HookThe night a million kettles switched on at once
On 4 July 1990, England lost a World Cup semi-final to West Germany on penalties. The moment the shoot-out ended, something extraordinary happened inside the National Grid's control room: demand for electricity leapt by roughly 2,800 megawatts — the largest 'TV pickup' ever recorded in Britain. Nothing mysterious caused it. A large slice of the country stood up, sighed, and put the kettle on. One 3 kW kettle is a modest machine; the best part of a million of them, switched on inside the same few minutes, is a power station's worth of sudden demand. To this day the Grid employs forecasters who study the television schedules, because the end of a big match or a soap wedding shows up on their demand graphs as reliably as sunrise.
Almost everything in P2 hides inside that surge. Each kettle pulls about 13 amps from the 230-volt mains — that is \(P = VI\) working backwards. The extra power raced across the country through 400,000-volt transmission lines, because moving energy at enormous potential difference and small current is the only way to stop the cables cooking themselves — that is \(P = I^2R\). And every one of those kettles sat behind a fuse chosen using the same two equations. Electricity at GCSE is four quantities — charge, current, potential difference and resistance — tied together by a handful of equations you must be able to run in any direction. Master the quartet and the whole topic opens.
ModelCircuit symbols — the grammar of the subject
A circuit diagram is a sentence, and the symbols are its words: cell, battery (two or more cells), open and closed switches, lamp, fuse, fixed resistor, variable resistor, thermistor, LDR, diode, LED, ammeter and voltmeter. You are expected to draw them and read them without hesitation, and examiners are unsentimental about sloppy grammar — wires drawn with gaps, components floating off the line, or a battery with its cells the same length all cost marks.
Two placement rules do most of the exam work. An ammeter measures the current through a component, so it must sit in series with it, in the same loop. A voltmeter measures the potential difference across a component — between one side and the other — so it connects in parallel, bridging the component. Get those two the wrong way round on a practical diagram and the circuit either shorts or reads nothing; get them right and half of Required practicals 15 and 16 is already drawn.
ModelCharge and current: Q = It
Current is not a substance; it is a rate. A current of one ampere means one coulomb of charge passing a point every second, exactly as a river's flow is water per second past a bridge. The equation is \(Q = It\): charge flow (coulombs) equals current (amperes) multiplied by time (seconds). In a metal the moving charges are electrons, drifting through a lattice of fixed positive ions; nothing flows at all unless the circuit is a closed loop with a source of potential difference pushing it.
The fact examiners test relentlessly: in a single series loop the current is the same at every point. An ammeter reads the same before the lamp, after the lamp, and next to the battery, because charge is not consumed anywhere — what the lamp takes from each passing coulomb is energy, not charge.
A kettle draws 13 A and takes 3.5 minutes to boil. First convert: \(t = 3.5 \times 60 = 210\) s. Then \[Q = It = 13 \times 210 = 2730\ \text{C}.\] Nearly three thousand coulombs — and since one coulomb is about \(6.25 \times 10^{18}\) electrons, that is a staggering number of individual charges shuffling through the element. Keep the habit of converting minutes to seconds before substituting; the mark scheme forgives almost nothing about units.
ModelPotential difference, resistance and V = IR
Potential difference (pd) is the push: the energy transferred per coulomb of charge passing between two points, measured in volts, where one volt is one joule per coulomb. Resistance is the opposition: how hard a component makes it for charge to pass, measured in ohms (Ω). The master equation \(V = IR\) links them, and you must be able to rearrange it on sight: \(I = V/R\), \(R = V/I\).
Hold the pd steady and the relationship is a see-saw — current is inversely proportional to resistance. Double the resistance in a loop and the current halves; that is why adding components in series dims every lamp in the chain. The tidy special case is the ohmic conductor: for a resistor at constant temperature, current is directly proportional to pd, so the ratio \(V/I\) never changes. The 'at constant temperature' clause is not decoration — it is the exact condition that fails in a filament lamp, as Required practical 16 makes visible.
DataRequired practical 15 — what sets a wire's resistance
The first half of RP15 asks how the length of a wire affects its resistance. A thin resistance wire (nichrome or constantan) is taped along a metre rule; a crocodile clip taps off the test length; an ammeter sits in series and a voltmeter across the tested section; at each length you record V and I and calculate \(R = V/I\). The independent variable is length, the dependent variable is resistance, and the control variables are the wire's material, its cross-sectional area (same wire throughout) and — the one students forget — its temperature.
Temperature is also the practical's built-in systematic error: the current heats the wire, and a hotter metal has a higher resistance, so readings creep upwards as you work. The fix is to keep the current small and switch off between readings so the wire cools. Plotted properly, resistance against length gives a straight line through the origin: resistance is directly proportional to length. The second half of the practical swaps the wire for resistor combinations and delivers the headline rules — two identical resistors in series double the resistance; the same two in parallel halve it.
Real numbers from the wire investigation. At a length of 0.500 m the meters read V = 2.0 V, I = 0.32 A, so \(R = V/I = 2.0 \div 0.32 = 6.25\ \Omega\). At 1.000 m, with the supply unchanged, the current falls to 0.16 A: \(R = 2.0 \div 0.16 = 12.5\ \Omega\). Doubling the length has doubled the resistance — the proportionality in two rows of data. And if your line of best fit refuses to pass through the origin, cutting the R-axis at, say, 0.5 Ω, that intercept is real physics too: the contact resistance of the crocodile clips, a systematic offset added to every reading.
DataRequired practical 16 — three components, three signatures
RP16 plots the I–V characteristic — current against potential difference — for a resistor, a filament lamp and a diode. A variable resistor in series sweeps the current up and down; reversing the supply connections gives the negative quadrant. Three shapes result, and you must both describe and explain them.
The ohmic resistor gives a straight line through the origin: current proportional to pd, resistance constant — provided the temperature stays constant. The filament lamp gives a curve that flattens as pd grows: the filament runs hotter and hotter, its ions vibrate more violently, electrons collide with them more often, and the resistance climbs. The diode conducts in one direction only — essentially no current until a small forward pd (around 0.7 V), then current rises steeply; in reverse, the current is negligible. It is a one-way valve for charge.
Two more components complete the set even though they star in sensing circuits rather than this practical: the LDR, whose resistance falls as light intensity rises (automatic streetlights), and the thermistor, whose resistance falls as temperature rises (thermostats). Both follow the same memory hook: dark and cold mean high resistance.
ModelSeries and parallel — two rule-sets, one logic
In series there is one loop, so the current is the same through every component; the supply pd is shared between components (bigger resistance takes the bigger share); and resistances simply add: \(R_{total} = R_1 + R_2\). In parallel every branch is connected straight across the supply, so each branch gets the full supply pd; the branch currents add up to the supply current; and the total resistance is less than the smallest branch resistance.
That last rule feels wrong until you see the logic, and 'state the rule, then explain it' is precisely what six-mark questions demand: adding a branch opens an extra path for charge. More paths means more total current drawn at the same pd — and since resistance is \(V/I\), a bigger current at fixed pd is, by definition, a smaller resistance. Nothing was removed from the circuit; you simply widened the road.
Take a 4 Ω and a 12 Ω resistor and a 12 V supply. In series: \(R_{total} = 4 + 12 = 16\ \Omega\), so \(I = V/R = 12 \div 16 = 0.75\) A everywhere in the loop. The pds follow from \(V = IR\): \(0.75 \times 4 = 3.0\) V across the small resistor and \(0.75 \times 12 = 9.0\) V across the large — and 3.0 + 9.0 = 12 V, the supply, exactly as the series rule promises. In parallel: each branch sees the full 12 V, so the currents are \(12 \div 4 = 3.0\) A and \(12 \div 12 = 1.0\) A, giving a supply current of 4.0 A. The effective resistance is \(R = V/I = 12 \div 4.0 = 3.0\ \Omega\) — smaller than either resistor on its own. Same two components, two completely different circuits.
MechanismMains electricity: ac, dc and the three-core cable
Cells and batteries supply direct current (dc): the pd has a fixed polarity, so charge flows one way. The mains is alternating current (ac): the pd reverses direction rhythmically — in the UK, 50 complete cycles per second (50 Hz) at about 230 V. Those two numbers are pure recall marks; never leave them blank.
An appliance's flex is a three-core cable, colour-coded so it cannot be mis-wired by anyone paying attention. The live wire (brown) carries the 230 V supply. The neutral wire (blue) completes the circuit and sits at or close to 0 V (earth potential). The earth wire (green and yellow stripes) is pure safety: connected to the metal casing, it normally carries no current at all, and springs into action only during a fault.
The danger logic earns the marks. Your body is connected to the ground, so touching a live wire puts a 230 V potential difference across you and drives a current through you — potentially lethal. Crucially, this is true even when the appliance is switched off: the switch breaks the current, but the live side stays at 230 V. And the earth-plus-fuse partnership: if the live wire ever touches the casing, a large current surges harmlessly to earth through the earth wire, the fuse wire melts, and the circuit is cut — leaving the casing safe to touch.
CasePower, energy and the National Grid
Electrical power — energy transferred per second, in watts — comes in two forms you choose between depending on what the question hands you: \(P = VI\), and (substituting \(V = IR\)) \(P = I^2R\). Energy transferred follows as \(E = Pt\), or directly from charge as \(E = QV\) — every coulomb delivers V joules. A kettle rated 3 kW transfers 3,000 J every second; power ratings are why the fuse in its plug, the thickness of its flex and the size of your electricity bill are all knowable in advance.
Now scale up to the National Grid — the network of cables and transformers linking power stations to homes, the machine that absorbed the 1990 kettle surge. Cables have resistance, and the heating loss in them is \(P = I^2R\): it grows with the square of the current. So the Grid's strategy is to make the current as small as possible: step-up transformers raise the pd to as much as 400,000 V for transmission — same power, higher V, therefore lower I — and step-down transformers drop it to a safe 230 V before it enters your street. High pd is not an engineering quirk; it is the entire reason grid transmission is efficient.
The kettle, then the country. Kettle: \(I = P/V = 3000 \div 230 = 13\) A, so it needs a 13 A fuse — the largest standard rating, sitting just above the operating current. Boiling for 210 s transfers \(E = Pt = 3000 \times 210 = 630{,}000\) J, and the cross-check via charge agrees: \(E = QV = 2730 \times 230 = 627{,}900\) J, the small gap being pure rounding of the current. Country: send 100 MW through transmission cables of total resistance 5 Ω. At 25 kV the current would be \(I = P/V = 100{,}000{,}000 \div 25{,}000 = 4000\) A, wasting \(P = I^2R = 4000^2 \times 5 = 80\) MW — four-fifths of everything sent. At 400 kV the current is only 250 A, wasting \(250^2 \times 5 = 312{,}500\) W, about 0.3 MW. Sixteen times the pd, \(16^2 = 256\) times less loss: the case for transformers in three lines of arithmetic.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
Calculations reward ritual. Write the equation, substitute in SI units, rearrange after substituting, give the answer with a unit — and convert prefixes before anything else, because kW left as kilowatts and minutes left as minutes are the two most expensive slips in P2. Then sanity-check the size: domestic appliances draw somewhere between about 0.1 A and 13 A, so a kettle answer of 2,000 A means a decimal has slipped, and spotting that is an AO3 habit worth marks across the paper.
Circuit questions follow a fixed choreography: quote the rule, then apply it with numbers. 'In series the pds add, so the resistor takes 12 − 9 = 3 V' scores; jumping straight to the number risks losing the working mark. Graph questions on RP16 are describe-and-explain: name the shape (straight through the origin; curve flattening; one-way conduction) and then give the mechanism — for the lamp, that current heats the filament, ions vibrate faster, collisions increase, resistance rises. On both required practicals, be ready to name independent, dependent and control variables, and to state the direction of the heating error in RP15: a warming wire reads a resistance that is too high, controlled by using small currents and switching off between readings.
The six-markers recycle three arguments, so rehearse them as causal chains: why parallel resistance is lower (extra path → more total current → lower V/I), why the Grid transmits at 400 kV (same power at higher pd → smaller current → I²R losses collapse), and the mains safety chain (fault → casing live → large current through earth wire → fuse melts → circuit isolated). Write them as 'because' sequences in order, not scattered facts. One boundary note for Trilogy students: static electricity is not in Combined Science — P2 ends at the National Grid — so spend those revision hours on I–V graphs instead.