Magnetism and Electromagnetism: notes

Magnets

• Poles: like poles repel; unlike poles attract.
• Magnetic materials: iron, steel, nickel and cobalt. Iron is a soft magnet (easy to magnetise and demagnetise); steel is a hard magnet (keeps its magnetism).
• Magnetic field: the region where a magnet exerts force. Field lines leave the north pole and enter the south pole. Closer lines mean a stronger field.

Electromagnetism

• Current makes a field: a straight wire has circular field lines around it. A solenoid has a field like a bar magnet.
• Electromagnet: a coil on a soft iron core. A stronger field comes from more turns, more current or an iron core.
• Motor effect: a wire with current in a magnetic field feels a force. Fleming's left-hand rule gives its direction.

Induction

• Electromagnetic induction: moving a wire or magnet so the field changes induces a voltage.
• Generator: turns a coil in a field to make alternating current.
• Transformer: Vs / Vp = Ns / Np. It works only with alternating current. Step-up raises the voltage; step-down lowers it.