Using Electrical Formulae Using the formulae you have

Using Electrical Formulae Using the formulae you have been taught (V=IR, P=VI, Energy in k. Wh = 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 12 v headlamp that takes 4 A? 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 k. Wh) is used up by a 10 k. W oven which is on for 4 hours? 5. A radio running off 240 v power supply has a resistance of 8 K , what is the current running through it? 6. A microwave operates at 800 W at full power. How many units are used when cooking a jacket potato at full power for 12 minutes? 7. If one unit costs 6 p, what is the cost of using a 1 k. W hairdryer for half an hour? 8. A fridge has a power rating of 1200 W and runs off mains electricity (240 v). 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. 5 A. What voltage is being supplied? 11. How many units are being used up when two 100 W 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 11 A from the mains supply at 240 V? 13. How long must you leave a 3 k. W 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 10 p. How long would you have to vacuum the house with a 0. 8 k. W vacuum cleaner to spend £ 1? 16. A power rating of 1 W means that the appliance uses energy at the rate of 1 joule per second. How many joules does a 1 k. W fire use up in a minute?

Too easy for ‘ya? Try these!! 1. A drill running of mains electricity (240 v) has a resistance of 192 ohms. What is its power rating in W? 2. A resistor is labelled ‘ 2 k ’. What voltage would need to be applied to produce a current of 10 m. A 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 5 p? 4. A 5 kg 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 2600 W. If a unit of electricity costs 6 p - calculate how much it costs to cook the turkey. 5. A computer that runs off 240 v uses up half a unit of electricity per hour. What fuse should be put in the computer’s plug? (3 A fuse, 10 A fuse or 13 A fuse).

Answers 1. 12/2 = 6 ohms 2. 12 x 4 = 48 W 3. 10/20 = 0. 5 A 4. 10 x 4 = 40 units 5. 240/8000 = 0. 03 A 6. 800/1000 = 0. 8 k. W 12/60 = 0. 2 hours 0. 8 x 0. 2 = 0. 16 units 7. 1 x 0. 5 x 6 = 3 p 8. 1200/240 = 5 A 9. 240/5 = 48 ohms 10. 10 x 0. 5 = 5 V 11. 100/1000 = 0. 1 k. W 0. 1 x 70 = 7 units for one light 12. 11 x 240 = 2640 W 13. 27/3 = 9 hours 14. 20/10 = 2 k. W 15. 10/0. 8 = 12. 5 hours 16. 1000 x 60 = 60 000 joules Hard Questions 1. 240/192 = 1. 25 A 240 v x 1. 25 A = 300 W 2. 2000 x 0. 01 = 20 V 3. 1 W = 0. 01 k. W x (10 x 365) = 36. 5 k. Wh = £ 18. 25 14 units for two