THE TRANSFORMER
4 marks · NormalA transformer is used to power a device that needs 6 V a.c. from the mains (230 V a.c.). The current in the primary coil is 0.10 A. Assume the transformer is 100% efficient.
Calculate the current in the secondary coil.
Explain why the secondary current is greater than the primary current.
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) 3.8 A · must show power in = power out
(b) Because the voltage decreases, the current must increase (power is conserved if the transformer is 100% efficient). · must show decrease in voltage, increase in current, conservation of power
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · States power conservation equation: .
M2 (a) · 1 mark · Correctly rearranges for or .
A1 (a) · 1 mark · Substitutes values to give (accept 3.8 or 3.83). (follow-through)
B1 (b) · 1 mark · States that as voltage decreases in a 100% efficient transformer, current increases to keep power the same (conservation of energy).
Worked solution
a) Assume 100% efficiency:
b) For the same power, if voltage decreases, current must increase so .