11 mark
A motor has an input energy of 800 J. The useful output energy is 500 J.
What is the wasted energy?
300 J
500 J
800 J
1300 J
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7.2 Power & Efficiency
11 mark
A gas fire, used to heat a room, has an input energy transfer of 180 000 J per minute.
The fire has an efficiency of 0.8.
Use the equation: Efficiency = Useful output energy transfer / Input energy transfer
Calculate the useful output energy transfer per minute.
600 J
2400 J
36 000 J
144 000 J
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2
1 mark
1 markA boiler has an input energy of 720 kJ from the gas it burns.
It transfers 540 kJ of useful energy to the home.
What is the efficiency of the boiler?
Use the equation: efficiency = useful output energy transfer ÷ total input energy transfer
0.12
0.75
0.90
1.33
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31 mark
A boiler has an input power of 12 kW from the gas it burns.
Its efficiency is 0.75.
Which row in the table shows the correct values for this boiler?
| Useful output power (kW) | Wasted output power (kW) | |
| A | 3 | 9 |
| B | 8 | 4 |
| C | 9 | 6 |
| D | 9 | 3 |
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41 mark
A vehicle has an input power from fuel of 20 kW and a useful output power of 6 kW.
Calculate the power it wastes.
3 kW
6 kW
14 kW
20 kW
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51 mark
A radio transfers 30 J of potential energy to 27 J of useful energy.
What is the efficiency and energy loss for the radio?
| efficiency | energy loss | |
| A | 10% | 3 J |
| B | 10% | 27 J |
| C | 90% | 3 J |
| D | 90% | 27 J |
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61 mark
Higher Only
Which row would result in an increase in the efficiency of a machine?
|
Increase the energy losses due to friction |
Increase the work output without changing the work input |
|
| A | Yes | Yes |
| B | Yes | No |
| C | No | No |
| D | No | Yes |
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71 mark
Which wall would allow the most heat transfer through the wall?
A thick wall made from a material with high thermalconductivity.
A thick wall made from a material with low thermal conductivity.
A thin wall made from a material with high thermal conductivity.
A thin wall made from a material with low thermal conductivity.
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