Poles, attraction and repulsion

A magnet has two poles — north (N) and south (S) — where the magnetic effect is strongest. The forces between poles follow one rule:

• Like poles repel (N–N or S–S push apart).
• Unlike poles attract (N–S pull together).

Magnetic materials: iron, steel, cobalt and nickel are attracted by magnets. Most other materials (plastic, wood, copper, aluminium) are not magnetic.

The field around a bar magnet

A magnetic field is the region where a magnetic force acts. We draw it with field lines. Two rules you must get right:

• Field lines run from N to S outside the magnet (and S to N inside).
• The lines are closest together where the field is strongest — at the poles.

Misconception: field lines never cross, and outside the magnet they always point N → S, never the other way.

Which way do they point?

Soft iron vs hard steel

Magnetic materials behave very differently when you magnetise or demagnetise them:

• Soft magnetic material (e.g. iron) — easy to magnetise but loses its magnetism easily. Ideal for an electromagnet core and the core of a transformer.
• Hard magnetic material (e.g. steel) — harder to magnetise but keeps its magnetism. Ideal for a permanent magnet.

Induced magnetism: a magnetic material placed in a field becomes a magnet itself (this is why a magnet can pick up a chain of unmagnetised nails). You can magnetise by stroking with a magnet or with a d.c. coil, and demagnetise by hammering, heating, or using a.c. in a coil.

Soft, hard or induced?

Tap a description on the left, then its matching term on the right.

Charge: positive and negative

There are two kinds of electric charge. Like charges repel; unlike charges attract — the same pattern as magnetic poles.

• Conductors (metals, graphite) let charge flow freely; insulators (plastic, rubber) do not.
• Charging by friction: rubbing transfers electrons. The object that gains electrons becomes negative; the one that loses them becomes positive. (Only electrons move — never protons.)

Electric field: a charge is surrounded by an electric field — the region where it exerts a force on other charges. Field lines point away from + and towards –.Supplement Charging an isolated conductor by electrostatic induction (without contact) is also Supplement.

Current is the flow of charge

Current (I) is the rate of flow of electric charge, measured in amperes (A) with an ammeter in series. Charge is measured in coulombs (C):

Conventional current vs electron flow: conventional current flows from + to – around the circuit. The electrons (the actual moving charge in a metal) drift the opposite way, from – to +. Both describe the same circuit — exam answers use conventional current.

Your turn — charge

Voltage and resistance

• e.m.f. (electromotive force) — the electrical work done by a source per unit charge driven round a complete circuit. Unit: volt (V) = joule per coulomb.
• Potential difference (p.d.) — the work done by the current per unit charge across a component. Measured with a voltmeter in parallel.
• Resistance (R) — how much a component opposes the current:

Supplement For a wire, resistance is proportional to length and inversely proportional to cross-sectional area: R = ρL/A. A longer, thinner wire has more resistance.

Your turn — rearranging R = V/I

I–V graphs: resistor vs lamp

Plotting current against potential difference tells you whether resistance is constant:

• A fixed (ohmic) resistor at constant temperature gives a straight line through the origin — constant resistance, I ∝ V.
• A filament lamp gives an S-shaped curve: as the current rises the filament gets hotter, so its resistance increases and the line bends away from the current axis.

Reading the lamp's graph

Electrical power and energy

Electrical power is the rate at which energy is transferred:

and the energy transferred over a time t:

Supplement Combining P = IV with V = IR gives two more forms: P = I²R and P = V²/R.

Your turn — electrical power

Your turn — electrical energy

Drawing circuit diagrams

You must recognise and draw the standard symbols. A few that catch people out:

Series vs parallel circuits

• Series: current is the same at every point; the e.m.f. is shared (p.d. splits) between components; total resistance R = R₁ + R₂ + …
• Parallel: the p.d. is the same across each branch; the current splits and recombines; the combined resistance is less than the smallest branch.Supplement 1/R = 1/R₁ + 1/R₂ + …
• Cells in series add their e.m.f.s (two 1.5 V cells give 3 V).

Misconceptions: current is not "used up" by the first bulb in series — it is the same all round. In parallel, p.d. is equal across the branches, not shared.

Your turn — combined resistance

Where does the voltage go?

Potential dividers & sensors Supplement

A potential divider is two resistors in series that share the supply voltage. The larger resistor takes the larger share of the p.d.

Swap one resistor for an input transducer and the output voltage responds to the surroundings:

• Thermistor (NTC): resistance falls as temperature rises — used in temperature sensors and fire alarms.
• Light-dependent resistor (LDR): resistance falls as light gets brighter — used in light sensors and automatic lights.

Hazards and safety features

Mains electricity is dangerous. The common hazards are: damaged insulation, overheating cables, damp conditions, and overloading sockets (too much current).

The UK mains plug uses three wires: live (brown), neutral (blue) and earth (green/yellow). Key safety features:

• Fuse — a thin wire in the live wire that melts and breaks the circuit if the current gets too high.
• Circuit breaker — a switch that trips on a large current (resets faster than a fuse).
• Earthing — connects a metal case to earth; if the live wire touches the case, a large current flows to earth and blows the fuse.
• Double insulation — a plastic case means no earth wire is needed (shown by the ▣ symbol).

Choosing a fuse: pick the standard fuse just above the appliance's normal current. For a 920 W heater on 230 V, I = P/V = 920/230 = 4 A, so a 5 A fuse is correct (a 13 A fuse would not protect it).

Choosing the right fuse

Inducing an e.m.f.

When a conductor cuts magnetic field lines (or the field through a coil changes), an e.m.f. is induced across it. If the circuit is complete, an induced current flows. The induced e.m.f. is larger when you:

• move the conductor (or magnet) faster;
• use a stronger magnet;
• use more turns on the coil.

Supplement The induced current always opposes the change that caused it (its direction is given by Lenz's law) — a consequence of conservation of energy.

The a.c. generator

Spin a coil in a magnetic field and the sides cut field lines, inducing an alternating e.m.f. The coil connects to the circuit through two slip rings and brushes.

The output reverses every half-turn, giving a sine-shaped alternating current (a.c.). Spinning faster increases both the frequency and the peak voltage.

The motor effect & Fleming's left-hand rule

A current-carrying conductor in a magnetic field feels a force (the motor effect). The force is biggest when the current is at right angles to the field.

Fleming's left-hand rule gives the direction. Hold the thumb and first two fingers of your left hand mutually at right angles:

• First finger → Field (N → S);
• seCond finger → Current (conventional, + → –);
• thuMb → Motion (the force).

Your turn — force on a conductor Supplement

The d.c. motor

A coil in a magnetic field carries a current. By Fleming's left-hand rule, the two sides feel opposite forces — one up, one down — so the coil turns.

A split-ring commutator reverses the current in the coil every half-turn, so the coil keeps spinning the same way instead of stopping. The motor turns faster with more current, a stronger field, or more turns.

Transformers

A transformer changes the size of an alternating voltage. It has a primary coil and a secondary coil wound on a soft iron core. The a.c. in the primary makes a changing magnetic field in the core, which induces an a.c. e.m.f. in the secondary.

Supplement For an ideal (100% efficient) transformer, power in = power out: V p I p = V s I s. So a step-up transformer that increases voltage decreases the current by the same factor.

Misconception: transformers only work with a.c. — a steady d.c. gives a constant field, no change of flux, and so no induced e.m.f. in the secondary.

Your turn — transformer turns

Transformers in the National Grid Supplement

Power stations send electricity across the country at very high voltage. Step-up transformers raise the voltage (and lower the current) for transmission; step-down transformers lower it again for homes.

Why high voltage? The power lost as heat in the cables is P = I²R. Transmitting at high voltage means a much smaller current for the same power, so far less energy is wasted heating the cables.

Why transmit at high voltage?

The equations to know

You've covered all five parts of Cambridge 0625 Topic 4 — magnetism, electrical quantities, circuits, safety and electromagnetic effects. Press Finish to see your score.