In GCSE Science students will spend some time looking at electrical energy. This is the second of six quizzes on that topic and it focusses on costing electricity and calculating energy transfers.

We often use electrical appliances because they transfer energy at the flick of a switch. The amount of energy transferred by the appliance depends on the power of the appliance and how long it is switched on. We can calculate how much energy is transferred by any appliance using a simple equation linking the power and the time it was switched on for:

energy transferred = power x time

You need to watch the units - if you need the energy transferred in kilowatt-hours, the power needs to be in kilowatts and the time in hours. If the power is given in watts and the time in seconds, the energy transferred will be in joules. Take care **not to mix** the units.

In the exams the examiners check who really understands about the correct units by perhaps giving a time in minutes. This would need converting to either a fraction of an hour or seconds, depending on what the question is asking you for. The same goes for the power. It may be given in watts but you are asked for an answer in kilowatt-hours in which case it will need converting from watts to kilowatts by dividing by 1,000 and vice versa.

From this, you can then work out how much the appliance will cost to run for the time it was on. Generally speaking, you pay for electricity measured in kilowatt-hours so questions involving cost calculations will be often be in the right units. To calculate the cost, you multiply the number of kilowatt-hours by the cost per kilowatt-hour and that is the answer. You will **not** be asked to convert between kilowatt-hours and joules.

**We recommend that you have pen, paper and a calculator handy before starting this quiz.**