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GCSE Standard Questions: The Motor Effect

Q7. Motors change electrical energy into kinetic energy.

Look at the information about five different electric motors in the table below.

Electric motor
Energy input per hour (J)
Useful energy output per hour (J)
Energy 'wasted' per hour (J)
A
72000
60000
12000
B
54000
36000
18000
C
18000
15000
3000
D
60000
48000
12000
E
54000
48000
6000

 

(a)

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

Energy input = Energy Output + 'Wasted' energy

Energy input = 48000 + 12000

Energy input = 60000 J

[2 marks]

(ii) Which electric motor has the lowest 'wasted' energy in one hour?

Fill in the table blank values on the exam paper to help you with this.

Motor C

[1 mark]

(iii) Which electric motor has the highest 'wasted' energy in one hour?

Motor B

[1 mark]

(iv) Describe how energy is 'wasted' in an electric motor.

Friction between moving parts causes unwanted heat and sound energy to be produced, and KE of particles to be passed to other particles in the surroundings.

[1 mark]

(v) Suggest how this 'wasted' energy can be reduced in an electric motor.

By reducing friction in the moving parts by using lubrication/oil.

[1 mark]

(b) Calculate the % efficiency of electric motor E.

Give your answer to two significant figures.

Efficiency = Useful output energy transfer / Input energy transfer

E = 48000/54000

E = 0.8888

E (%) = 0.8888 x 100 = 88.88

E (%) = 89%

[4 marks]

(Total 10 marks)