Magnetism and electromagnetism
Magnetism and magnetic fields
Magnets have a north and a south pole: like poles repel, unlike poles attract. Magnets attract magnetic materials (iron, steel, nickel, cobalt) by inducing magnetism in them. The space around a magnet where a force acts is its magnetic field, shown by field lines from N to S. Soft iron makes temporary magnets; steel makes permanent ones.
≈ 25 minElectromagnetic induction and generators
Move a wire across a magnetic field, or change the field through a coil, and an e.m.f. is induced. It's bigger with faster movement, a stronger magnet and more turns, and it reverses if the motion reverses. A generator spins a coil in a magnetic field to produce alternating current, whose trace you can read on an oscilloscope.
≈ 30 minElectromagnets and motors
A current produces a magnetic field: circles round a straight wire, and a bar-magnet pattern from a solenoid. Wind a solenoid round soft iron and you have an electromagnet. A current-carrying wire in a magnetic field feels a force, and in a d.c. motor that force turns a coil, with a split-ring commutator reversing the current every half turn.
≈ 30 minTransformers
A transformer has two coils wound on a soft iron core. An alternating current in the primary coil makes a changing magnetic field that induces an alternating e.m.f. in the secondary. The voltages are in the same ratio as the turns: . Power stations step the voltage up for transmission so the current, and the energy wasted heating the cables, is small.
≈ 30 min