Ohm's law and resistors
Physics · Key Stage 3 · The School
Ohm's law
For a resistor at a steady temperature: voltage = current × resistance, written V = IR. A 4 ohm resistor with 2 A flowing through it has a voltage of 2 × 4 = 8 V across it. Rearrange the same equation and you get current = voltage ÷ resistance, and resistance = voltage ÷ current. One equation, three uses.
Reading a current-voltage graph
Plot current against voltage for a resistor at a steady temperature, and you get a straight line through zero. A steeper line means LOWER resistance — more current flows for the same voltage. A shallower line means HIGHER resistance.
Why the graph can curve
Not every graph is a straight line. A filament lamp's graph curves. As more current flows, the filament heats up, and its resistance rises. The straight-line rule only holds while resistance stays steady. So "Ohm's law" really describes a resistor at fixed temperature, not every component.
A cell gives 12 V across one resistor, and the current is 3 A. Resistance = voltage ÷ current = 12 ÷ 3 = 4 ohms. Swap in a different cell that gives just 6 V. The resistance has not changed, so current = voltage ÷ resistance = 6 ÷ 4 = 1.5 A. Halving the voltage exactly halved the current, because resistance stayed the same the whole time. That is Ohm's law, doing its job.