Magnetic fields and forces — Physics, 14–17 years
Magnets and electric currents can exert forces without touching. A magnetic field maps where those effects act, and the motor effect explains how a current-carrying wire can move in a field.
A field and the motor effect
A magnetic field is a region where a magnet or moving charge can feel a magnetic force. When a current flows through a wire in a magnetic field, the wire may be pushed sideways. The force is greatest when the wire and field are at right angles, and its direction depends on both.
Why fields were introduced
Magnets seemed to act through empty space, which made a simple contact-force picture incomplete. The field idea gives each point in space a strength and direction, so we can predict what happens before placing a wire or magnet there. Experiments with compasses and currents revealed the link between electricity and magnetism.
The force on a wire
A 0.20 m wire carries 3.0 A at right angles to a 0.50 T magnetic field. Use F = BIL. Substitute: F = 0.50 × 3.0 × 0.20 = 0.30 N. If the wire were parallel to the field, the angle would remove this sideways force instead of giving the maximum value.
The common trap
It is easy to think that every current-carrying wire is pushed by a magnet. The condition is more specific: the wire must be in a magnetic field, and a parallel arrangement produces no sideways force. This mistake is reasonable because the current and the magnet are both present, but direction matters as much as size.
Where it is used
An electric motor uses this force on coils carrying current, turning electrical energy into rotation. Loudspeakers use a related moving coil to vibrate a cone and make sound. Motors also need arrangements that keep the turning force going, so one straight wire in one field is only the essential starting idea.
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