A2 Daily A Level Physics question
A bench transformer has 1200 turns on the primary and 60 on the secondary. It is connected to 240 V rms and powers a 12 V halogen lamp drawing 2.0 A rms at its rated brightness. The transformer is 80% efficient under this load. Which value must the rms current in the primary coil have?
Answer
The correct answer is C.
Correct: C — 0.125 A. Output power is 12 V × 2.0 A = 24 W, so input power must be 24 W / 0.80 = 30 W; hence primary current = 30 W / 240 V = 0.125 A. A is the ideal-lossless case (24 W/240 V = 0.10 A), ignoring that input power must exceed output power when efficiency is 80%. B adds 20% to the ideal 0.10 A (giving 0.12 A) instead of dividing by 0.80; 1/0.80 = 1.25, not 1.20. C uses the correct reasoning: input power is higher than output by a factor of 1/0.80, then divides by the primary voltage. D confuses primary and secondary currents; in a step-down transformer the secondary current is larger, not equal to the primary.