An electric motor takes in $2000$ W and delivers a useful output of $1500$ W. What is its efficiency?
A$75\%$CORRECT
B$25\%$
C$50\%$
D$133\%$
JUPEBPhysicsWork, Energy and Power
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Why the answer is A, and why the others tempt you.
Efficiency $= \dfrac{\text{useful output}}{\text{input}} \times 100\% = \dfrac{1500}{2000} \times 100\% = 75\%$. Inverting the ratio gives 133%, which is impossible, and 25% is the percentage of energy wasted.
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