Homopolar Motor Experiment: The Simplest Motor to Build

There is no coil. There are no brushes, no commutator, no magnets glued to a rotor. A homopolar motor experiment uses three objects, takes about four minutes to build, and starts spinning the instant the wire touches down. That immediacy is the attraction, and it is also the warning: the thing you have just built is a deliberate short circuit across a battery, drawing amps, and it gets hot. Run it for ten seconds, lift the wire off, let it cool. Done that way it is the clearest demonstration of the motor effect available in a kitchen.

Key Takeaways

• You’ll balance a bent copper wire on an AA cell with a magnet on its base, and watch it spin.
• It teaches the motor effect: a current crossing a magnetic field feels a force at right angles to both.
• Best from about age 10, with an adult present for the whole run.
• What usually goes wrong: the wire is too heavy or badly balanced, so it either falls off or will not turn.
• This shorts the battery. The cell, the wire and the magnet all get hot. Seconds, not minutes.

What You’ll Need

Item	How much	Where to get it	If you don’t have it
Alkaline AA cell	1–2, fresh	Supermarket	A fresh alkaline AAA works. Never a lithium or rechargeable cell
Neodymium disc magnet	One, 12–20 mm across, 3–5 mm thick	Hardware shop, craft shop, or salvaged from a dead hard drive	A stack of two thin ones. Ferrite fridge magnets are too weak
Bare copper wire	30 cm, 0.8–1.2 mm thick	Hardware store: strip a length of solid house wiring	Un-insulated jewellery wire of the same gauge
Pliers, small	One pair	Toolbox	—
Sandpaper	A scrap	Toolbox	A nail file, for cleaning wire ends
Oven glove or tongs	One	Kitchen	Needed for lifting a hot assembly
Timer	One	Phone	—

Time: prep 10 min, experiment 20 min, cleanup 2 min Cost: about 5 to 8 dollars, mostly the magnet Mess level: low, but set it on a heatproof surface

Do the Experiment: Build the Homopolar Motor Experiment

Step 1 — Stick the magnet to the flat end of the cell

The flat end is the negative terminal. The magnet should sit centred and flush. Put the assembly magnet-down on a tile, a wooden board or a metal tray, not on a laminate worktop you care about, because it will get hot.

Step 2 — Shape the wire into a balanced loop

Bend 30 cm of bare copper wire into a shape with a small dimple or loop at the top that will sit on the battery’s raised positive terminal, two arms running down past the sides of the cell, and the two ends bent inwards so they just graze the rim of the magnet. Sand the ends and the dimple until bright.

The shape matters more than anything else here. Aim for symmetry, keep the wire light, and get the centre of mass as low as you can without the arms gripping the battery.

Step 3 — Ask for the prediction

Which way will it spin, and what happens if we flip the magnet over? Get an answer before the wire touches down. Most people have no intuition for this, which makes it an honest prediction rather than a recall question.

Step 4 — Set the wire on the terminal and start the timer

Balance the dimple on the positive terminal so the wire ends lightly touch the magnet’s edge. Start a ten-second timer as it makes contact.

What you should see: rotation starting almost instantly and building to a fast, slightly wobbling spin, usually a few hundred revolutions per minute. If the ends scrape, the spin is jerky. If it is balanced well it looks almost frictionless.

Step 5 — Lift the wire off at ten seconds and feel the parts

Use tongs or the oven glove. Touch the battery’s side briefly with the back of a finger.

What you should see: the cell is noticeably warm after ten seconds and hot after thirty. The wire ends are warm where they contact. This is the part most guides leave out, and it is the reason for the ten-second rule.

Step 6 — Reverse the magnet, then reverse the battery, then both

Let everything cool for a minute between runs. Flip the magnet so the other face is against the cell and run it again. Then put the magnet back as it was and turn the battery upside down. Then reverse both at once.

What you should see: flipping the magnet reverses the spin. Flipping the battery reverses the spin. Flipping both leaves the direction unchanged. That last result is the one worth stopping on, because it rules out every explanation except the right one.

What’s Actually Going On

A current-carrying wire sitting in a magnetic field feels a force. The size is given by the magnitude of the field multiplied by the current, the length of wire in the field, and the sine of the angle between the current and the field. The direction is perpendicular to both the current and the field (HyperPhysics, Georgia State University). That is all a motor is, and this one has nothing else in it.

Trace the circuit. Current leaves the positive terminal, travels through the dimple into the wire, runs down the arms, and crosses onto the magnet where the wire ends touch its rim. The magnet is a conductor, so current then flows through it and into the battery’s negative terminal. The key step is the one at the magnet’s rim: there the current is flowing roughly radially, inward toward the axis, while the magnet’s own field there is roughly axial, along the battery. Radial current crossed with axial field gives a tangential force, so each arm is pushed sideways around the axis. Both arms get pushed the same way round, and the thing spins.

Nothing needs to switch. In an ordinary DC motor a commutator reverses the current in the coil twice per revolution, because otherwise the torque would reverse too. Here the geometry keeps the current radial and the field axial no matter where the wire has rotated to, so the force stays tangential in the same sense forever. That is what “homopolar” means: the polarity the conductor experiences never changes. David Kagan published a tidy upright version of exactly this arrangement in The Physics Teacher, which is worth reading if you want the balance geometry refined (Kagan, 2005).

The flipping experiments in step 6 are the proof. The force depends on the product of current direction and field direction, so reversing either one flips the force, and reversing both flips it twice and leaves it where it started. Any explanation that appeals to the magnet “attracting” the wire fails this test immediately, because attraction does not reverse when you turn a battery around.

One honest complication. The energy cost is appalling. The wire and magnet together have a resistance of a few hundredths of an ohm, so this is close to a dead short across a 1.5-volt cell, and an alkaline AA will push somewhere around 3 to 5 amps into it. Essentially all of that becomes heat rather than motion, which is why the cell warms so fast and why a fresh AA can be flattened in a few minutes of running. The homopolar motor is a beautiful demonstration and a terrible machine, and saying both is more interesting than saying only the first. The fact that the same configuration is used in serious research on relativistic rotation effects is a nice footnote for an older kid (Guala-Valverde, Mazzoni & Achilles, 2002).

How to Explain It to Your Kid

Ages 5–8: Watching only, with one sentence that lands

This is not a five-year-old’s build. The parts get hot, the magnet is a swallowing hazard, and the mechanism sits well above the age. A younger sibling can watch from across the table, and the one thing worth saying to them is this: the wire is not being pulled toward the magnet or pushed away from it, it is being shoved sideways, which is why it goes round and round instead of just sticking.

Then give them their own activity rather than a spectator seat. Iron filings revealing a magnetic field is the right prerequisite for this anyway, it is safe at that age in the sealed version, and it means the afternoon has something for both of them.

Ages 9–12: Push at right angles

Electricity flowing through a wire makes its own magnetism around it. Put that wire near a strong magnet and the two magnetisms shove each other, but not in the direction you expect. The push is sideways to both, which is why the wire goes round in a circle instead of toward or away from the magnet. Have them do the three flipping tests and write down the spin direction for each of the four combinations. The pattern in that table is the physics.

Ages 13+: The cross product, and the energy audit

Give them the full force law and have them identify the current direction and field direction at the contact point, then predict the torque direction with their right hand. Then do the energy accounting, which is the more valuable exercise. Measure the time to a noticeable warming, estimate the current from the cell’s known internal resistance, and compare the electrical power going in with any reasonable estimate of the mechanical power coming out. The efficiency is close to nothing, and working out why forces a real understanding of where resistance lives in this circuit.

The question to ask: “If we used two magnets stacked instead of one, what changes: the speed, the direction, the heat, or all three?”

Why Teaching This Early Pays Off

The motor effect is the single idea behind every electric motor on the planet, from a phone’s vibration alarm to a train. A child who has felt a wire shove itself sideways has a physical referent for it, which matters because the mathematics of the cross product arrives later and lands badly on students with no picture to attach it to.

There is a second, less obvious payoff, and it is about honesty in engineering. This motor works and is also useless, because nearly all its input becomes heat. Noticing that gap between “it works” and “it is good” is a genuinely transferable habit. It is also worth saying that this is not an experiment to hand over and walk away from. Reviews of minimally guided instruction have argued consistently that leaving novices to discover for themselves underperforms direct guidance, particularly where safety and procedure matter (Kirschner, Sweller & Clark, 2006). Here, the adult staying at the table is not just pedagogy. It is because the battery gets hot.

Before You Start: What Parents Should Know

Best age	10 and up, with an adult present for every run. Under 10, watch only
Supervision	Continuous. The adult holds the timer, lifts the hot assembly and owns the magnet
Real hazards	Heat: the cell, wire ends and magnet all get hot within seconds, and a cell run for minutes can get hot enough to burn and in the worst case to vent. Magnets: neodymium discs snap together hard enough to pinch fingers and chip into sharp fragments. Ingestion: two or more swallowed magnets attract through tissue and frequently require surgery, as the US Consumer Product Safety Commission states plainly. Keep them accounted for and away from under-6s and pets
Don’t	Never use a lithium primary cell, a lithium-ion cell or a rechargeable NiMH cell. Those can deliver far higher currents into a short and can vent, leak or catch fire. Use a plain alkaline AA. And never, under any circumstances, power this or any home-built circuit from a wall socket or mains adapter: this is a battery-only experiment and the entire design assumes 1.5 volts

When It Doesn’t Work

• The wire falls off immediately. Centre of mass is too high or the dimple is too shallow. Make a tighter dimple that cups the terminal, and shorten the arms so there is less weight hanging out to the sides.
• It sits there and does nothing. Check contact at both ends. Copper oxidises, and a dull film is enough to block several amps. Sand the dimple and both wire tips until they are bright pink, and make sure the ends are touching the magnet’s rim rather than resting on the tile.
• It twitches but will not keep turning. The arms are gripping the battery or scraping the magnet face. The ends should barely graze the rim. Bend them out a fraction and try again.
• It worked and now the cell is dead. Expected. This is a short circuit, and a few minutes of cumulative running will flatten an alkaline AA. Budget two cells per session and treat a dying spin as a signal to stop rather than to press harder.
• A ferrite fridge magnet does nothing. Field strength is the problem, not your technique. Ceramic ferrite magnets are roughly an order of magnitude weaker than neodymium, and the force scales directly with field. Neodymium is the one item here with no good substitute.

Take It Further

Count the revolutions: film a run at high frame rate with a marker dot on one arm, then count frames per revolution to get the speed. Repeat with one magnet, two magnets stacked and three. Plot speed against magnet count. The relationship will not be a clean straight line, and arguing about why is better physics than a tidy graph would be.

Change the wire: build three loops of the same shape from thin, medium and thick copper wire. Thicker wire means lower resistance and more current, so more force, but also more mass to accelerate. Time how long each takes to reach full speed and record how warm the cell gets in ten seconds. There is a trade-off in there to find.

Where the Homopolar Motor Experiment Belongs in a Sequence

Do magnetism first. A child who has seen field lines drawn out by iron filings around a magnet has somewhere to put the word “axial”, and one who has watched a magnet fall slowly down a copper pipe already knows that magnets and moving charge interact. Afterwards, the obvious next step is a motor with a coil and a commutator, because the contrast is the lesson: our guides to how electric motors work and building a simple electric motor at home both cover the version that needs switching, and the question “why does that one need brushes and this one doesn’t?” is the best thing a child can ask after an afternoon like this.

What to Watch For Over the Next 3 Months

• Week 4: they predict a direction correctly before a run, using the flipping rule rather than guessing.
• Month 2: the phrase “at right angles” shows up when they talk about magnets, which is the hard part of the idea.
• Month 3: they notice that something that works is still wasteful, in some unrelated context. That transfer is rarer and more valuable than the motor.

Frequently Asked Questions

Why does my battery get so hot?

Because the motor is a short circuit. The wire and magnet together offer a few hundredths of an ohm, so an alkaline AA pushes several amps through it and nearly all of that energy becomes heat. Ten-second runs with a cooling gap between them keep the cell in a safe range.

Can I use a rechargeable battery to save money?

No. Rechargeable NiMH and lithium-ion cells can deliver much higher short-circuit currents than alkaline cells, which means more heat, faster, with a real risk of venting or fire. A plain alkaline AA is the correct and the safe choice here.

Are neodymium magnets safe to have in the house?

They are safe with rules. Keep them counted and stored high, never let two of them near a small child, and keep them away from phones, bank cards, pacemakers and hard drives. Two or more swallowed magnets attract through the gut wall and are a surgical emergency, and paediatric emergency presentations for magnet ingestion rose sharply once small powerful magnets became common (Abbas et al., 2013).

Why doesn’t it need brushes like a real motor?

Because the geometry never needs the current reversed. In this arrangement the current stays radial and the field stays axial regardless of rotation, so the force stays tangential in the same direction. A coil motor has to reverse its current twice per turn, which is what a commutator and brushes are for.

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About the author

Ricky Flores is the founder of HiWave Makers and an electrical engineer with 15+ years of experience building consumer technology at Apple, Samsung, and Texas Instruments. He writes about how kids learn to build, think, and create in a tech-saturated world. Read more at hiwavemakers.com.

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Sources

1. Kagan, D. (2005). “Upright Homopolar Motor.” The Physics Teacher, 43(2), 68. https://doi.org/10.1119/1.1855734
2. Guala-Valverde, J., Mazzoni, P., & Achilles, R. (2002). “The homopolar motor: A true relativistic engine.” American Journal of Physics, 70(10), 1052–1055. https://doi.org/10.1119/1.1498857
3. Nave, R. “Magnetic Force.” HyperPhysics, Georgia State University. http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magfor.html
4. U.S. Consumer Product Safety Commission. “Magnets.” https://www.cpsc.gov/Safety-Education/Safety-Education-Centers/Magnets
5. Abbas, M. I., Oliva-Hemker, M., Choi, J., Lustik, M., Gilger, M. A., & Noel, R. A. (2013). “Magnet Ingestions in Children Presenting to US Emergency Departments, 2002–2011.” Journal of Pediatric Gastroenterology and Nutrition, 57(1), 18–22. https://doi.org/10.1097/MPG.0b013e3182952ee5
6. Kirschner, P. A., Sweller, J., & Clark, R. E. (2006). “Why Minimal Guidance During Instruction Does Not Work.” Educational Psychologist, 41(2), 75–86. https://doi.org/10.1207/s15326985ep4102_1