Build a Homopolar Motor and Magnetic Train with Neodymium Magnets
A battery, a strong neodymium magnet and a piece of bare copper wire are all it takes to build two of the most satisfying science demos around: a homopolar motor that spins on its own and a tiny electric train that zooms through a copper coil. Both take about 10 minutes and look great on video.
- 10 minBuild Time per Project
- 3Parts per Project
- 1 PackMagnets for Both Builds
- 10+Ages, with Adult Supervision
Why These Two Experiments Are Classics
Both projects turn electricity into motion right in front of you, with no switches, gears or circuit boards. The only moving part is the one you built. That makes them perfect for science fairs, classrooms, homeschool lessons and short videos that stop people mid-scroll.
They also teach the same big idea. When electric current flows through a magnetic field, it feels a push. That push, called the Lorentz force, is the principle behind every electric motor, from a desk fan to an electric car. Michael Faraday demonstrated the first homopolar motor in 1821, and the setup you'll build today works the same way.
The Magnet for Both Projects
Both builds work best with a strong, nickel-plated disc that is slightly wider than an AA battery. Our 5/8 × 1/4 Inch N52 Disc Magnets are an ideal fit. At 5/8 inch (15.9 mm) across, they're just wider than an AA battery (about 14.5 mm), which is exactly what the train needs to touch the coil. One 10-pack covers a homopolar motor, a train, and spares.
5/8 × 1/4 Inch N52 Neodymium Disc Magnets (10 Pack)
| Size | 5/8 Inch diameter × 1/4 Inch thick (15.9 × 6.35 mm) |
|---|---|
| Grade | N52 |
| Pull force | 19 lbs per magnet |
| Magnetization | Axial (poles on the flat faces) |
| Coating | Nickel-copper-nickel (Ni-Cu-Ni) |
| Max operating temp | 176°F (80°C) |
Why the coating matters: Electric current has to flow through the magnet in both projects. Our standard nickel-copper-nickel plating conducts electricity. Epoxy, rubber and plastic coated magnets are insulated, so they won't work for these builds.
What You'll Need
Homopolar Motor
- 1 AA alkaline battery
- 1 5/8 × 1/4 Inch Disc Magnet
- About 8 inches of bare copper wire, 14 to 18 gauge
- Needle-nose pliers
Magnetic Train
- 1 AA battery (rechargeable NiMH works best)
- 2 5/8 × 1/4 Inch Disc Magnets
- 20 to 30 feet of bare copper wire, 18 to 20 gauge
- A 3/4 inch wooden dowel or pipe to wind the coil
The wire must be bare, uninsulated copper. Enameled "magnet wire" and plastic-coated wire look similar but won't conduct where the magnets touch it. You can find bare copper wire at most hardware and craft stores.
Project 1: Build a Homopolar Motor
A homopolar motor is the simplest electric motor you can make. Current flows from the battery, down the wire, through the magnet and back into the battery. Because that current crosses the magnet's field, the wire gets a sideways push and starts spinning around the battery.
- Set the battery on the magnet. Place the disc magnet on a flat, non-metal surface and stand the battery on top of it, flat (negative) end down. The magnet will grab the battery and hold it upright.
- Shape the wire. Find the middle of the wire and bend a small dimple or point into it. This point will balance on the battery's positive terminal. Bend the two ends down and around the battery in a heart, spiral or rectangle shape.
- Adjust the ends. Curl the ends of the wire so they lightly brush the sides of the magnet at the bottom. They should touch without squeezing.
- Let it go. Balance the wire's center point on top of the battery. As soon as both ends touch the magnet, the wire should start spinning. If it stalls, loosen the ends slightly or balance the shape so both sides weigh about the same.
Pro Tip: Try the even faster screw version. Stick the magnet to the flat head of a steel drywall screw, then touch the screw's point to the battery's negative end so it hangs there. Touch one end of a short wire to the positive terminal and the other to the edge of the magnet. The screw and magnet will spin fast enough to blur.
Project 2: Build a Magnetic Coil Train
The "world's simplest electric train" is a battery with a magnet on each end, dropped into a coil of bare copper wire. The magnets touch the coil and let current flow through the section of coil between them. That section becomes a small electromagnet, which pushes on one magnet and pulls on the other, so the whole train shoots forward.
- Wind the coil. Wrap the bare copper wire tightly around a 3/4 inch dowel or pipe, keeping the loops close together and evenly spaced. A cordless drill on slow speed turning the dowel makes this quick. Slide the coil off when you're done.
- Check the fit. The coil's inside diameter should be just a bit bigger than the magnets, so they slide through easily while still touching the wire. Stretch or squeeze the coil to fine-tune it.
- Attach the magnets. Stick one magnet on each end of the battery. The magnets must face opposite directions, so the same pole points outward on both ends. To check, hold a spare magnet near each end: it should be attracted to both ends, or pushed away from both.
- Send the train in. Push the train into one end of the coil. It should zip straight through. If it moves backward, turn the train around. If it doesn't move at all, flip one of the magnets.
Magnet orientation: put the same pole facing outward on both ends. If the train won't move, one magnet is flipped.
Pro Tip: Bend the coil into a circle, join the ends, and the train will keep looping. Clear tubing or a cardboard tunnel on top makes a great video set.
Troubleshooting
| Problem | Fix |
|---|---|
| The motor wire won't spin | Make sure the wire is bare copper and both ends lightly touch the magnet. Sand off any oxidation, and balance the shape so it isn't leaning. |
| The train doesn't move | One magnet is facing the wrong way. Pull it off, flip it, and put it back. |
| The train moves backward | Nothing is wrong. Turn the train around, or flip both magnets. |
| The train gets stuck in the coil | The coil is too tight or uneven. Rewind it on a slightly larger form, or gently stretch the tight spots. |
| Everything works, then slows down | The battery is draining fast. That's normal for both builds. Swap in a fresh or recharged battery. |
Take It Further
- Race wire shapes. Build hearts, spirals and double loops for the homopolar motor and time which spins fastest.
- Test magnet size. Compare the 5/8 Inch disc with a 1/2 × 1/8 Inch Disc. The smaller size also makes a great train with a AAA battery.
- Science fair version. Measure how far the train travels on one battery, or how the number of loops per inch changes its speed.
- Film it. Both projects look best in slow motion with a dark background and side lighting on the copper.
Neodymium Magnet and Battery Safety
- Adult supervision required. Both projects short-circuit the battery on purpose, so the battery and wire can get hot within a minute or two. Run them in short bursts, and never leave them running.
- Use AA or AAA alkaline or NiMH batteries only. Never use lithium-ion cells like 18650s. They can overheat dangerously when shorted.
- Keep magnets away from young children and pets. Swallowing two or more magnets can cause serious internal injury.
- Handle magnets carefully. Strong magnets can pinch fingers and chip if they snap together. Keep them away from pacemakers and electronics.
- Mind the heat. Neodymium magnets lose strength above 176°F (80°C), so let a hot battery cool before running it again.
Read our full magnet safety information before you start.
Frequently Asked Questions
What magnet do I need for a homopolar motor?
A strong, nickel-plated neodymium disc about the width of the battery or wider. Our 5/8 × 1/4 Inch N52 Discs work well with a AA battery. The coating must conduct electricity, so avoid epoxy or plastic-coated magnets.
What size magnets does a magnetic coil train need?
The magnets must be slightly wider than the battery so they touch the coil. For a AA battery, a 5/8 Inch (15.9 mm) disc is ideal. For a AAA battery, try a 1/2 Inch (12.7 mm) disc.
Why won't my magnetic train move?
Usually one magnet is facing the wrong way. Both magnets need the same pole facing outward. Flip one magnet and try again. Also check that the wire is bare copper and that the coil loops are touching the magnets.
Can I use enameled magnet wire?
No. Magnet wire has a thin insulating coating that stops current from flowing where the magnets touch it. Use bare, uninsulated copper wire for both projects.
How does a homopolar motor work?
Current flows from the battery through the wire and the magnet. Where that current crosses the magnet's field, it feels a sideways push called the Lorentz force. That push makes the wire spin around the battery.
Is this experiment safe for kids?
Yes, with adult supervision. The battery and wire can get hot because the circuit is a short, so run it in short bursts. Keep loose magnets away from young children, who could swallow them.
Ready to Build?
Grab the magnets used in this guide. All U.S. orders ship free, and most orders placed before 1 PM Mountain Time on weekdays ship the same day.
Shop 5/8 × 1/4 Inch Magnets Browse Disc MagnetsTeaching a class or running a STEM program? Volume discounts are built into our product pages, and we can quote larger orders. Contact our team and tell us what you're building.