The motor effect — Physical education, 14–17 years
How a magnetic field pushes a current-carrying wire, producing the force that makes an electric motor turn.
The idea
A current in a wire creates moving charges, and a magnetic field can push on those charges. The wire then experiences a force at right angles to both the current and the field. For a straight wire, the size of the force is F = BILsinθ.
Why we need it
Electricity can transfer energy, but a circuit alone does not explain how that energy becomes rotation. The motor effect solves this problem by making a current-carrying coil experience forces in a magnetic field. Early experiments showing that current could deflect a compass revealed a link between electricity and magnetism.
A worked example
A wire of length 0.40 m carries 3.0 A through a magnetic field of 0.20 T at 90°. Since sin90° = 1, use F = BIL = 0.20 × 3.0 × 0.40. The force is 0.24 N, and its direction is found with the motor-effect hand rule.
The common trap
A common mistake is to expect a force whenever a wire carries current near a magnet. That seems reasonable because both ingredients are present, but the angle matters: a wire parallel to the field has θ = 0° and sin0° = 0, so the motor force is zero. The wire must cut across the field direction.
Outside school
Electric motors use a current-carrying coil in a magnetic field to produce continuous rotation. They power fans, washing machines, electric bikes and many tools. A motor uses the motor effect to turn electrical energy into movement; its design includes switching or alternating current so the turning force keeps the same rotational sense.
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