GCSE Combined Science Magnetism & motors revision

Every magnet has two poles — north and south. Unlike poles attract; like poles repel, and the forces act without touching, through the magnetic field: the region around the magnet in which a magnetic material feels a force. Only a handful of materials are magnetic — iron, steel, cobalt and nickel (and some alloys). Copper, aluminium, brass and plastics are not, however strongly you magnetise them.

Field lines map the field: they always leave the north pole and enter the south pole outside the magnet, and the closer the lines, the stronger the field. Plotting the field is practical work you should be able to describe: place a bar magnet on paper, put a small plotting compass near the north pole, mark where the needle points (dot the tail and tip), slide the compass along to the next position and repeat, then join the dots into smooth field lines running N → S. The needle behaves this way because it is itself a tiny magnet, free to rotate until it aligns with the field.

Bring a piece of iron near a magnet and the iron itself becomes a magnet — an induced magnet. That is why paperclips hang in chains and why the nail in a nail-and-magnet pick-up keeps picking up other nails while attached. Induced magnetism is temporary: remove the inducing magnet and most of the magnetism fades. Permanent magnets (hard steel alloys) keep theirs; that contrast — temporary versus permanent — is a favourite one-mark question.

A current in a straight wire makes a circular magnetic field around the wire — the Oersted discovery behind every electromagnet. Coil the wire into a solenoid and the fields of all the loops add together into a strong, uniform field inside the coil, with a north pole at one end and a south pole at the other: the solenoid's field is the same shape as a bar magnet's. Reverse the current and the poles swap.

Three ways to strengthen the field, all examinable: more turns of wire, a larger current, and sliding an iron core inside the coil (the core itself becomes an induced magnet and concentrates the field). The result is an electromagnet: as strong as you need, and off when you switch the current off. Applications follow from switchability: scrapyard cranes that release cars on command, relays that let a small current close a switch in a big circuit, electric door locks and bells, and the头 magnets of junkyard separators. A permanent magnet could do none of the release jobs.

Place a wire carrying a current between magnet poles and the wire feels a force — the motor effect. Three factors set its size: the magnetic flux density (how strong the field is), the current, and the length of wire inside the field. Direction matters too: the force is largest when the wire is at right angles to the field and zero when the wire is parallel to it — a wire lying along the field lines feels nothing.

The force reverses if you reverse either ingredient alone: swap the wires to the battery (current reversal) or flip the magnet over (field reversal). Reverse both and the force stays the same direction — two negatives cancel. Predicting directions needs Fleming's left-hand rule (Higher): hold your left hand flat — first finger = Field (N → S), second finger = Current (+ → −), thumb = Motion (force) — all three at right angles.

Bend the current-carrying wire into a coil and mount it between magnet poles. Current flows up one side of the coil and down the other, so (by Fleming's left-hand rule) one side is pushed up while the other is pushed down — a turning couple that spins the coil. Half a turn later the same side needs a downward push to keep rotating the same way, but the current in it is now flowing the "wrong" way. The split-ring commutator fixes this: a metal ring split into two halves, connected to brushes, that swaps the current's direction in the coil every half turn — so the left side is always pushed up and the right side always down, and the spin continues in one direction.

To make a motor turn faster: increase the current, use a stronger field, or add more turns to the coil. To make it reverse: swap the current (or flip the field) — reversing both changes nothing, exactly as with the single wire. Practical motors add many turns and an iron core to multiply the force. Every fan, pump, drill, electric car and phone vibrator in your life is this exact arrangement, scaled.