ENERGY IN TRANSFORMERS
3 marks · NormalA transformer used in a mobile phone charger has an input voltage of 220 V and input current of 0.05 A. If the transformer is 100% efficient and the output power required is 8.8 W, what is the output current?
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0.8 A (±0.01)
Common mistakes: Dividing voltage by current instead of power by voltage
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Marking points
M1 · 1 mark · Recognises use of output power to find output current.
M2 · 1 mark · Correct substitution: I_s = output power / Vs (Is = 8.8 / 11 if Vs is stated or found). (follow-through)
A1 · 1 mark · Evaluates Is = 0.8 A. (follow-through)
Worked solution
For a 100% efficient transformer, output power is same as input power. Output power = Vs × Is, so Is = Output Power / Vs.
If Vs is not given, assume Vs = input power / output current. But with 220 V × 0.05 A = 11 W input power, which is larger than output power required (8.8 W). Since transformer output power is given (8.8 W):