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18 Cards in this Set
- Front
- Back
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Give examples of appliances that transfer electrical energy into: Light energy - Heat energy - Sound energy - Kinetic energy - |
Light energy - lightbulbs, torches, car headlights etc.
Heat energy - ovens, irons, toasters etc. Sound energy - headphones, speakers, car horns Kinetic energy - segways, power drills, food mixers etc. |
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What are the units of power? |
Power is measured in watts or kilowatts. To convert from W to kW you must divide by 1,000. |
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What is 2000 watts in kilowatts? |
2,000 W = 2,000 ÷ 1,000 = 2 kW.
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What are the units of energy? |
Energy is measured in joules (J). |
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State the relationship between watts and joules. |
1 watt = 1 joule of energy transferred per second. |
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Which formula/equation is used to work out the energy transferred? |
E = P × t
E is the energy transferred in joules, J P is the power in watts, W T is the time in seconds, s. |
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What is the energy transferred by a 20 W lamp lit for 100 seconds? |
20 × 100 = 2,000 J (2 kJ). |
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A 20 W lamp is switched on for 5 hours. How much energy does it transfer? |
5 hours = 5 x 3600 = 18000 seconds energy = 20 x 18000 = 360,000 J (or 360 kJ) |
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A 3 kW heater uses 8000 kJ of energy. How many minutes was it switched on for? |
E = P x t ..... therefore t = E ÷ P t = 8,000,000 ÷ 3000 = 2667 seconds
2667 seconds = 44.45 minutes (or 44 minutes 27 seconds) |
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What energy transfers are the following devices designed to bring about? (a) electric drill (b) fan heater (c)television. |
(a) electric drill - Electric to kinetic (b) fan heater - Electric to kinetic and heat (c) television. - Electric to light and sound. |
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How is energy wasted in the following devices?
(a) electric drill (b) fan heater (c) television. |
(a) Heat and sound. (b) Sound. (c) Heat.. |
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Name two electrical devices (other than those in question before) that are designed to transfer electrical energy into: (a) heating (b) light (c) sound (d) movement |
(a) heating - Any two from: cooker, toaster, microwave, electric fire, hair dryer, iron, etc.
(b) light - Any two from: light bulb, LED, computer, etc. (c) sound - Any two from: radio, computer, stereo system, MP3 player, etc. (d) movement. - Any two from: food mixer, hair dryer, lawnmower, electric saw (or other DIY tools), etc.; not television (some students think the ‘moving’ picture means that kinetic energy is involved) unless electrons in CRT are specified. |
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A 50 W radio is switched on for 3 hours. How much energy does it use? |
Energy = power × time = 50 W × (3 × 3600) s = 540 000 J (or 540 kJ). |
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A tumble dryer uses 2700 kJ of electrical energy when it is used for half an hour. What power is the tumble dryer? |
Power = energy/time = 2 700 000 J/(30 × 60) s = 1500 W (or 1.5 kW). |
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A computer uses 2400 kJ of energy. Its power is 65 W. How long was it switched on for? Give your answer in hours, minutes and seconds to the nearest second. |
Time = energy/power = 2 400 000 J/65 W = 36 923 seconds. This is 10.256 hours = 10 hours 15.38 minutes = 10 h 15 m 23 s. |
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An electric oven has a power of 3 kW. A cake takes 40 minutes to bake. (a) What is the maximum amount of energy the oven will use to bake the cake? (b) Explain why this is a maximum amount. (Hint: think about what the thermostat in an oven does.) |
Energy = power × time = 3000 W × (40 × 60) s = 7 200 000 J (or 7200 kJ). The oven takes some time to warm up, so some energy will have been used before the cake is put in. Once the oven has reached the correct temperature the heating element is switched off until the temperature drops a little, so the oven is not using electricity for all the time it is switched on. |
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‘The increasing ownership of toasters could help to reduce the amount of electricity used.’ Explain this statement. (Hint: think about how you would make toast if you did not have a toaster.) |
A toaster has the heating elements very close to the bread, so not much energy is wasted. A grill is usually further away from the bread, and has a much larger heating element and so is likely to have a higher power. Unless a lot of toast is being made some of the element will just be heating air and so wasting energy. So, although some electricity will have been used when the toaster was made, taking into account the savings in electricity use when a toaster is used instead of a grill,owning a toaster might save on electricity use overall. |
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What are the key points to remember? |
Everyday electrical devices can bring about many different types of energy transformation. The energy used by an appliance depends on its power and how long it is switched on. The power of an appliance is the rate at which it transforms energy. E= p x t Energy is measured in joules (J) or kilojoules (kJ). Power is measured in watts (W) or kilowatts (kW). I W = 1 J/s |
