A transformer connected to a 240 V mains supply draws an input current of 0.50 A. If the secondary coil delivers a current of 4.0 A at 24 V to a resistive load, what is the electrical efficiency of the transformer?
Answer and explanation
Correct answer: B. 80%
- Step 1: Primary input power: P_in = V_p × I_p = 240 × 0.50 = 120 W. Step 2. Secondary output power: P_out = V_s × I_s = 24 × 4.0 = 96 W. Step 3. Efficiency: η = (P_out / P_in) × 100% = (96 / 120) × 100% = 80%. Answer: 80%. Why not the others: 75% uses P_out = 90 W; 85% is an arithmetic estimate; 90% assumes P_out = 108 W.
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