Fun with Less Kilowatts: The Homopolar Motor

By Vernon Trollinger, April 18, 2017, Events & Fun, Family, News

Welcome to Fun with Less Kilowatts! We believe that science experiments at home can be a creative way to engage kids in learning while having fun. They can be educational AND great activities to keep your kids busy and away from the television. Each month, we’ll feature a new science experiment that can be a great resource for parents and teachers.

The Homopolar Motor

Sarah Dees at Frugal Fun For Boys and Girls posted this classic electric motor experiment. Originally developed in 1821 by legendary British electricity pioneer, Michael Faraday to demonstrate electro-magnetic rotation, the Homopolar Motor continues to offer important basic scientific principles that are being used right now for cutting-edge technology. In fact, a seriously high-voltage version of this experiment is the basis behind the US Navy’s railgun. The railgun’ s projectile is actually the conductor of this system. When fired, the projectile is accelerated to 6 times the speed of sound (Mach 6) and is capable of traveling to its target over 160 miles away (the distance from Washington, DC to Philadelphia, PA). The impact has been described as that of a small meteor.

In this experiment, we just want our conductor to spin.

This motor is different from the Beakman motor we covered earlier. The Beakman used two paper clips that acted as motor “brushes” that contacted the coil’s axles, which acted as commutators. That kind of design is practically standard for all DC motor design. Since the Homopolar motor doesn’t use commutators, it’s actually even easier to build.

The Materials

Fun with Less Kilowatts: The Homopolar Motor | Bounce Energy Blog

  • One pair of needle-nose pliers
  • 8 inches of 18 gauge copper wire

Note— craft wire maybe lacquer-coated. Lacquer coatings will insulate the wire and for this experiment, it’s best to use bare wire. If your wire is lacquer-coated, sand the entire entire length with small piece of fine grit sandpaper to remove the coating.

  • Two or three rare earth 3/8 diameter magnets. These (and the wire) are available at local hardware stores for under $10.

WARNING—These are EXTREMELY powerful little magnets. Keep them away from credit cards and cell phones as their magnetic fields can damage them.

  • One AA battery

The Directions

1)  Cut off 8 inches of wire. Find the middle and wind it around the shaft of a small screwdriver.

2)  Remove wire from the screwdriver. The wire is the motor conductor. This forms a little loop that will need to balance on top (at the positive terminal) of the battery. TIP—Use the screwdriver point and gently press it into the positive terminal dome to make a shallow dent in the battery. This makes a little socket for loop to spin in and stay in place. Be careful as you don’t want to pierce the metal battery casing.

3)  Put the loop on top of the positive terminal and shape the wire leads so that they run down each side of the battery, but do not touch the battery.

4)   Wind the last 1/2 inch of each wire lead one half turn around the battery to create a nice curve. Bend these curves at 90°. Bend any excess wire out ward. The wire leads should come close together but not touch.

Fun with Less Kilowatts: The Homopolar Motor | Bounce Energy Blog

5)  Put three rare earth batteries on the negative end of the battery.

6)  Place the wire conductor over the battery so that the loop contacts the positive dome (and rides in the socket if you made one). At the bottom, the conductor’s leads need to be close to but not touching the magnets. The magnetic force will complete the circuit for the battery.

The Result


Fun with Less Kilowatts: The Homopolar Motor

The conductor spins. In fact, it should spin surprisingly fast! It may even fall off. You’ll need to tweak and bend the conductor so that it stays in place as it spins. Note, too, that the conductor will get warm to the touch starting at the positive (top) end.

The Science

Homopolar motors are an example of the Lorentz Force in operation. That is, when you have current flowing through a conductor (like our wire) and place it in a magnetic field that is perpendicular (that is, cutting across at 90°), the current feels force coming from a third direction. This force is coming from a direction that is perpendicular to both the magnetic field and the current flowing in the wire. To kind of get an idea of what that looks like, try out the Flemming Left Hand Rule for Motors:

  1. Make a pistol with your left hand.
  2. Extend your middle finger straight out so that it’s 90° from your pointer finger.
  3. Your pointing finger is the magnetic force. Your middle finger is the current. Your thumb is the force being exerted.

That force creates torque and causes the conductor to spin. Or, in the case of a railgun, is the force that launches the projectile.

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A native of Wyomissing Hills, PA, Vernon Trollinger studied writing and film at the University of Iowa, later earning his MA in writing there as well. Following a decade of digging in CRM archaeology, he now writes about green energy technology, home energy efficiency, DIY projects, the natural gas industry, and the electrical grid.

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