Homopolar Motor Lab — 3D Electromagnetism

Physics in the homopolar motor

A homopolar motor is one of the simplest electric motors: a battery supplies current, a permanent magnet supplies a magnetic field, and a conductor experiences a force. In this model, follow the amber current, green magnetic-field loops, and red tangential force to see how a straight-line force becomes rotation.

Closed circuit and current

Conventional current leaves the lower positive terminal, enters the wire through the magnet contacts, travels upward along both side arms, and returns at the upper negative terminal. No complete path means no sustained current and no motor action.

Closed magnetic-field loops

Outside a permanent magnet, field lines are directed from north to south; inside the magnet they return from south to north. The green curves show the external part leaving the upper pole, bowing around the magnet, and entering the lower pole.

Lorentz force

Near the upper pole, the field has an outward radial component where it crosses each lower side wire. The cross product I × B sets the tangential direction; the scalar conductor length L scales the force magnitude.

Torque and rotation

The two side forces point in opposite tangential directions but produce the same rotational sense about the axis. Increasing current, field strength, active wire length, or rotor radius increases the idealized torque.

Cause-and-effect testing

Reverse either the current or magnet polarity and the rotation reverses. Reverse both and the two changes cancel, so the original rotation direction returns.