HookThe evening a lightning strike stopped a million kettles
At 4:52 p.m. on Friday 9 August 2019, lightning struck a transmission line near Eaton Socon in Cambridgeshire. The strike itself was routine and the line recovered in seconds — but in the disturbance, two large generators tripped off almost together: the Hornsea offshore wind farm and the Little Barford gas station, together nearly 1900 MW of supply, gone in under a minute. With that much generation missing, the grid's frequency began to fall from its steady 50 hertz. When it dropped past 48.8 Hz, automatic protection systems shed load to save the rest of the network, cutting power to about a million people, stranding trains that needed a manual reset, and darkening part of a London hospital.
Almost every idea in P2 is written into that timeline. Britain's mains is alternating current at 230 V and 50 Hz, and it was that frequency — a direct measure of the balance between supply and demand — that betrayed the fault. The electricity had reached those homes through the National Grid, stepped up to hundreds of kilovolts by transformers to keep transmission losses low, then stepped back down for the street. Inside every affected house sat circuits obeying \(V = IR\), appliances rated in watts by \(P = VI\), and safety systems built around the live, neutral and earth wires. Learn to read a circuit as charge flowing under a push called potential difference, meeting a hindrance called resistance, and the rest of this topic falls into place.
ModelCharge, current and the language of circuits
Electric current is the rate of flow of electric charge. Charge is measured in coulombs (C), current in amperes or amps (A), and the two are linked by \(Q = It\): the charge that passes is the current multiplied by the time it flows. One amp is one coulomb per second. For charge to flow you need a complete, unbroken loop and a source of potential difference (voltage), measured in volts (V) — the electrical 'push' that does work on the charges as they move round.
Circuits are drawn with agreed symbols so that any engineer, anywhere, reads the same diagram: a cell (one long line, one short), a battery (two or more cells), a switch, a fixed resistor (a rectangle), a variable resistor, a filament lamp (a cross in a circle), a diode (a triangle pointing to a bar), an LED, a fuse, an ammeter (A in a circle) and a voltmeter (V in a circle). The one rule that trips people up is placement: an ammeter measures current, so it goes in series, in the line of flow; a voltmeter measures potential difference across a component, so it goes in parallel, straddling it.
Think of it with a gentle analogy: potential difference is the pressure, current is the rate of flow, and charge is the water itself. It is only an analogy — but it correctly predicts that nothing flows without a push, and that the current is the same all the way round a single loop.
A torch bulb carries a steady current of 0.30 A. How much charge flows through it in 5 minutes, and how many electrons is that? First convert the time: 5 minutes is 300 s. Then \[Q = It = 0.30 \times 300 = 90\ \text{C}.\] Ninety coulombs of charge have flowed. Since each electron carries \(1.6 \times 10^{-19}\) C, the number of electrons is \(90 \div (1.6 \times 10^{-19}) \approx 5.6 \times 10^{20}\) — an unimaginable swarm, which is why we count charge in coulombs rather than electrons. The mark most often dropped here is the time conversion: minutes must become seconds before you multiply.
ModelResistance and Ohm's law
Resistance, measured in ohms (Ω), is how strongly a component opposes the flow of charge. The central equation of the whole topic is \(V = IR\): the potential difference across a component equals the current through it multiplied by its resistance. Rearranged, \(I = V/R\) says that for a fixed potential difference, current is inversely proportional to resistance — double the resistance and you halve the current.
A component is called an ohmic conductor if its resistance stays constant whatever the current, which happens only when its temperature is held steady. A length of resistance wire at constant temperature is the standard example: plot current against potential difference and you get a straight line through the origin, because \(I\) and \(V\) rise in strict proportion.
The reason resistance exists at all is collisions: as charge flows, the moving charges collide with the fixed ions of the metal, transferring energy to them and making the component warm. This is why a wire heats up when current flows, and — crucially — why many components are not ohmic: as they warm, their resistance changes, and the neat straight line bends.
A resistor is connected to a 6.0 V supply and the ammeter reads 0.25 A. Its resistance is \[R = \dfrac{V}{I} = \dfrac{6.0}{0.25} = 24\ \text{(ohms)}.\] Now suppose you keep the same 6.0 V but swap in a resistor of double the resistance, 48 Ω. The current becomes \(I = V/R = 6.0 \div 48 = 0.125\) A — exactly half, because at fixed potential difference current is inversely proportional to resistance. Stating the equation, substituting, then rearranging keeps this reliable; guessing which way the fraction goes is where marks are lost.
MechanismComponents and their I–V graphs — Required practical 4
Required practical 4 measures the I–V characteristic of three components by varying the potential difference and recording the current, then plotting current (y) against potential difference (x). Each component tells its own story.
A fixed resistor at constant temperature gives a straight line through the origin — ohmic, constant resistance. A filament lamp gives an S-shaped curve that flattens at the ends: as more current flows the filament heats up, the metal ions vibrate more, and resistance increases, so the current rises less and less steeply. A diode gives a line that stays flat (almost no current) for one direction of potential difference and then rises sharply for the other: a diode has very high resistance one way and low resistance the other, so it only lets charge flow in a single direction.
Two more components respond to their environment rather than to current. A thermistor's resistance falls as temperature rises, which makes it a natural temperature sensor in thermostats and fire alarms. A light-dependent resistor (LDR)'s resistance falls as light gets brighter, which is why LDRs switch on street lamps at dusk. In both, remember the direction: more heat, or more light, means less resistance.
Imagine testing a filament lamp. At 2 V the current is 0.40 A, giving a resistance of \(R = V/I = 2 \div 0.40 = 5.0\) Ω. Turn the supply up to 8 V and the current is only 1.0 A, so now \(R = 8 \div 1.0 = 8.0\) Ω. The resistance has risen from 5.0 Ω to 8.0 Ω because the filament is hotter — and that is exactly why the I–V line curves rather than staying straight. Reading a graph, the giveaway is the gradient: for the resistor the line is straight (constant resistance); for the lamp it bends over as resistance climbs; for the diode it hugs the axis one way and shoots up the other.
ModelSeries and parallel circuits — Required practical 3
In a series circuit — one single loop — the current is the same everywhere. The potential differences across the components add up to the supply potential difference, and the total resistance is just the sum: \(R_\text{total} = R_1 + R_2 + \dots\). Add another resistor in series and the total resistance always rises, so the current always falls.
In a parallel circuit, each branch connects across the same two points, so every branch feels the full supply potential difference. The currents in the branches add up to the total current drawn from the source. The surprising rule is that the total resistance of parallel resistors is less than the smallest single resistance, found from \(\dfrac{1}{R_\text{total}} = \dfrac{1}{R_1} + \dfrac{1}{R_2} + \dots\). Adding a parallel branch gives charge another route, so more current can flow — resistance goes down, not up. This is why household sockets are wired in parallel: each appliance gets the full 230 V and can be switched independently.
Required practical 3 investigates what affects resistance. In its first part you vary the length of a wire and find that resistance is directly proportional to length — a straight line through the origin — because a longer wire means more collisions. In its second part you combine identical resistors in series and in parallel and confirm the two rules: series resistances add, parallel resistance drops.
Take a 6 Ω and a 3 Ω resistor. Wired in series, the total resistance is \(6 + 3 = 9\) Ω, so a 9 V supply drives a current of \(I = V/R = 9 \div 9 = 1\) A through both. Wired in parallel, \[\dfrac{1}{R_\text{total}} = \dfrac{1}{6} + \dfrac{1}{3} = \dfrac{1}{6} + \dfrac{2}{6} = \dfrac{3}{6} = \dfrac{1}{2},\] so \(R_\text{total} = 2\) Ω — less than either resistor on its own. The same 9 V now drives a total current of \(9 \div 2 = 4.5\) A. Same two resistors, opposite behaviour: series adds up and throttles the current; parallel opens extra paths and lets more through.
MechanismMains electricity, ac and dc, and staying alive
There are two kinds of supply. Direct current (dc), from cells and batteries, flows one way only; the potential difference has a fixed positive and negative terminal. Alternating current (ac) reverses direction many times a second, produced by generators. UK mains electricity is ac at about 230 V and a frequency of 50 Hz — meaning the current changes direction fifty times over and back each second. That 50 Hz is what the 2019 blackout was really about: frequency is the heartbeat of the grid, and it falls the instant demand outstrips supply.
Mains appliances connect through a three-core cable whose three wires are colour-coded and each do a job. The live wire (brown) carries the alternating potential difference from the supply and sits at about 230 V — it is the dangerous one. The neutral wire (blue) completes the circuit and stays at close to 0 V. The earth wire (green-and-yellow) is a safety wire at 0 V that carries no current in normal use; it only carries current if a fault connects the live wire to the metal casing, giving the current a safe path to ground and blowing the fuse.
The danger of the live wire is that your body, and the ground you stand on, are both near 0 V. Touch the live wire and there is a large potential difference across you, so a current flows through you to earth — even when a switch is off, the live wire can still be at 230 V. This is why you isolate the supply before working on it, and why earthing, fuses and circuit breakers exist: they cut the current before it can harm you.
ModelPower and energy in appliances
The power of an appliance — the rate at which it transfers energy — can be found two ways. \(P = VI\) uses the potential difference and current; \(P = I^2R\) uses the current and resistance, and is handy when you know the current flowing through a known resistance. Both give power in watts.
Once you know the power, the energy transferred over a time is \(E = Pt\) (energy = power × time). There is also a charge-based version, \(E = QV\): the energy transferred when a charge \(Q\) moves through a potential difference \(V\). This last equation says something deep — the potential difference is really the energy transferred per coulomb of charge, which is why a 12 V battery gives each coulomb twice the energy of a 6 V one.
These equations decide which appliance suits a job. A kettle needs a high power to heat water fast, so it draws a large current; a phone charger needs little. The National Grid, meanwhile, uses \(P = I^2R\) as its guiding principle in reverse: because the energy wasted in a cable depends on the square of the current, keeping the current low is everything — which is the whole reason transformers exist.
An electric heater has a resistance of 23 Ω and is plugged into the 230 V mains. The current is \(I = V/R = 230 \div 23 = 10\) A. Its power can be found two ways, and they must agree: \[P = VI = 230 \times 10 = 2300\ \text{W}, \qquad P = I^2R = 10^2 \times 23 = 2300\ \text{W}.\] Run it for 5 minutes (300 s) and the energy transferred is \(E = Pt = 2300 \times 300 = 690000\) J = 690 kJ. Cross-check with charge: \(Q = It = 10 \times 300 = 3000\) C, so \(E = QV = 3000 \times 230 = 690000\) J — the same 690 kJ. When two routes to the same number agree, you know the working is sound.
CaseThe National Grid and transformers
The National Grid is the network of cables and transformers that carries electricity from power stations to homes. It faces one enemy: energy lost heating the transmission cables, given by \(P = I^2R\). Because the loss depends on the square of the current, halving the current cuts the loss to a quarter. The grid cannot easily change the cables' resistance, so instead it slashes the current — by cranking the voltage right up.
That is what transformers do. For a fixed power \(P = VI\), raising the voltage lowers the current in exact proportion. A step-up transformer at the power station raises the voltage to 275 kV or 400 kV for long-distance transmission, so the current — and the \(I^2R\) loss — becomes tiny. A step-down transformer near your home drops it back to a safe 230 V for the sockets. Transmitting at high voltage is not more wasteful, as intuition suggests — it is dramatically less wasteful, and that single trick is why grids can span a country.
The 2019 blackout is the grid seen from the inside. When Hornsea and Little Barford dropped off, the surviving stations could not instantly make up the shortfall, so frequency sagged and automatic disconnection shed load to protect equipment from damage. It restored within about forty-five minutes — a demonstration that the grid's job is not just to deliver power, but to keep supply and demand, and therefore frequency, in constant balance.
MechanismStatic electricity and electric fields (physics only)
Rub two insulating materials together — a polythene rod with a duster, your shoes on a nylon carpet — and friction transfers electrons from one to the other. The material that gains electrons becomes negatively charged; the one that loses them is left positively charged. Only electrons move; the positive nuclei stay put. This is why charging always produces equal and opposite charges, and why it works only for insulators, which cannot let the charge flow away.
Charged objects exert forces without touching: like charges repel, unlike charges attract. A charged object also creates an electric field in the space around it. We draw the field as lines pointing away from a positive charge and towards a negative one; the lines are closer together where the field is stronger, which is near the charge. Any other charge placed in that field feels a force — the field is simply a map of the force a charge would feel at each point.
If enough charge builds up, the potential difference between the object and a nearby earthed conductor can grow so large that the electric field ionises the air, and charge leaps across as a spark. That is the physics of a static shock from a car door, and, on a colossal scale, of lightning. It is also a real hazard: fuel tankers and aircraft are earthed during refuelling so that charge cannot build up and spark near flammable vapour.
Pull off a jumper in the dark and you may see tiny sparks and feel a crackle: friction has transferred electrons between the wool and your shirt, leaving each charged. Now the safety version. As fuel flows through a hose it rubs against the pipe and charge builds on the tanker. If that charge is left to accumulate, the potential difference between the tanker and the earthed ground can rise until the surrounding electric field ionises the air and a spark jumps — next to petrol vapour, a disaster. The cure is an earthing strap: a conductor connects the tanker to the ground so charge drains away continuously and the field never grows large enough to spark. Same physics as the jumper, opposite intention.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
Circuit calculations reward the same discipline every time: identify whether components are in series or parallel first, because that decides the rules. In series, current is your fixed quantity and potential differences add; in parallel, potential difference is fixed and currents add. State V = IR, substitute, then rearrange — and convert minutes to seconds before ever touching Q = It or E = Pt.
Graph questions on components are really resistance questions in disguise. For any point on an I–V graph, resistance is V ÷ I (not the gradient), so a curve that bends over means resistance is changing. Describe the filament lamp as 'resistance increases because the filament heats up and the ions vibrate more', and the diode as 'very high resistance one way, low the other, so current flows only one way'. For thermistors and LDRs, state the direction plainly: more heat or more light means less resistance.
Mains and grid questions want precise safety language and the I²R argument. Name each wire, its colour, its normal potential difference and its role, and explain earthing and fuses as a route that blows the circuit before the casing becomes live. For the National Grid, the full-mark chain is: transformers step voltage up, so for a fixed power the current falls, so the I²R heating loss in the cables falls — reference a real event like the 9 August 2019 frequency drop to show you understand the grid as a live balance of supply and demand.