GCSE Combined Science Electric circuits revision

Electric current is the rate of flow of charge — how many coulombs pass a point each second. Its unit is the ampere (A), and an ammeter measures it in series, so the charge must flow through the meter. The link: Q = It. A 1.5 A phone charger left for 40 s delivers Q = 1.5 × 40 = 60 C of charge.

Potential difference (p.d.) — voltage — is the energy transferred per coulomb of charge as it passes through a component. One volt means one joule per coulomb. A voltmeter measures p.d. in parallel, clipped across the component in question, comparing the energy before and after. The three quantities lock together in V = IR: potential difference = current × resistance.

Series — one loop only. The current is identical at every point (charge has nowhere else to go). The supply p.d. is shared among components in proportion to their resistances. Resistances simply add: 3 Ω + 4 Ω + 5 Ω = 12 Ω total.

Parallel — branches across the supply. Each branch receives the full supply p.d. The supply current is the sum of the branch currents. And a curiosity worth memorising: two resistors in parallel behave like a smaller single resistor — two 10 Ω resistors act as 5 Ω. (Higher: for a pair, R = (R₁ × R₂) ÷ (R₁ + R₂); for 6 Ω ∥ 3 Ω: 18 ÷ 9 = 2 Ω.) More paths for charge means more flow for the same push — that is why parallel wiring lowers total resistance and why mains sockets are wired in parallel so each appliance gets 230 V.

Plot current against potential difference and each component signs its name. A fixed resistor at constant temperature draws a straight line through the origin — current is proportional to p.d., so resistance is constant (Ohm's law behaviour). A filament lamp draws a curve that flattens at higher p.d.: the current heats the filament thousands of degrees, the ions in the metal vibrate harder, electrons collide with them more often — so resistance rises as it brightens. A diode allows current in one direction only: essentially nothing flows backwards, and forwards it switches on above about 0.7 V, after which the current climbs steeply.

Two components turn circuits into sensors. An NTC thermistor's resistance falls as temperature rises — feed it into a divider and a heating system can switch off when the room is warm enough (a thermostat). An LDR's resistance falls as light level rises — in darkness its resistance soars, which is how an automatic night light knows to switch on: the falling LDR resistance after dusk rebalances the circuit and the lamp receives its p.d.

Required practical 1 — resistance of a wire: clip a wire along a metre rule; connect a power supply, ammeter (series) and voltmeter (parallel across the tested length); record I and V for lengths such as 0.20, 0.40, 0.60, 0.80 m; compute R = V ÷ I for each. The independent variable is the length; controls are the wire material, thickness, temperature (brief currents only) — the graph of R against length is a straight line through the origin, showing proportionality: at 0.30 m a wire might read 1.8 Ω, so at 0.60 m it reads 3.6 Ω. A related investigation swaps the wire for resistor combinations in series and parallel.

Required practical 2 — I–V characteristics: build a circuit with the component, an ammeter, a voltmeter and a variable resistor; use the variable resistor to sweep the current through a range of values, recording matching I and V pairs (then reverse the connections for negative values). Plot I against V for a fixed resistor, a filament lamp and a diode to obtain the three signatures above. Keep currents small so temperature stays controlled for the resistor.

(Higher) Why length and area matter: a current is electrons drifting through a lattice of vibrating ions. Double the length and each electron faces twice as many collisions — double the resistance. Thicken the wire and there are more parallel routes through the same lattice — resistance falls with cross-sectional area.