Magnetism and electromagnetism
Physics · GCSE · The School
Fields, permanent and induced
A magnetic field runs from north to south outside the magnet, and its strength falls with distance. A PERMANENT magnet produces its own field. An INDUCED magnet only becomes magnetic in another field and loses it — which is why a paperclip sticks to a magnet and then falls off, and why induced magnetism is always attractive.
Current makes a field
A current in a wire produces a circular field around it; reverse the current and the field reverses. Coil the wire into a SOLENOID and the fields add to give a strong, uniform field inside and a field outside like a bar magnet's. Add an iron core and it becomes an electromagnet — which can be switched off, and that is what makes it useful.
The motor effect
Put a current-carrying wire in a magnetic field and the two fields interact, producing a force on the wire. Force = magnetic flux density × current × length (F = BIL). FLEMING'S LEFT-HAND RULE gives the direction: first finger Field, second finger Current, thumb Motion. A motor is that force applied to a coil that keeps turning.
A wire runs horizontally between the poles of a magnet, with the field pointing from left to right and a current flowing away from you. Left hand: FIRST finger along the Field, left to right. SECond finger for Current, pointing away from you. Your thuMb now points down — that is the Motion. Reverse either the current or the field and the force reverses; reverse BOTH and it points down again. That last check is the one exam questions are built on.
Run the motor effect backwards: the generator effect
Move a wire through a magnetic field, or change the field through a coil, and a voltage is INDUCED across it — no battery needed. Push the wire faster, use a stronger magnet, or add more turns to the coil, and the induced voltage grows. This is the motor effect in reverse, and it is how a dynamo or a power station generator turns movement into electricity rather than electricity into movement.
The motor effect at work: loudspeakers
A loudspeaker coil sits in a magnetic field and carries a current that varies with the audio signal; the motor effect pushes and pulls it back and forth in step with that current, and the attached cone moves the air to make sound. Reverse the field or the current briefly and the cone moves the other way — which is exactly what an alternating audio signal makes it do, thousands of times a second.