Units (Electricity)
4 marks · EasyA brand-new electric iron is labelled '230 V, 1200 W'. It has a metal case and is connected to the mains by a three-core cable.
State the purpose of the earth wire in this appliance.
If the iron is switched on for 3 minutes, calculate the total energy transferred to 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) To provide a low resistance path for current to flow safely to earth if the metal case becomes live, protecting the user from electric shock. · must show mention of protection, low resistance path
(b) 216,000 J
Common mistakes: Mixing up live, neutral, and earth roles; Mistaking kW for W
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Marking points
B1 (a) · 1 mark · States earth wire provides a low resistance path to earth in a fault.
B2 (a) · 1 mark · Explains how this protects user from electric shock if the case becomes live.
M1 (b) · 1 mark · Uses E = P × t with correct unit conversion.
A1 (b) · 1 mark · Correct answer 216,000 J (accept 216 kJ if unit given correctly).
Worked solution
a) The earth wire gives any current from a fault a path to earth. This prevents the metal case from becoming live, reducing risk of electric shock.
b) E = P × t = 1200 × (3 × 60) = 1200 × 180 = 216,000 J