# 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 current, or moving electrons. 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 current, because there is a lot of moving charge, can also generate 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

A homopolar motor is a simple motor that uses a large current to generate mechanical movement. Current is a flow of charge, namely electrons, and is measured in amperes or amps. Typical AA Batteries are designed to give between 1000 and 1500 mAh, or "milli-amp hours", meaning they can provide a steady 1 amp current for 1 to 1.5 hours. However, if you increase the amperage needed, then the lifetime of the battery will reduce with it. Our homopolar motor draws a lot of current in order to get spinning and to stay spinning, which heats the system up as well. This happens because of what is called the Joule-Lenz Law, or resistive heating:

 Q ∝ I2 * R Q = Heat Energy: Energy lost to resistive heating ∝ = Proportional To I = Current: How much charge is flowing in the system R = Resistance: How hard it is for charge to flow in the system.

Copper wire has low resistance, but since there is still some resistance the wire will start to heat up. Metal conductors become more resistant to charge as they heat up, so the wire will get hotter the longer the motor runs. This means that the amount of current given has to also increase the longer the battery runs! This self-destructive cycle results in the battery depleting itself in a very short amount of time, which is why homopolar motors are not commonly used.

After depleting the battery, if you used a rechargeable battery, you could recharge it partially by manually rotating the wire counterclockwise. However, we would not be able to simply set the system up in reverse and have it recharge itself. The reason for this is because the discharged end of the battery does not naturally attract the charge back to itself. If that was the case, then the battery wouldn't discharge at all! Instead, you have to provide some additional energy to the system in order to have a current flow in the reverse direction. The common way that this is done is with most phone chargers, which use a constant direct current (DC) to recharge the battery directly. Other ways that this can be done is like with a AA battery charger, which uses DC to create a reverse-current for the battery that will recharge it fully, and resupply the energy lost to heat.

This demonstration is also a great way to become familiar with the Left Hand Rule and the Right Hand Rule. These rules are handy ways to remember how the electromagnetic field being generated by the current is applying a force on the system. With either of the hand rules, you do the same steps: You point your thumb along the length of the wire, tracing the direction of the flow of current. You then point your index finger straight out, and curl the other fingers. Your fingers are now representing the following:

 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.