Recall and use the equations: electrical power (watt, W) = current…

The idea

Electrical power tells you how many joules of energy a device transfers each second, so 1 W = 1 J/s. Two equations describe it. The first, P = I × V, applies to any component: multiply the current in amperes by the potential difference in volts to get power in watts. The second, P = I² × R, is useful when you know the current through a resistor and its resistance but not the voltage across it; square the current first, then multiply by resistance in ohms.

For example: A current of 2 A through a 3 Ω resistor gives P = 2² × 3 = 4 × 3 = 12 W. Check units: A × V = W and A² × Ω = W. Choose the equation that matches the quantities you are given.

What earns the marks

• Selects P = I × V when the question supplies current and potential difference.
• Selects P = I² × R when the question supplies current and resistance.
• Squares the current before multiplying by resistance in the P = I² × R calculation.
• States power in watts and keeps the units of current, voltage and resistance consistent.

Common mistakes and how to put them right

• Multiplies current by resistance instead of squaring the current.

  Put it right: Square the current first in P = I² × R.

• Adds current and voltage rather than multiplying them.

  Put it right: Multiply the two quantities in P = I × V.

• Uses the wrong equation for the data given.

  Put it right: Match the equation to the quantities supplied.

• Forgets to square the current and treats 2 A as 2 rather than 4 in the resistance equation.

  Put it right: Apply the exponent to the current value.

Exam tips

• Write down the quantities you are given with their units before choosing an equation.
• Substitute numbers only after rearranging, and carry the squared term carefully.
• Check the answer's unit is the watt (W) and that its size is sensible.