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.