GCSE Physics Magnetism revision

Magnets attract iron, steel, nickel and cobalt, and their fields fill the space around them. Field lines leave the north pole and enter the south pole; the closer the lines, the stronger the field. A plotting compass is a tiny magnet — its needle points along the field line, north end in field direction. Unlike poles attract, like poles repel; a piece of unmagnetised iron touching a magnet becomes an induced magnet, which is why paperclips hang in chains.

A current always makes a magnetic field. Wrap the wire into a solenoid and the fields of each loop add into a bar-magnet shape — one end north, the other south, field lines running inside and looping outside. Add an iron core and you have an electromagnet: switchable, and strengthened by more turns, a larger current or the core itself. Uses run from scrapyard cranes and relays to electric door locks and MRI machines. The strength of the field inside grows with the number of turns per metre and the current.

Put a wire carrying a current inside a magnetic field and it feels a force — the motor effect. The force is biggest when the wire is at right angles to the field and vanishes when they are parallel. Its direction comes from Fleming’s left-hand rule: first finger = field (N → S), second finger = current, thumb = motion (force). Its size is F = B I L (Higher): B the flux density in tesla (T), I the current in amps, L the length of wire in the field in metres. Swap the current direction or flip the magnet and the force reverses.

A DC motor exploits the effect twice: the two sides of a current-carrying coil in the field feel opposite forces, spinning the coil. Once past half a turn the forces would pull it back — so a split-ring commutator swaps the current direction every half turn, keeping the torque in one direction. More turns, a stronger field or more current all make the motor faster and stronger.

Run the motor effect backwards and you generate electricity: any relative motion between a magnet and a conductor — or any changing magnetic field through a coil — induces a potential difference across it; connect a complete circuit and a current flows. This is the generator effect. The induced potential grows with faster movement, a stronger magnet, more turns, or a larger coil area. An alternator spins a coil in a field with slip rings, giving alternating current; a dynamo uses a split-ring commutator to deliver direct current. Direction flips if the motion reverses — and Lenz’s law shows up as the induced current always opposing the change that made it.

Physics only: a microphone is a generator that hears — sound waves vibrate a coil attached to a diaphragm in a magnetic field, inducing a potential that copies the sound waveform. A loudspeaker is a motor that sings — a current copying the waveform flows through a coil in a field, the motor effect pushes the coil in and out, and the attached cone vibrates the air. One converts sound to electricity, the other electricity to sound.

A transformer (Physics only) is two coils sharing an iron core. Alternating current in the primary makes a continuously changing magnetic field in the core; that changing field induces an alternating potential in the secondary. With DC there is no change once switched on, so transformers do not work with DC. The turns ratio sets the voltages (Higher): Vs/Vp = Ns/Np. More turns on the secondary → step-up (voltage up, current down); fewer → step-down. Assuming 100% efficiency, input power = output power, so whatever the voltage gains, the current loses.

The National Grid (Physics only) runs on this: power stations generate at about 25 kV, step-up transformers push it to 400 kV for transmission, and local step-down transformers bring it to 230 V for homes. The point is heat: cable losses are P = I²R, so cutting the current cuts losses brutally — a 100× smaller current makes losses 10 000× smaller for the same cable.