Conservation of EnergyEdexcel GCSE Physics: Topic test

20 questions, 54 marks

Edexcel GCSE Physics

Conservation of Energy topic test

Total 54 marks

Name

Class

Date

1
A hydroelectric power station releases water that falls through a height of 50 m before reaching turbines at the bottom of the dam. For each 1000 kg of water that falls, 500 000 J of gravitational potential energy is lost, and the turbines and generator convert 490 000 J of this into electrical energy (g = 10 N/kg).
(a)
As the water falls through the dam, which statement correctly describes the energy transfer taking place?
[1 mark]
AThe energy in the gravitational potential energy store decreases, and the energy in the kinetic energy store increases
BThe energy in the kinetic energy store decreases, and the energy in the gravitational potential energy store increases
CBoth the gravitational potential energy store and the kinetic energy store increase together
DEnergy is created as the water falls, increasing the total energy of the system
(b)
What is the efficiency of the turbines and generator at converting the gravitational potential energy lost into electrical energy?
[1 mark]
A102%
B98%
C49%
D2%
(c)
State where the energy that is not converted into electrical energy (the difference between 500 000 J and 490 000 J) is most likely transferred to, and give one reason why hydroelectric power is still considered a good source of renewable energy despite this small loss.
[2 marks]
Total for question 1: 4 marks
2
A theme park 'drop tower' ride carries a capsule of mass 400 kg (including passengers) to the top of a 45 m tower, then releases it to fall freely under gravity. A magnetic brake begins slowing the capsule for the final 5 m before it reaches the ground (g = 10 N/kg).
(a)
How much energy is transferred from the gravitational potential energy store to the kinetic energy store during the first 40 m of free fall, before the brake engages (ignore air resistance)?
[1 mark]
A16 000 J
B180 000 J
C160 000 J
D4000 J
(b)
Assuming all the gravitational potential energy lost in the first 40 m is transferred to kinetic energy, what is the capsule's speed just before the magnetic brake engages?
[1 mark]
A800 m/s
B20 m/s
C56 m/s
D28 m/s
(c)
Explain what happens to the capsule's kinetic energy store during the final 5 m as the magnetic brake slows it down, and state where this energy is transferred to.
[2 marks]
Total for question 2: 4 marks
3
A pogo stick's spring is compressed, storing 45 J of elastic potential energy. When released, the spring pushes a child and pogo stick (combined mass 30 kg) vertically upwards. Assume, for part (a) only, that all of the elastic potential energy is transferred to the gravitational potential energy store at the highest point of the bounce (g = 10 N/kg).
(a)
Assuming all 45 J of elastic potential energy is transferred to the gravitational potential energy store at the highest point of the bounce, calculate the maximum height gained by the child and pogo stick.
[3 marks]
(b)
In reality, only 36 J of the spring's 45 J of stored elastic potential energy is actually transferred to the gravitational potential energy store at the highest point, the rest being dissipated. Calculate the real maximum height gained, and explain why this is lower than your answer to part (a).
[4 marks]
Total for question 3: 7 marks
4
A pumped-storage hydroelectric plant pumps 2 000 000 kg of water up into a reservoir 90 m above a lower lake during periods of low electricity demand, using 2 100 000 000 J of electrical energy to do so. Later, during periods of high demand, the same 2 000 000 kg of water flows back down through turbines, generating 1 500 000 000 J of electrical energy (g = 10 N/kg).
(a)
Calculate the gravitational potential energy gained by the water when it is pumped up to the reservoir, and hence calculate the efficiency of the pumping process as a percentage. Explain what happens to the energy that is not usefully transferred to the water's GPE store.
[6 marks]
(b)
When the water later flows back down and generates 1 500 000 000 J of electrical energy from the 1.8 × 10⁹ J of gravitational potential energy released, calculate the efficiency of this generating stage, and hence calculate the overall efficiency of the whole pump-then-generate cycle (electrical energy out ÷ electrical energy originally put in). Explain why this overall efficiency is lower than either individual stage's efficiency.
[6 marks]
Total for question 4: 12 marks
5
A university campus energy manager is comparing two heating systems for a new building: a gas boiler that is 92% efficient, and a biomass boiler (burning wood pellets) that is 78% efficient. Each boiler is supplied with 500 000 J of chemical energy from its fuel per hour.
(a)
How much useful heat energy does the gas boiler actually deliver per hour?
[1 mark]
A460 000 J
B500 000 J
C40 000 J
D920 000 J
(b)
How much useful heat energy does the biomass boiler actually deliver per hour?
[1 mark]
A500 000 J
B390 000 J
C110 000 J
D780 000 J
(c)
State which boiler wastes more energy per hour in absolute terms (not just by comparing efficiency percentages), showing a calculation for each, and suggest one reason, other than efficiency, why the campus might still choose the biomass boiler.
[2 marks]
Total for question 5: 4 marks
6
A fairground 'pirate ship' ride swings like a giant pendulum. A rider of mass 60 kg sits at the lowest point of the swing, moving at 8.0 m/s. The ship then swings upward through an arc, rising through a vertical height before momentarily coming to rest at its highest point (g = 10 N/kg).
(a)
What is the kinetic energy of the rider at the lowest point of the swing, when moving at 8.0 m/s?
[1 mark]
A480 J
B3840 J
C1920 J
D240 J
(b)
Assuming all of this kinetic energy is transferred to the gravitational potential energy store, what maximum height above the lowest point would the ship rise to?
[1 mark]
A0.32 m
B32 m
C6.4 m
D3.2 m
(c)
In reality the ship rises to a height of only 2.8 m, less than your answer to part (b). Explain, in terms of energy transfers, why the real height reached is lower, and state one place the 'missing' energy is transferred to.
[2 marks]
Total for question 6: 4 marks
7
An electric car's home charger draws 7.5 kWh of electrical energy from the mains to fully charge the car's battery. Of this, only 6.9 kWh actually ends up stored usefully in the battery, the rest being dissipated as heat in the charger and cables.
(a)
Calculate the efficiency of the charging process as a percentage.
[3 marks]
(b)
The car's owner charges the car every day for a year (365 days). Calculate the total electrical energy wasted (not stored in the battery) over the year, in kWh, and suggest one practical way the owner could reduce this wasted energy.
[4 marks]
Total for question 7: 7 marks
8
A wind farm generates electrical energy from the kinetic energy store of moving air. On a windy day, the turbines' blades are struck by air with a combined kinetic energy of 8 000 000 J each second, and the turbines convert 2 800 000 J of this into electrical energy each second. Some of this electrical energy is used to charge a large battery storage system so that energy is available when the wind drops; the battery stores energy with an efficiency of 90%.
(a)
Calculate the efficiency of the wind turbines at converting the kinetic energy of the air into electrical energy. Explain, in terms of energy stores, why a wind turbine can never be 100% efficient at this conversion, even in principle.
[6 marks]
(b)
If 1 000 000 J of the 2 800 000 J generated each second is directed into the battery storage system, calculate how much energy is usefully stored in the battery each second. Explain why storing surplus wind energy in batteries, despite the storage system not being 100% efficient, is still considered a useful approach for managing electricity supply from a wind farm.
[6 marks]
Total for question 8: 12 marks

End of questions