Learnabout Electronics

- Circuits and Resistors

Module 3.5

Power & Energy Quiz

Try a few calculations based on Power and Energy, using the information on the Power and energy page. Some are easy, some are not. Because more than one formula may be needed to solve some of the problems its important to remember to use the correct formula at the right time.

Before starting, think about these few tips, they will make the problems easier if followed carefully.

1. Work out the answers using pencil and paper; otherwise you WILL get mixed up half way through and end up with the wrong answer.

2. Of course the answer is not just a number, it will be a certain number of Watts or Joules, don't forget to show the correct unit (e.g. W mW J etc.) or your answer is meaningless.

3. Convert all sub units such as mW to Watts before you put them into the appropriate formula. A slip up here can give really stupid answers, thousands of times too big or too small.

To help you on the right track why not read about our ‘Maths Tips’ booklet, as described in the Resistors & Circuits Module 3.4 Power and Energy page.

OK so now you have read these instructions, you are ready to start.


Note: If you are using Calc98 for your calculations you need to set the View>Option>Display menu to Engineering (under the "Decimal" choices) and it would be a good idea whilst you are in this menu to select 2 from the Decimals drop down box to set the number of digits after the decimal place. This will round your answer down to two decimal places, which is sufficiently accurate for most uses and stops you getting silly answers such as 4.66666666667mW, which would be far too accurate for any practical scale of measurement we would be likely to use.

Power and Energy Quiz

(Calculate your answers with pencil, paper and calculator, then check your answers below.)


What is the power dissipated by a circuit that passes a current of 1.6A when a voltage of 6V is connected across it?


What power is dissipated by a 40 Ω resistor when a voltage of 6V is connected across it?


How much power is dissipated by a 150 Ω resistor when a current of 100 mA flows through it?


A resistor is needed to reduce the voltage supplying a circuit by 7V when the circuit draws a current of 100mA. Choose the best resistor for the job from the values below.


How much energy is used by a light emitting diode when it is illuminated by a 10 mA current at a voltage of 2 V for 30 minutes?