Sec 4 Combined Science (Physics) Magnetism notes

A magnet attracts the magnetic materials — iron, steel, cobalt, nickel — and the pull is strongest at its two poles. Suspended freely, a magnet settles pointing north–south: the pole seeking north is the north-seeking (N) pole. The interaction rule: like poles repel, unlike poles attract — and repulsion is the only sure test of a magnet, since attraction also happens between a magnet and unmagnetised iron.

Bring a magnetic material near a magnet and it becomes a magnet itself — induced magnetism. The touching end always becomes the opposite pole (a N pole induces a S pole at the contact), which is why a chain of paperclips hangs from a magnet: each clip induces poles in the next. Iron is soft magnetic — the induced magnetism vanishes when the magnet leaves (temporary magnets). Steel is hard magnetic — it keeps its magnetism and makes permanent magnets.

The space around a magnet is its magnetic field, drawn as field lines that by convention leave the N pole and enter the S pole, never crossing, closest where the field is strongest (at the poles). A compass — a small pivoting magnet — simply aligns itself with the field at its position: away from magnets it points north–south (Earth's field); near a magnet it swings to follow the local field.

To plot a bar magnet's field: place the magnet on paper, set a plotting compass near one pole, mark the direction the needle points with two dots, move the compass so its tail sits on the last dot, and repeat — join the dots into smooth curves that fan from N to S. For two like poles nearby the lines bend away from each other, leaving a point between them where the fields cancel (a neutral point); for unlike poles the lines run directly from N to S.

A current always wraps itself in magnetism. A straight wire carries concentric circular field lines around it, drawn looking down the wire; their direction follows the current. Bigger current → stronger (closer-set) field; reversing the current reverses the field direction.

Coil the wire into a solenoid and the circles from every turn add up: strong and nearly uniform inside, and outside exactly like a bar magnet — one end N, the other S. Three levers strengthen it: more current, more turns, and an iron core inside (the induced magnetism of the iron concentrates the field enormously). Swap the battery over and every pole swaps end. This is the electromagnet: controllable, reversible, and off when the current is off.

Put a current-carrying wire at right angles to a magnetic field and the field pushes the wire sideways — a real, measurable force. The classic experiment: a stiff wire (or aluminium foil strip) clamped between magnet poles and wired to a battery and switch; close the switch and the wire jumps sideways. The force grows with a bigger current and a stronger field, and acts largest when the wire is perpendicular to the field.

Direction experiments: reverse the current (swap the battery over) and the force reverses; reverse the field (swap the magnet poles) and the force reverses; reverse both at once and the force is unchanged — two flips cancel.

The directions tie together in Fleming’s left-hand rule. Hold your left hand with thumb, first finger and second finger all at right angles: first finger = field (N to S), second finger = current (+ to −, conventional), thumb = force (the motion the wire takes). Line up two of them with the apparatus; the third reads off the answer.

Bend the wire into a rectangular coil and set it in the field. Current travels up one side of the coil and down the other — opposite directions — so Fleming’s rule gives opposite forces on the two sides: one side pushed up, the other pulled down. The pair forms a turning effect (a couple) and the coil rotates — this is the principle of the electric motor, energy transferred from electrical to kinetic stores.