Electrical Energy Calculations

The equation for electrical energy is tricky because it contains four components whereas many of the common GCSE equations contain only three components – or can easily be reduced to three because one of the components can be treated as a constant.

The best way to handle a four-component equation is to fill-in the values that you know and do as much of the calculation as you can. Since there will only be one unknown, you will always end up with two of the values combining, leaving just three terms to handle.

Let’s look at an example…

A certain pocket calculator contains a 4.5 V battery and draws a current of 0.15 A. How long would it take the calculator to transfer 1215 J?

The calculation to use is;

electrical energy (J) = current (A) x potential difference (V) x time (s)

Your first step should always be to check that the values stated in the question have the correct SI units (as required by the equation) and to perform any conversions if necessary. In this case, no conversions are needed so we can get straight into the calculation and insert the values that we have been given…

1215 = 0.15 x 4.5 x time (s)

This can be simplified by calculating the values that we already know;

1215 = 0.675 x time (s)

Effectively, this is now a three-component equation and we could put the values into an equation triangle to check the method for calculating the final answer. Or we might just immediately see that the time will be equal to 1215 / 0.675, giving a value of 1800 s.

In everyday language, we would refer to 1800 seconds as 30 minutes, which is a sensible amount of time to be using a pocket calculator. This is important because the exam board must always set questions that give sensible answers. So if you had miscalculated and obtained an answer of 1.8 s then you really should doubt this value because it’s unlikely that anybody would be using a pocket calculator for such a short time!

To help you practise using this four-part equation, I have created an online worksheet that is hosted on liveworksheets.com. As usual with these worksheets, you will be able to check your work as soon as you finish it and, if you select the option to send the work to your teacher, I will also get a notification. To access the worksheet, click here.