Power & Resistance (WJEC GCSE Physics): Revision Note

A current of 6 A passes through a 20 Ω fixed resistor. 

Calculate the power of the resistor.

Answer: 

Step 1: List the known quantities

• Current, $I$ = 6 A

• Resistance, R = 20 Ω

Step 2: Write out the relevant equation

$P=I^{2}R$

Step 3: Substitute in the known values

$P={\left(6\right)}^2\times 20$

$P=720\mathrm{W}$

There are several different equations for calculating power. Always begin any calculation by writing out the known quantities with their symbols. Then you can choose an equation that contains the quantities that you know. 

For example:

• If you are given resistance, R, and current, $I$, you would use $P=I^{2}R$

• If you are given current, $I$, and voltage, V, you would use $P=IV$