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13 Cards in this Set
- Front
- Back
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What is power? |
Power is a measure of how quickly work is being done and so how quickly energy is being transferred.
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What unit is power measured in? |
Power is measured in Watts. |
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What is a watt? |
One Watt is equal to one joule per second. |
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What equations are used to calculate power? |
Power (watts, W) = work done (joule, J) / time taken (seconds, s) Power (watts, W) = Energy (joule, J) / time taken (seconds, s) |
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The energy transferred by a small electric motor is 10 J and it takes 5 seconds to move a lever. What is the power of the motor? |
2 W |
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Twocar engines are working at full capacity. The engine in the red car transfers 50 000 J in 5 seconds and the engine in the blue car transfers 20 000 J in 1 second. Which engine is more powerful? Show your working to calculate the power of each engine. |
Red car engine: 50 000/5 = 10 000 W. Blue car engine: 20 000/1 = 20 000 W. The blue car engine is more powerful. |
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How long will it take a 500 W heater to give out 3000 J of energy? |
3000/500 = 6 s. |
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How much light energy does the bulb in the Figure give out in 1 minute? |
5% of 100 W is 5 W. In 60 s the bulb will give out 5 × 60 = 300 J. |
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A new car has an 80 kW engine. It has a top speed of 190 km/h. Explain why it cannot go any faster than this. Calculate how much energy is being transferred and explain where the energy is going. |
The engine transfers 80 000 J each second. This energy is being dissipated as heat. |
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In a typical car engine, the chemical energy in the petrol is about four times greater than the amount of energy transformed into useful work by the engine. Explain why. |
The output power of a typical engine is 80 kW but the engine is quite inefficient: less than 25% of the energy that goes into the engine is output as useful energy. |
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A small electric motor can output 2 watts of energy. It can be made to do the same job of moving a CD tray as a more powerful 15 W motor. Use the relationship between power, energy and time to describe the advantage of using a more powerful motor. |
A more powerful motor can do the job more quickly. As power increases, the time it takes to output the same amount of energy shortens. |
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The following wording is taken from an advert for batteries. ‘This is the most powerful battery we have ever made! It lasts up to five times longer than an ordinary battery.’ Use your understanding of energy and power to explain to the advertising department of the company what they have got wrong in their advert and construct a scientifically correct advert to replace it. |
The fact that it lasts longer does not affect the power of the battery. Power is the rate at which energy is converted. What your battery claims to do is last longer. If this is the case it delivers more energy because it does so for longer. The advert should read: “This is the most energetic battery we haveever made. It keeps on delivering the same power as an ordinary battery for upto five times longer than ordinary batteries.” |
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What are the key points to remember? |
Power is the work done, or the energy transferred, in a given time. P = E/t. Power is measured in watts (W). 1 W = 1 J/s. |
