Magnetism and Electromagnetism

Magnets

• Like poles repel; unlike poles attract.
• Permanent magnet – produces its own field. Induced magnet – only magnetic in a field.
• Field lines run north → south; closer lines = stronger field.
• Magnetic materials: iron, steel, cobalt, nickel.

Electromagnets

A current through a wire creates a magnetic field. Coil it into a solenoid with an iron core for a strong electromagnet.

• Strengthen it: more current, more turns, an iron core.
• Advantage: can be switched on/off and its strength varied — used in cranes, relays, motors, loudspeakers.

The motor effect

A current-carrying wire in a magnetic field experiences a force:

F = B × I × L

(B = magnetic flux density in tesla, I = current, L = length of wire in the field)

Use Fleming's left-hand rule for the direction (thumb = force, first finger = field, second finger = current).

Worked example

A 0.5 m wire carrying 3 A in a 0.4 T field:

F = B I L = 0.4 × 3 × 0.5 = 0.6 N

Exam tip

The big advantage of an electromagnet over a permanent magnet is that it can be turned on/off and its strength controlled. Force is greatest when the wire is at 90° to the field.