1.5 Conservation and dissipation of energy
9 exam questions worth 17 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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State the law of conservation of energy.
Mark scheme
energy cannot be created or destroyed, only transferred from one store to another / total energy is constant
Model answer
Energy cannot be created or destroyed. It can only be transferred from one store to another.
A phone charger transfers 10 J of electrical energy. 8 J is stored in the phone battery. Calculate the energy wasted.
Mark scheme
2 J
Model answer
10 − 8 = 2 J.
Explain what is meant by the energy being 'dissipated'.
Mark scheme
energy is spread out / transferred to the surroundings (1); as thermal energy / so it is not useful / hard to get back (1)
Model answer
The energy is transferred to the surroundings and spread out, mainly as thermal energy, so it cannot easily be used again.
Explain how putting oil on the moving parts of a machine reduces wasted energy.
Mark scheme
reduces friction (1); less energy transferred to the thermal store / less heating (1)
Model answer
Oil reduces friction between the moving parts, so less energy is transferred to the thermal store of the machine and the surroundings.
A motor takes in 200 J of electrical energy. It produces 150 J of useful kinetic energy and 10 J of sound. Calculate how much energy is dissipated as thermal energy.
Mark scheme
200 − 150 − 10 (1); 40 J (1)
Model answer
Total in = useful out + wasted, so thermal = 200 − 150 − 10 = 40 J.
Which of the following reduces the energy wasted by friction in a bicycle?
Choose one answer.
Correct Not quite: the answer is Oiling the chain
Mark scheme
B: oiling the chain
Model answer
Oiling the chain (B), because lubrication reduces friction.
A television takes in 400 J of electrical energy. It transfers 120 J as light and 30 J as sound. The rest is transferred to the thermal store. Calculate how much energy is transferred to the thermal store.
Mark scheme
400 − 120 − 30 (1); 250 J (1)
Model answer
Energy to the thermal store = 400 − 120 − 30 = 250 J.
A student says that he has built a machine that will keep running for ever without any energy being supplied. Use ideas about energy to explain why this cannot be true.
Mark scheme
every machine transfers some energy to the thermal store of the surroundings / energy is dissipated (1); this energy cannot be recovered / the machine has no new energy supply (1); so it will eventually stop (energy cannot be created) (1)
Model answer
Energy is always dissipated to the surroundings by friction, so the machine would lose energy all the time. With no new supply of energy, and because energy cannot be created, the machine would eventually stop.
A basketball is dropped from a height of 1.5 m and bounces lower each time. Explain, in terms of energy, why the bounces get lower.
Mark scheme
at each bounce some kinetic energy is transferred to the thermal store of the ball and floor / to sound (1); so less energy is transferred to the gravitational potential store (1); the energy is dissipated, not destroyed / total energy is conserved (1)
Model answer
At each bounce some of the kinetic energy is transferred to the thermal stores of the ball and floor, and to sound. Less energy is left to be transferred to the gravitational potential store, so the ball does not rise as high. The energy has been dissipated, not destroyed.
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