Module 3.10-3.11 Magnetism and inductance

Knowledge levels

Item	A	B1 / B2 / B2L	B3
3.10(a) Theory of magnetism, magnets, Earth's field, magnetisation, shielding, materials, electromagnets, hand rules	—	2	1
3.10(b) MMF, field strength, flux density, permeability, hysteresis, retentivity, coercive force, reluctance, saturation, eddy currents, care of magnets	—	2	1
3.11 Inductance: Faraday, Lenz, mutual and self-induction, back EMF, uses	—	2	1

Magnets and magnetic fields

• Every magnet has a north and a south pole. Like poles repel, unlike poles attract.
• Field lines run from N to S outside the magnet and S to N inside; they never cross. Their density shows the field strength.
• A freely suspended magnet aligns with the Earth's magnetic field; its north-seeking pole points to magnetic north (the Earth's magnetic pole in the north is therefore a magnetic south pole). This is the principle of the compass (Module 13/11 instruments) - affected by variation and deviation.
• Domain theory: in magnetic materials, groups of atoms (domains) act as tiny magnets. Magnetising aligns the domains; heating, hammering or an alternating decreasing field demagnetises them.

Magnetic materials

Type	Behaviour	Examples
Ferromagnetic	strongly attracted, easily magnetised	iron, nickel, cobalt and alloys
Paramagnetic	very weakly attracted	aluminium, platinum
Diamagnetic	very weakly repelled	copper, bismuth

• Soft magnetic materials (soft iron, silicon steel) magnetise and demagnetise easily - transformer cores, relays.
• Hard magnetic materials (cobalt steel, Alnico, rare-earth magnets) keep their magnetism - permanent magnets in magnetos and PMGs.
• Magnetic shielding: a high-permeability case (soft iron, mu-metal) diverts flux around sensitive instruments.
• Storage: keep magnets with keepers across the poles; avoid heat and shock.

Electromagnetism

• A current-carrying conductor is surrounded by a circular magnetic field.
• Right-hand grip rule (handclasp rule): grip the conductor with the right hand, thumb in the direction of conventional current - the fingers show the field direction.
• Coil (solenoid): grip the coil with the right hand, fingers in the direction of current - the thumb points to the north pole.
• Field strength of an electromagnet increases with current, number of turns, and a ferromagnetic core. Uses: relays, solenoids, contactors.