The motor effect: how electricity makes things turn
Physics · Key Stage 3 · The School
A force from two fields combining
A wire with current in it makes its own small magnetic field. Put that wire inside a bigger, outside magnetic field, and the two fields interact. This gives a real push — a FORCE — on the wire, at right angles to both fields. This is called the MOTOR EFFECT.
Fleming's left-hand rule
Hold your left hand with your thumb, first finger and second finger all at right angles. Point your first finger along the Field. Point your second finger along the Current. Your thumb then points along the Motion — the force. Field, Current, Motion, in that order. This rule gives the direction of the force, not its size.
What increases the force
The force gets bigger if the current goes up. It gets bigger if the magnetic field is made stronger. It gets bigger if the wire is made longer inside the field. Any one of these three helps. Do more than one at once, and the effect multiplies.
A simple electric motor uses a coil of wire shaped like a rectangle, sitting between two magnets, free to spin on an axle. Current runs one way along one side of the coil. It runs back the other way along the far side. By the motor effect, the two sides feel forces in OPPOSITE directions — one pushed up, one pushed down. Because they sit on opposite sides of the axle, those opposite pushes turn the coil. A split-ring commutator then flips the current direction every half turn. That keeps the push working the same rotational way, instead of the coil settling into one fixed spot. That one repeated flip is what keeps the motor spinning, rather than twitching once and stopping.