Spectrum Scientifics Blog

A homopolar motor is an easy and fun science project that takes only a few items to create.
Most of the items you probably have around the house already! The homopolar is a great way to demonstrate electromotive force.

To make a homopolar motor you will need the following items:

A Neodymium Magnet disk

A single AA Battery or AAA Battery

A piece of copper electrical wire, 3-6″ long with the ends exposed.

A flat-headed steel screw, at least 1″ long (not longer than 2-1/2″)

Optional: Some white-out or paint to mark the screw. Some electrical tape can also be useful.

Once you have your parts together take the magnet and attach it to the flat head of the steel screw. Then, ’stick’ the pointy end of the screw onto the negative end of the battery. The neodymium magnet is powerful enough to magnetize the entire screw so it will attach itself nicely to the battery. The screw, with the magnet, should be dangling from the negative of the battery.

Now take one end of the wire and attach one end to the positive end of the battery. You can hold it there with your finger (but it can get hot, be warned!) or tape it on with the electrical tape.

Now, to get the motor to run, take the other end of the wire and touch it to the edge of the magnet disk. This should make the screw start to spin. It may take some practice to get a real good ’spin’ out of the motor. The trick is to get the ‘tip’ of the screw to sit in the groove of the negative end of the battery so that it is still making contact with the battery (the casing does not carry electricity) and can still spin. From this video we made you can see it takes a few tries to get it right (about a minute of fidgeting with the motor):

So how does the homopolar motor work? The electricity actually travels through the disk magnet in a circular current. Since the magnet has a magnetic field and the electrical currect produces its own magnetic field the result is combined forces that produce torque in the magnet. Since the screw is free to move (its only attachement is where the tip touches the battery) it starts to spin.

The homopolar magnet can spin at up to several thousand RPM, and there are many variations on the design. Try varying to experiment to see what you can come up with!

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Filed under: Magnets, Physics, electricity