variety of things. It holds bunches of atoms together to form matter, which means you can’t fall through the floor or walk through a wall, but can push through water and air. It also happens to have a lot to do with how light is made, how electricity works and why magnets stick together or push apart.
As such, electricity and magnetism are related to one another. We can make magnets out of a battery, a nail and a coil of wire. See our Science by Email activity where you can make your very own electromagnet. As electricity flows through a conductor, it creates a magnetic field, ‘pushing’ in an anticlockwise direction around the wire (if you imagine the negative electrons moving down the wire towards you).
On the other hand, a moving magnet will also jiggle the charges in some materials, creating electricity. So a moving magnet makes electricity, which in turn creates another magnetic field.
Our dropped magnet won’t stick to the aluminium because it’s not made of the right material. The only elements that can attract magnets are iron, nickel and cobalt. However, some metals, such as copper and aluminium, will still conduct electricity if a magnet is moved over it. As the magnet falls, it moves past the aluminium, making a small amount of electricity. This electricity creates its own tiny magnetic field that pushes up against the falling magnet, slowing it down.
Applications
This is called ‘magnetic braking’, which is quite useful in situations where normal brakes aren’t as effective. For instance, most mechanical brakes rely on friction. Yet if the surfaces are too smooth or covered in a lubricant such as water or oil, mechanical brakes can slip and fail. Many modern roller coaster cars therefore rely on magnetic brakes to slow them down.
Some brakes are similar to the one in our experiment, relying on the movement of neodymium magnets to create what are called ‘eddy currents’. The faster the magnets are moving, the stronger the eddy currents and the harder they’ll push, controlling the speed of the vehicle. Since they don’t rely on an electrical supply and work in most environments, they are a reliable way to make sure your speeding roller coaster doesn’t malfunction when you least want it to.
