The Simplest Motor in the World, Rebuilt in 3D: A Homopolar Motor You Can Interrogate

A battery, a neodymium magnet, and a bent piece of copper wire. No commutator, no brushes, no coils of insulated wire wound a hundred times. Balance the wire on the battery's terminal, let its arms touch the magnet, and it spins — seemingly by magic. This is the homopolar motor, the oldest electric motor design in existence (Faraday, 1821), and it remains one of the best five-minute demonstrations in all of physics teaching.

But here is the problem every teacher knows: the demo spins, students are delighted, and then someone asks "why does it turn that way?" — and suddenly you are waving your left hand in the air, trying to describe an invisible radial magnetic field with your fingers. The physics that makes the motor work is fundamentally three-dimensional, and a whiteboard is not.

That gap is what our new simulation, the Homopolar Motor 3D Explorer, is built to close.

Seeing the invisible triad: B, I and F

The simulation renders the classic tabletop setup — battery standing on a disc magnet, copper wire loop pirouetting around it — in a fully orbitable 3D scene. Three things that are invisible in the real demo are made explicit:

• The magnetic field B (blue). Field lines are numerically traced from the magnet's dipole field, and one family of lines is deliberately seeded to pass exactly through the point on the wire where the B vector is drawn. Students can see, on a single field line, that the arrow on the coil is the local direction of the magnet's field — not a separate abstraction.
• The current I (yellow). Animated markers flow from the battery's positive terminal down both arms of the wire into the magnet, at a speed proportional to the current. Reverse the current and watch the markers turn around.
• The force F (magenta). At each wire arm, the force is computed live as F = (I × B) L — a real cross product of the local current direction and the local field, not a canned arrow. It reverses when it should, and only when it should.

Because the force is genuinely computed rather than drawn, the simulation is honest in a way that matters pedagogically. Press Reverse Current and the spin reverses. Press Flip Magnet and it reverses again. Press both — and the motor stubbornly keeps spinning the same way, exactly as Fleming's left-hand rule demands. Students can test every combination themselves and check each prediction against the arrows.

Small design decisions with large payoffs

Two details are worth highlighting for fellow simulation builders. First, the steady-state spin rate is not scripted: torque from the magnetic force competes against a frictional torque proportional to angular velocity, so the motor accelerates and settles at a terminal angular speed — doubling the current doubles the steady spin rate, a quantitative relationship students can verify with the sliders. Set the current to zero and the wire coasts to a stop under friction alone, an unplanned but welcome illustration of rotational inertia.

Second, the whole thing runs from a single folder with a fully localised three.js library — no CDN, no internet dependency. It works on a school laptop with no Wi-Fi, which is where demos are most often needed.

Five questions to leave your students with

The simulation page ends with five check-your-understanding questions with click-to-reveal answers — from predicting the spin direction with Fleming's left-hand rule, to explaining why a homopolar motor uniquely needs no commutator (the current crosses the field the same way at every rotation angle, so the torque never reverses). The last one is the deepest: it is the question that separates "I saw it spin" from "I understand why it spins".

Try it: Open the Homopolar Motor 3D Explorer, download the complete ZIP package, or browse the Electromagnetism collection at Open Source Physics @ Singapore.

Made by lookang, created using Claude Fable 5. Concept reference: JavaLab's homopolar motor. For more interactive resources, visit iwant2study.org.