A torch uses 15watts of energy. It uses two 1.5volt batteries connected in series. What is the resistance in the circuit of the torch?

In order to answer this question, we need a formula which relates resistance to voltage and power. We don't already know this formula, but can create it from two formulae we already know: P=IV & V=IR. As we do not know what the current is, we can remove it from these equations using the elimination method. In this case, that means dividing one forumla by another. This gives us a new formula: P/V=V/R. We want R to be the subject of the formula, so we can rearrange, using algebra, to make: R=V²/P. We can now work out the resistance.

Remember that the voltage is 3, as we have two 1.5 volt batteries connected in series, which means their voltages are added. Substituting these numbers into the forumla give R=0.6Ω