Units (Energy)

3 marks · Normal

A lamp uses 120 joules of energy in 60 seconds.

(a)

State the formula for power and the correct SI unit.

[1 mark]

(b)

Calculate the power output of the lamp.

[2 marks]

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.

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(a) Power = energy / time ; unit: watt (W)

(b) 2 W

Also accept: 2 W; 2.0 W

Common mistakes: Dividing time by energy

Show mark scheme and worked solution

Marking points

B1 (a) · 1 mark · States correct formula AND unit

M1 (b) · 1 mark · Correctly substitutes values into power = energy / time

A1 (b) · 1 mark · Correct answer (2 W)

Worked solution

a) Power = energy / time, unit is watt (W).

b) Power = 120 J / 60 s = 2 W.