Circuits — current, voltage, resistance
Physics · GCSE · The School
Three quantities, one relationship
CURRENT (I, amps) is the flow of charge. POTENTIAL DIFFERENCE (V, volts) is the push — energy transferred per unit of charge. RESISTANCE (R, ohms) is the opposition to flow. They lock together in V = I × R: more push means more current; more resistance means less.
Using V = IR
A lamp with 6 V across it and 2 A through it has resistance R = V/I = 3 Ω. Rearrange as needed: I = V/R, V = IR. Units discipline (volts, amps, ohms) prevents most errors — write them in every line.
Series vs parallel
In SERIES, components sit on one loop: the same current passes through each, voltages share out, and one broken bulb kills the lot. In PARALLEL, each component has its own branch: full supply voltage across each, currents add at junctions, and a broken branch leaves the rest lit. Homes wire in parallel for exactly that reason.
Two circuits, worked with numbers
SERIES first. A 12 V supply with a 3 Ω and a 5 Ω resistor in one loop. Resistances add: R = 3 + 5 = 8 Ω. Current is the same everywhere: I = V/R = 12/8 = 1.5 A. The voltages share out in proportion to resistance — 1.5 × 3 = 4.5 V across the first and 1.5 × 5 = 7.5 V across the second — and 4.5 + 7.5 = 12, which is the check. PARALLEL next. The same two resistors on their own branches across 12 V. Each gets the FULL 12 V, so the branches carry 12/3 = 4 A and 12/5 = 2.4 A, and the supply delivers their sum, 6.4 A. Notice the total current is larger than through either resistor alone: adding a parallel branch lowers total resistance, because it opens another route.