How do you calculate resistance using Ohm's law?
Short answer
Ohm's law links voltage, current and resistance: V = IR, where V is the potential difference in volts (V), I is the current in amps (A) and R is the resistance in ohms (Ω). To find resistance, divide the voltage by the current: R = V ÷ I.
Step by step
- Identify the potential difference across the component and the current through it.
- Convert units: milliamps to amps (divide by 1000), kilovolts to volts.
- Use R = V ÷ I.
- Give the answer in ohms (Ω).
Worked example
A lamp has 6 V across it and a current of 250 mA. Find its resistance.
- I = 250 mA = 0.25 A
- R = V ÷ I = 6 ÷ 0.25
- R = 24 Ω
Common mistakes
- Forgetting to convert milliamps to amps, which makes the answer 1000 times too small.
- Rearranging the formula incorrectly. A formula triangle with V on top helps.
- Assuming every component obeys Ohm's law. A filament lamp's resistance rises as it heats up.
Useful for:GCSEIGCSEWAECMatric
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