Saturday Science Sessions – Neodymium Magnets in a Copper Tube

I saw this physics experiment via a YouTube video a few months ago and wanted to try it out myself. All that you need is a copper pipe (conductive but not magnetic) and a neodymium or rare Earth magnet. Keep in mind that the magnet must be smaller than the diameter of the pipe!

This experiment is a demonstration of Lenz’s law: An electric current induced by a changing magnetic field will flow such that it will create its own magnetic field that opposes the magnetic field that created it.

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Two things to note first:

1. The copper pipe will not stick to the magnet, because copper does not have magnetic properties.
2. If you drop a non-magnetic object down the copper pipe, it falls to the floor.

Now take the same copper tube and drop a magnet through it. What happens? You can calculate the time it takes for the magnet to fall through the pipe.

As the magnet falls through the pipe, its magnetic field passes different locations along the length of the pipe. This changing magnetic field induces a current in the copper pipe. The induced current in the copper tube creates a magnetic field, which opposes the field of the magnet (Lenz’s Law!). So both a downwards gravitational force and an upwards magnetic force are acting on the magnet, causing it to fall slowly.

The best part of this experiment is that I want to try it again by varying the parameters. It was difficult to capture decent video footage, so this is definitely a must-try with a larger diameter pipe!


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