AS Daily A Level Physics question
A small DC motor lifts a 0.60 kg load vertically at constant speed, raising it 1.5 m in 5.0 s. During the lift an ammeter in series with the battery reads 1.6 A and a voltmeter across the battery terminals reads 3.0 V. Which statement must be true?
Answer
The correct answer is A.
Correct: A — The efficiency from the battery’s electrical output at its terminals to the mechanical output on the load is about 37%. Mechanical output per second is (m g h)/t = (0.60 × 9.8 × 1.5)/5.0 ≈ 1.76 W, electrical input at the battery terminals is VI = 3.0 × 1.6 = 4.8 W, so efficiency ≈ 1.76/4.8 ≈ 0.37 (≈37%). B is wrong because it treats electrical energy as V × t (ignoring current), giving 8.82/15 ≈ 59%, which omits the factor of I. C is wrong because it uses V/I as the electrical power (confusing resistance with power), leading to ≈94%. D is wrong because it compares mechanical energy mgh directly with electrical power VI (forgetting to divide by time), giving an impossible >100% value.