Units (Electricity)
5 marks · NormalA heater is rated at 230 V, 2.3 kW. During a cold evening, it is left switched on for 15 minutes.
Calculate the resistance of the heater.
Calculate the energy transferred by the heater in 15 minutes.
State the correct SI unit for 'energy transferred'. Explain why the heater becomes hot when current flows through it.
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) 23 Ω
(b) 2,070,000 J
(c) Energy is measured in joules (J). The heater gets hot because current passes through a high-resistance wire, which causes electrical energy to be transferred to heat energy.
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Uses P = V^2 / R or rearrangement of V=IR and P=VI. (follow-through)
A1 (a) · 1 mark · Correct substitution to get 23 Ω (accept answers within tolerance).
M2 (b) · 1 mark · Substitutes P and t into E = Pt (t in seconds). (follow-through)
A2 (b) · 1 mark · Correct answer: 2,070,000 J (tolerance allowed). (follow-through)
B1 (c) · 1 mark · States 'joule' as SI unit for energy. Explains the heating effect correctly.
Worked solution
a) Resistance, R = V^2 / P = (230^2) / 2300 = 52900 / 2300 = 23 Ω
b) Energy transferred: E = P x t = 2,300 W x (15 x 60) s = 2,300 x 900 = 2,070,000 J
c) The unit of energy is joule (J). The heater wire's resistance causes energy to be transferred to heat as current flows, making it hot.