Magnetism and electromagnetism

Permanent magnets

Permanent magnets always produce a magnetic field without needing electricity. Materials: iron, steel, nickel, cobalt (ferromagnetic materials).

Magnetic poles: every magnet has a North (N) and South (S) pole.

• Like poles repel.
• Unlike poles attract.

Magnetic field lines:

• Show the direction a North pole would move.
• Point from North to South outside the magnet.
• Closer lines = stronger field.
• Never cross.

Magnetic field around a current-carrying wire

A current-carrying wire produces a circular magnetic field around it. Use the right-hand grip rule: point thumb in direction of current; fingers curl in the direction of field lines.

Solenoid (coil of wire): produces a magnetic field like a bar magnet (field lines enter one end, exit the other). Adding an iron core → much stronger field → electromagnet.

Electromagnets

Electromagnets can be switched on/off by switching the current. Strength increased by:

• Increasing current.
• Increasing number of turns.
• Adding a soft iron core.

Uses: electric bells, cranes in scrap yards, MRI scanners, loudspeakers.

Soft iron: easily magnetised and demagnetised → ideal core for electromagnets. Steel: keeps its magnetism → used in permanent magnets.

The motor effect

A current-carrying conductor in a magnetic field experiences a force. This is the motor effect.

Fleming's left-hand rule gives the direction of the force:

• First finger: direction of magnetic Field (N to S).
• seCond finger: direction of Current (positive to negative / conventional current direction).
• thuMb: direction of Motion (force on conductor).

F = BIL (force = magnetic field strength × current × length of conductor) Where B is in tesla (T), I in amperes A, L in metres (m), F in newtons (N).

The electric motor

A coil of wire in a magnetic field, connected to a DC supply:

• Current in the top wire → upward force (by Fleming's left-hand rule).
• Current in the bottom wire → downward force.
• The coil rotates.
• A split-ring commutator reverses current direction every half-turn → continuous rotation in one direction.

Electromagnetic induction

Moving a magnet into a coil (or moving a conductor through a magnetic field) induces an electromotive force (EMF) and current. This is electromagnetic induction (Faraday's law).

The induced EMF is greater when:

• The magnet moves faster.
• The magnetic field is stronger.
• There are more turns in the coil.

Reversing the direction of movement reverses the induced current.

Generators and transformers

Generator: uses electromagnetic induction to convert kinetic energy to electrical energy. The coil rotates in a magnetic field, inducing an alternating current AC.

Transformer: changes AC voltage using two coils wound on an iron core. Vs/Vp = Ns/Np (secondary voltage / primary voltage = secondary turns / primary turns) Step-up transformer: Ns > Np → Vs > Vp. Step-down transformer: Ns < Np → Vs < Vp.