Sec 4 Physics Electromagnetism notes

Run a current through a straight wire and a magnetic field appears around it: concentric circles centred on the wire, strongest close to it and weaker further out. The sense of the circles is given by the right-hand grip rule: grip the wire with your right hand, thumb along the conventional current, and your curled fingers point the way the field circulates. Reverse the current and the circles reverse. Plotting compasses placed around the wire confirm it — this is Oersted’s discovery, the hinge on which the whole of electromagnetism swings.

Bend the wire into a solenoid — a long coil — and every turn’s circular field adds up: inside the solenoid the field is strong and nearly uniform along the axis; outside it matches a bar magnet, with a north pole at one end and a south at the other. Which end is north? Grip the solenoid with the right hand, fingers along the current in the coils, and the thumb points to the north end. A suspended solenoid aligns north–south exactly like a bar magnet: electricity has manufactured magnetism, switchable at will.

An electromagnet is a solenoid wound on a soft-iron core. Its strength grows with more turns per unit length, a larger current, and the soft-iron core itself, which channels and multiplies the field. Why soft iron (from Chapter 9): it magnetises fully the instant current flows and demagnetises fully the instant it stops. Steel would stay magnetised and the device would jam on.

Two classic applications. The relay lets a small, safe control current switch a dangerous high-current or high-voltage circuit: the coil magnetises its core, the core pulls a soft-iron armature, and the armature snaps the main-circuit contacts shut — user and mains never touch. The electric bell is a self-interrupting loop: the electromagnet pulls the armature, the hammer strikes the gong, but the same movement breaks the contact feeding the coil, so the electromagnet releases and the spring returns the armature — contact closes again, and the cycle repeats for as long as the button is held.

Place a current-carrying wire inside another magnetic field and the two fields interact: the wire feels a force perpendicular to both. That is the motor effect, and its size is F = BIL — B the flux density in tesla, I the current in amps, L the length of wire inside the field in metres. Crucially the wire must be perpendicular to the field for the full formula; at angle θ the force is BIL sin θ, so a wire parallel to the field feels nothing.

Direction: Fleming’s left-hand rule. Left hand: First finger = Field (N to S), seCond finger = Current (conventional, + to −), thuMb = Motion (the force). Arrange the first two fingers, and the thumb gives the shove. Two shortcuts for the exam: reverse the current or the field alone and the force reverses; reverse both and the force keeps its direction. And two parallel wires attract if their currents run the same way, repel if opposite — the same left-hand rule applied twice.

Build it: a rectangular coil free to rotate between the poles of a magnet, fed through carbon brushes pressing on a split-ring commutator. Switch on and each side of the coil carries current across the field, so each feels a motor-effect force. Fleming’s left-hand rule gives the two sides opposite forces — an equal, opposite pair acting on opposite sides of the axle: a couple, which turns the coil.

Without help the coil would overshoot the upright position, the forces would reverse, and it would rock to a stop with its plane vertical. The split-ring commutator fixes that: the ring is split into two halves insulated from each other, and every half revolution the brushes swap which half feeds which side — reversing the coil’s current exactly when the geometry would reverse the forces. Net result: forces that always push the same way, and continuous rotation. Speed up the motor with a bigger current, a stronger field, or more turns on the coil; reverse it by swapping either the current or the magnet’s poles — never both.

Beyond the motor: loudspeakers drive a coil in a permanent magnet’s field so the cone vibrates with the audio current; magnetic relays and circuit breakers (Chapter 8’s trip unit is an electromagnet) switch power safely; scrapyard cranes and MRI-scale separators move ferrous scrap. In each case the design question is the same: how much force or pull per amp, and how cleanly it switches off.

Exam habits that earn marks: always convert centimetres to metres before F = BIL (5.0 cm → 0.050 m); quote the force in newtons to 2–3 s.f.; when asked to "state the direction", name it relative to the diagram (e.g. "vertically upwards", "towards the north pole") — a bare "into the page" earns nothing if the diagram defines a different view. And never mix up the left hand (motor, current you supply) with the right hand — the right-hand rule returns in the next chapter for generators, where the motion is supplied to you.