Why energy is always conserved but not always useful, how dissipation happens, reducing unwanted transfers, and calculating efficiency.
You can study this deck without signing up — your progress is kept in this browser. An account saves it properly, syncs it across devices, and lets you edit the cards.
1.What does dissipated mean?
Energy has been transferred to the surroundings in a way that makes it spread out and no longer useful.
2.Give an example of dissipated energy.
Energy transferred to the thermal store of the surroundings by friction in a machine.
3.Is dissipated energy destroyed?
No. It still exists, but it is spread out so thinly that it cannot be usefully used.
4.What is meant by wasted energy?
Energy transferred to stores that are not useful for the purpose of the device.
5.Give two ways to reduce unwanted energy transfers.
Lubrication to reduce friction, and thermal insulation to reduce the rate of energy transfer by heating.
6.How does lubrication reduce waste?
It reduces friction between moving surfaces, so less energy is transferred to thermal stores.
7.What two properties of a wall reduce the rate of cooling of a building?
Thicker walls, and walls made of material with a lower thermal conductivity.
8.What is thermal conductivity?
A measure of how readily a material transfers energy by heating. A high value means energy passes through quickly.
9.Why does cavity wall insulation work?
The trapped air and insulating material have a low thermal conductivity, so energy transfers through the wall more slowly.
10.State the equation for efficiency using energy.
efficiency = useful output energy transfer divided by total input energy transfer
11.State the equation for efficiency using power.
efficiency = useful power output divided by total power input
12.How is efficiency written as a percentage?
Multiply the decimal value by 100.
13.A device takes in 200 J and usefully transfers 150 J. Calculate the efficiency.
150 divided by 200 = 0.75, so 75 per cent.
14.A motor has a power input of 500 W and useful output of 400 W. Calculate the efficiency.
400 divided by 500 = 0.8, so 80 per cent.
15.Can any device be 100 per cent efficient?
No real device is, because some energy is always dissipated, usually to thermal stores.
16.If a device is 60 per cent efficient and takes in 1000 J, how much is wasted?
Useful output is 600 J, so 400 J is wasted.
17.Give three ways to increase the efficiency of a machine.
Lubricate moving parts, insulate to reduce energy transfer by heating, and streamline to reduce air resistance.
18.Why do modern light bulbs waste less energy than filament bulbs?
Filament bulbs transfer a large proportion of energy to thermal stores, whereas LEDs transfer far more of it to light.
19.What is a Sankey diagram?
A diagram where the width of each arrow represents the amount of energy transferred to each store.
20.What does the width of the input arrow show?
The total energy supplied to the device.
21.How can you find efficiency from a Sankey diagram?
Divide the width of the useful output arrow by the width of the input arrow.
22.Why is thermal energy usually the wasted store?
Friction and electrical resistance transfer energy to thermal stores, and it then spreads into the surroundings.
23.Why does insulation not stop energy transfer completely?
It only reduces the rate of transfer. Given enough time, energy still moves from hotter to cooler.
24.What happens to the useful energy output if efficiency stays the same but input doubles?
It doubles as well, since the proportion transferred usefully is unchanged.
25.A machine is 25 per cent efficient. What does that mean in plain terms?
For every 100 joules put in, only 25 joules do the intended job and 75 joules are wasted.
26.Why does a device get warm when it is running?
Some of the energy supplied is dissipated to the thermal store of the device rather than being transferred usefully.
You remember far more by trying to recall an answer than by reading it. Run these as flashcards and the ones you keep getting wrong will come back more often.
Start studyingOr see the other Energy decks.