AS Daily A Level Physics question
In a lab test, a 6.0 V DC supply powers a small winch to raise a 1.2 kg mass vertically. Over 5.0 s the mass rises 2.0 m at approximately constant speed. The current from the supply is 1.8 A for the first 3.0 s and 1.2 A for the next 2.0 s. Using this information, which estimate for the overall efficiency of the lift is most reasonable?
Answer
The correct answer is A.
Correct: A — About 50% efficient. A: Output energy ≈ mgh = 1.2×9.8×2.0 ≈ 24 J; input electrical energy ≈ V×(area under I–t) = 6.0×(1.8×3.0 + 1.2×2.0) = 6.0×7.8 ≈ 47 J, so efficiency ≈ 24/47 ≈ 0.5 (≈50%). B: About 44% arises from incorrectly taking 1.8 A for all 5.0 s so Ein ≈ 6×1.8×5 = 54 J giving 24/54 ≈ 44%. C: About 67% comes from wrongly assuming 1.2 A for all 5.0 s so Ein ≈ 6×1.2×5 = 36 J giving 24/36 ≈ 67%. D: Close to 100% ignores losses and contradicts the data since measured electrical energy (≈47 J) clearly exceeds the gain in gravitational potential energy (≈24 J).