What are eddy currents and why do they make a magnet fall slowly through a copper pipe
Magnets are fascinating – there are many things about magnets for which the only explanation seems to be magic! The best place to find out about magnet-related phenomena is of course here, on our blog – and YouTube. The most viewed videos showing super-strong magnets falling through copper pipes alone have amassed an incredible 8 million views and counting.
What you are seeing in these videos is of course a demonstration of eddy currents in action. Here we will explain what is going on.
What is an eddy current?
Not actually named after a fellow called Eddie – an eddy current is a swirling electrical current that is created in a conducting metal such as copper or steel by moving the metal through a magnetic field, or a magnetic field over the metal. The currents generated produce their very own magnetic field, just like any current flowing in any conductor. This newly created magnetic field opposes (repels) the original magnetic field that created the currents in the first place, creating a damping effect.
Eddy currents are strongest on the surface of a conducting material and flow in small circles representing ‘eddies’ in a stream, hence the name ‘eddy current’.
So why does a magnet fall slowly through a copper pipe?
As a result of the magnetic repulsion created, a damping effect is seen. This is why when you drop a strong magnet down a thick copper pipe the magnet will appear to descend slower than you would expect. So what’s going on here? Well, it isn’t witchcraft or magic it is science! As the magnet falls, the magnetic field around it constantly changes position and this moving magnetic field creates the flow of eddy currents on the surface of the copper pipe. All conducting materials, including copper, create their own magnetic field when a current is passed through them, like the eddy currents created. As gravity pulls the magnet downwards through the pipe, the magnetic field created by the eddy currents resist the magnetic field produced by the magnet, slowing it down. It makes really addictive viewing!
How are eddy currents used?
Apart from cool experiments, showing off in front of your friends and creating YouTube videos eddy currents have many practical uses.
If you’ve been on a rollercoaster, you will have witnessed eddy currents in action as their electromagnetic braking systems rely on them to slow the carriages. Because there is no contact between brake shoes or drums there is no mechanical wear.
Vending machines also use eddy currents to detect counterfeit coins. When you put a coin in a machine there’s a whole world of science going on behind that shiny front. When a fake coin is placed into a machine it passes a magnet, and because the fake coin is made of a conductive material, unlike real coins, it is slowed down by the eddy currents created in the coin and diverted into a rejection shoot.
Possibly, the most important use of eddy currents is their use in material testing. As cracks in the surface of a material will stop eddy currents forming in the area that is damage, eddy currents can be used to detect damage or cracked material. Particularly useful if you are making things such as aeroplane wings!
How to create your own eddy current experiment
To demonstrate eddy currents, you don’t need to be a scientist or an engineer. All you need is a strong neodymium magnet and a pipe made from a conducting material such as copper. The video above shows a 23mm diameter x 20mm thick neodymium magnet being dropped through a large, thick copper pipe.
We think a better way of showing eddy currents in action would be to use a thinner copper pipe and a neodymium ring magnet, just like in the image at the top of the page. It is important that the diameter of the copper pipe and the inner diameter of the ring magnet are very similar, so there is just enough space between to slide the ring over the pipe.
Try this for yourself with our 25mm diameter neodymium ring magnet with a 16mm inner diameter, ideal for use with 15mm copper tube. If you really want to amaze your friends or family, you can perform the demonstration with an unmagnetised neodymium ring magnet and watch it fall straight to the floor. If you then sneakily exchange the two rings and repeat with the magnet your audience will be captivated as the magnet descends slowly down the pipe!