Ohm's law and electrical power
V = I R and P = I2 R
Two equations, one topic — and P = I^2 R needs the current squared, not doubled.
Know the equation
| Symbol | Quantity | Unit |
|---|---|---|
| V | potential difference | V |
| I | current | A |
| R | resistance | ohm |
| P | power | W |
Rearrangements
- I = V / R
- R = V / I
- P = V I
- P = V2 / R
- R = P / I2
Apply it — mark your own working
Work each one out on paper first, then reveal the mark scheme and tick the marks you actually earned. That is exactly how you should mark past papers.
A 12 ohm resistor has a current of 0.50 A flowing through it. Calculate the potential difference across the resistor.
Do the calculation on paper first — then mark it.
A resistor of resistance 20 ohm carries a current of 3.0 A. Calculate the power dissipated in the resistor.
Do the calculation on paper first — then mark it.
Where the marks get lost
- In P = I2 R, doubling the current instead of squaring it.
- Mixing up which quantity is which — label V, I and R from the question before substituting.
- Leaving resistance in kilo-ohms; convert to ohms first (1 kohm = 1000 ohm).
Exam tip: If a question gives you current and resistance and asks for power, go straight to P = I2 R rather than finding V first — it is one step, not two.
Still losing marks on the calculations?
I'll go through your working line by line and show you exactly where the marks are — your first lesson is free.