Using Electrical Formulae Using the formulae you have been taught (V=IR, P=VI, Energy in kWh = kilowatts x hours) first of all work out which formula you need and then use it to work out the answer. Remember Put in units (amps, volts, watts, ohms etc.) You may have to convert some numbers to the right units (e.g. minutes to hours) 1. What is the resistance of a lamp that takes a 2 amps from a 12 volt supply? 2. What is the power of needed to run a 12v headlamp that takes 4A? 3. What current would flow through a 20 ohm resistor if it is given a voltage of 10 volts? 4. How many units (1 unit = 1 kWh) is used up by a 10kW oven which is on for 4 hours? 5. A radio running off 240v power supply has a resistance of 8K, what is the current running through it? 6. A microwave operates at 800W at full power. How many units are used when cooking a jacket potato at full power for 12 minutes? 7. If one unit costs 6p, what is the cost of using a 1kW hairdryer for half an hour? 8. A fridge has a power rating of 1200W and runs off mains electricity (240v). What is the current running though it? 9. What is the fridge’s resistance? 10. A fixed resistor of 10 ohms is in a circuit that has a current of 0.5A. What voltage is being supplied? 11. How many units are being used up when two 100W electric lights are left on for 10 hours a day for a week? 12. How much power is needed to run a washing machine heater that takes a current of 11A from the mains supply at 240V? 13. How long must you leave a 3kW fire on for it to use up 27 units of electricity? 14. 20 units of electricity were used up by a storage heater when it was left on overnight for 10 hours. What is its power rating? 15. One unit costs 10p. How long would you have to vacuum the house with a 0.8kW vacuum cleaner to spend £1? 16. A power rating of 1W means that the appliance uses energy at the rate of 1 joule per second. How many joules does a 1kW fire use up in a minute?

Too easy for ‘ya? Try these!! 1. A drill running of mains electricity (240v) has a resistance of 192 ohms. What is its power rating in W? 2. A resistor is labelled ‘2k’. What voltage would need to be applied to produce a current of 10mA in it? 3. A television left on stand-by uses up energy at a rate of 60 joules a minute. If a television if left on stand-by for 10 hours a night for a year - how much money is wasted if a unit costs 5p? 4. A 5kg Christmas turkey needs to be cooked for 40 minutes for each kg in weight. The oven it is to be cooked in has a power rating of 2600W. If a unit of electricity costs 6p - calculate how much it costs to cook the turkey. 5. A computer that runs off 240v uses up half a unit of electricity per hour. What fuse should be put in the computer’s plug? (3A fuse, 10A fuse or 13A fuse).

Answers 1. 12/2 = 6 ohms 2. 12 x 4 = 48W 3. 10/20 = 0.5A 4. 10 x 4 = 40 units 5. 240/8000 = 0.03A 6. 800/1000 = 0.8kW 12/60 = 0.2 hours 0.8 x 0.2 = 0.16 units 7. 1 x 0.5 x 6 = 3p 8. 1200/240 = 5A 9. 240/5 = 48 ohms 10. 10 x 0.5 = 5V 11. 100/1000 = 0.1kW 0.1 x 70 = 7 units for one light 14 units for two 12. 11 x 240 = 2640W 13. 27/3 = 9 hours 14. 20/10 = 2kW 15. 10/0.8 = 12.5 hours 16. 1000 x 60 = 60 000 joules Hard Questions 1. 240/192 = 1.25A 240v x 1.25A = 300W 2. 2000 x 0.01 = 20V 3. 1W = 0.01kW x (10 x 365) = 36.5 kWh = £18.25