In a previous blog, we completed a neodymium magnet experiment to see what happens when you drop a bar magnet into a copper tube. Spoiler alert — if you want to try the experiment for yourself first, stop reading and head to this blog. When you complete the age-old “drop a magnet down a tube” trick, you’ll notice something strange. The magnet unusually slows down, pushing against the forces of gravity. This is an example of eddy currents and Lenz’s law in action, as the electrons from the magnet and the tube counteract each other.

So...What Are Eddy Currents? 

Eddy currents are circular electric currents produced by conductors in a changing magnetic field and, like magnetism, are strongly tied to electricity. An easy example of eddy currents in action is in today’s roller coasters. The coasters — and other high-speed rail vehicles, like trains — may use a secondary brake system designed around eddy currents. Why? Even if a roller coaster loses power, the magnetic breaking system can slow it down. This is because it doesn’t require electricity to function. When it comes to electricity, eddy currents can also be a bad thing. For example, you wouldn’t want eddy currents in transformers, devices that help raise or lower electric power voltages. The presence of a current here could also cause energy losses. To combat this, transformers are designed with slots in the metal tubing or special insulation.

And What About Lenz’s Law? 

Eddy currents aren’t the only reason for the magnet to defy gravity. To get a full understanding, you have to think about Lenz’s Law as well. Lenz’s law was named for the German physicist who discovered it, Heinrich Friedrich Emil Lenz. The discovery of Lenz’s law was one of the most influential insights into the relationship between electricity and magnetism. The law states that: The induced electromotive force with different polarities induces a current whose magnetic field opposes the change in magnetic flux through the loop in order to ensure that original flux is maintained through the loop when current flows in it.  MagLab gives a great understanding of Lenz’s law, saying: “In short, Lenz’s law is a consequence of the conservation of energy. According to the law, the total amount of energy in the universe must remain constant. If the magnetic field associated with the current moves in the same direction as the change in the magnetic field that created it, these two magnetic fields would combine to create a net magnetic field that would induce a current with twice the magnitude.”  This brings us back to the copper tube and neodymium magnet experiment. When you drop a magnet down a copper tube, the non-magnetic metal is being placed next to the magnet’s magnetic field. This will induce an electric field (a voltage difference) in the metal. This subsequently generates a magnetic field with an opposite orientation. Metals don’t like having electric or magnetic fields inside of them, so they’ll try to cancel out the difference in electric potential by moving electrons around — a.k.a. Lenz’s Law. So, the magnetic field induced in the metal will attract the falling magnet, creating the resistance that you see. 

Discover More With Apex Magnets

There you have it! An age-old “trick” explained! If you want to stay up-to-date on the latest magnetic discoveries, sign up for our newsletter to get all the magnetic history, neodymium magnet experiments, and DIYs you can think of sent straight to your inbox each month.