Power & Efficiency (OCR Gateway GCSE Physics)

Exam Questions

49 mins13 questions
1a2 marks

A student investigates how the thickness of insulation affects the cooling of a cup of tea.

Fig. 16.1 is a diagram of her apparatus.

fig16-1-paper2-june2098-ocr-gcse-physics-1

Fig. 16.1

The student wraps a layer of insulation around a cup containing 200 cm3 of hot tea.

She measures the temperature of the tea at the start of the experiment and after 10 minutes.

She repeats the experiment with different thicknesses of the insulation.

Table 16.1 shows her results.

Thickness of the
insulation (mm)
Temperature of tea (°C)
Start End Difference
2 90 65 25
4 88 66 22
6 91 72 19
8 89 73 16
10 98 84 14
12 100 60  

  
Table 16.1

a)
i)
Calculate the temperature difference when the thickness of insulation is 12 mm.

Temperature difference = ................................°C [1]

ii)
The result when the thickness of the insulation is 12 mm is anomalous.

Suggest a reason why this result appears to be anomalous.

[1]

1b2 marks

Plot a graph of the results in Table 16.1 and draw a line of best fit.

Ignore the anomalous result for 12 mm.

q16b-paper2-june2098-ocr-gcse-physics

[2]

1c1 mark
c)
Describe how the temperature difference is affected as the thickness of the insulation increases.
[1]
1d1 mark
d)
Suggest how the thickness of the insulation affects the rate of cooling of the tea.
[1]
1e2 marks
e)
This experiment could be improved.

Describe two different ways of improving the experiment.

1 ....................................................................................................................

2 ....................................................................................................................

[2]

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1a6 marks

Look at the information about different electric motors.

Electric
motor
Energy input
per hour (J)
Useful energy
output per hour (J)
Energy ‘wasted’
per hour (J)
A 72000 60000  
B 54000 36000  
C 18000   3000
D   48000 12 000
E 54000 48000  

i)
Calculate the energy input per hour in J for electric motor D.

Answer = ......................... J [2]

ii)
Which electric motor has the lowest ‘wasted’ energy in one hour?

[1]

iii)
Which electric motor has the highest ‘wasted’ energy in one hour?

[1]

iv)
Describe how energy is ‘wasted’ in an electric motor.

[1]

v)
Suggest how this ‘wasted’ energy can be reduced in an electric motor.

[1]

1b3 marks

Calculate the % efficiency of electric motor E.

Use the equation: Efficiency = Useful output energy transfer / Input energy transfer

Give your answer to 2 significant figures.

Answer = .........................% 

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2a3 marks

A TV has a power rating of 0.2 kW.

Calculate the energy transferred, in kWh, if the TV is switched on for 4 hours.


Energy transferred = ................................ kWh

2b3 marks

A different TV works with a 12.0 V battery. It has a current of 3.19 A.

Calculate the power rating of the TV.


Power = ................................ W 

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3a2 marks

A car on a roller coaster is stationary at the top of a slope.

The car has a weight of 6 500 N and a potential energy of 217 000 J.

Calculate the car’s height above the ground.

Answer = ......................... m 
3b2 marks

Higher Only

 

The diagram shows the roller coaster car moving down a slope.
q23b-paper2-specimen-ocr-gcse-physics

The energy at the bottom of the slope is lower than expected.

Suggest two ways to improve the efficiency of the roller coaster car.

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1a6 marks
a)
This question is about an electric kettle.

i)
An electric kettle is filled with water, connected to the mains and switched on.

The electricity for the kettle is generated in a coal-fired power station.

Describe the energy transfer that occurs when the kettle is switched on.

Include ideas about energy stores in your answer.

 [2]

ii)
The mains supply has a potential difference of 230 V.

The kettle has a current of 5.0 A. The kettle is switched on for 2.0 minutes.

Calculate the total energy transferred to the kettle in 2.0 minutes.

Total energy transferred = ........................................................ J [4]

1b6 marks
b)


i)
Table 21.1 gives some information about a different kettle.
Energy transferred to the kettle 525 000 J
Mass of water 1.2 kg
Starting temperature of water 25 °C
Final temperature of water 100 °C
Specific heat capacity of water 4200 J/ kg °C

Table 21.1

Calculate the efficiency of the kettle described in Table 21.1.

Give your answer as a percentage.

Use an equation from the data sheet.

Efficiency = ...................................................... % [5]

ii)
Explain why the efficiency of the kettle is less than 100%.


[1]

1c2 marks

Higher Only

 

c)
The water can also be heated using a 12 V heater.

A transformer is used to change a 120 V supply into 12 V.
The current in the secondary coil is 9.0 A.

Calculate the current in the primary coil.

Use an equation from the data sheet.

Current = ....................................................... A [2]

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