Energy Transfers

3 marks · Normal

A laptop charger takes in 120 kJ of electrical energy from the mains and transfers 21 kJ of that to heat, which is not useful. Calculate the efficiency of the charger, giving your answer as a percentage.

Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.

Show answer

82.5% (±0.5)

Also accept: 82.5 %; 0.825

Common mistakes: Using wasted energy in numerator; Incorrectly adding wasted and useful outputs

Show mark scheme and worked solution

Marking points

M1 · 1 mark · Finds useful energy output (120 – 21 = 99 kJ). (follow-through)

M1 · 1 mark · Substitutes into efficiency formula: (useful output/total input) x 100%. (follow-through)

A1 · 1 mark · Correct final answer 82.5%. (follow-through)

Worked solution

Useful energy output = 120 kJ – 21 kJ = 99 kJ