How does an eddy current brake work?

What do you need?

Roll of aluminium foil (from the supermarket)
Neodymium sphere magnet 20 mm

Let's look at the following experiment with a magnetic sphere and an aluminium roll:

A neodymium magnetic sphere with a diameter of 20 mm and a weight of approx. 30 g is dropped through a vertically held roll of household aluminium foil.

The aluminium roll has an inner diameter of approx. 25 mm. It is therefore so large that it does not hinder the magnetic ball mechanically and therefore does not slow it down. The length of the aluminium roller is approx. 300 mm

Theory

According to the laws of free fall (s=1/2*g*t^2; s=distance, g = acceleration due to gravity, t=time), the time required by the ball in free fall for the distance of 0.3 m is approx. 0.24 s.
The frictional forces of the air can be neglected in this experiment due to the weight of the ball.

Practice

Now we measure the time that the magnetic ball (neodymium magnet) needs to fall through the aluminium roll.
We measure approx. 1.3 seconds and thus more than 5 times the time compared to the free fall of the magnetic sphere.

How does this effect come about?

The aluminium foil consisting of quasi pure aluminium is not ferromagnetic. We can easily check this by testing whether the foil is attracted to the neodymium magnet.
This is indeed not the case.
However, the aluminium foil is indeed electrically conductive, which we can quickly confirm with an ohm meter.

So what happens?

The explanation is relatively simple:
The magnetic field, which changes locally due to the movement of the magnetic sphere, generates an electric current in the aluminium foil conductor in accordance with the law of induction.
Strictly speaking, these are eddy currents that run through the aluminium in a circle.
These eddy currents in turn generate a magnetic field that is opposite to the magnetic field generated by the magnetic sphere. Due to the opposite direction of the two magnetic fields, they attract each other and the magnetic sphere is held in place by the induced magnetic field.
The result is a braked falling magnetic sphere.
As the eddy currents and therefore the opposing magnetic field are stronger the faster the magnetic sphere falls, a constant falling speed results after a short time, in contrast to free fall, in which the falling body is constantly accelerating.

Eddy current brakes in vehicles such as buses or trains work according to the same principle.

If you want to recreate this magnetic experiment, you need a strong magnet, preferably a neodymium magnet. Ferrite magnets and the induced eddy currents, as well as the associated magnetic field, are significantly weaker, which means that the braking effect is also significantly weaker.
Furthermore, the size of the magnet should be chosen so that the distance between the magnet and the aluminium foil is not too great.


A similar experiment can also be carried out using an aluminium rail as an inclined plane. In this case, a sphere magnet is rolled along the inclined magnetic rail.

 

Here you can see (even without measuring the time) that the ball rolls down the plane considerably slower than would normally be the case on a wooden or plastic rail, for example.
The explanation for this effect is the same as described above. The eddy currents generated in the track by the rolling magnetic ball build up a magnetic field that slows down the rolling magnetic ball.



Products used from our shop:

Magnetic sphere / sphere magnet Ø 20.0 mm neodymium nickel-plated N40 - holds 6.1 kg