Homopolar Motor

Engineering:	Simple Motors, Hand Rules, Current
Grade Range:	Elementary School, Middle School, High School
Format:	Hands-on

This simple motor is easy for students to make and fairly inexpensive. This demonstration can relate to a large variety of topics, so there are plenty of options to you on when to include it in a lesson. Be aware that the system will warm up when running, so let it cool down before handling it.

Materials

• AA Batteries
• Neodymium Magnets (small)
• Non-Insulated Wire

Safety Precautions

Please read the General Safety section of the Demonstration Safety page before performing this demonstration.

Demonstration

1. Provide each student group with a battery, a foot if wire and a magnet. Have them place the magnet on the positive end of the battery and stand it on the magnet.
2. Have students bend their wires for the demonstration. They should make a point in the middle of the wire to stand on top of the battery, and have the two ends of the wire bend down and inward towards the battery. Have them bend the two tips in so they can both be touching the magnet at the same time.
3. Once they have it set up, have them try to spin the wire counter-clockwise, then clockwise. When they spin the wire clockwise, it should start to spin on its own really fast!

Why This Works

Short Explanation

This is a simple motor, known as a homopolar motor. A homopolar motor creates movement by having a lot of moving current. Our battery has charge built in the positive end, and the charge wants to move to the negative end. When we connect the ends with the wire, the charge is able to start moving through it, and will do so really fast! This moving charge is what we call the current, and because there is a lot of moving charge it is generating an electromagnetic force. This electromagnetic force is really strong, and starts to push the wire around the magnet, resulting in the spinning wire we see!

The battery runs out pretty quickly, and the wire will stop spinning after a minute. If we used a rechargeable battery for this, then we could recharge the battery by manually spinning the wire counter-clockwise! However, we would have to manually spin it, and that would take a really long time. You might think that we could just set the system up the other way, with the magnet on the negative side and the wire on the positive side, and that we could get the battery to recharge itself. Unfortunately, that is not the case. The moving charge lost a lot of energy in making the wire spin, so we would have to put energy into the system in order for it to go in reverse. Not only that, but when a battery discharges, it means that both sides of the battery are now equal in charge, rather than one side having all the charge and the other having no charge. Since there isn't a charge difference, there won't be any current.

Full Explanation

COMING SOON

Left Hand Rule:	Right Hand Rule:
Thumb: Current	Thumb: Current
Index: Magnetic Field Direction	Index: Magnetic Force Direction
Curl: Magnetic Force Movement	Curl: Magnetic Field Movement

By using the Right-Hand Rule, we can see how the magnetic field lines move counter-clockwise around the motor while the electromagnetic force is in the direction of the battery and magnet setup. By using the Left-Hand Rule, we can see that the electromagnetic force moves the wire clockwise around the motor while the magnetic field points towards the battery and magnet.