First slide

Heat engine

Question

The second law of thermodynamics implies

Easy

A

whole of heat can be converted into mechanical energy
B

no heat engine can be 100% efficient
C

every heat engine has an efficiency of 100%
D

a refrigerator can reduce the temperature to absolute zero

Solution

Efficiency of an ideal heat engine $η = (1 - \frac{T_{2}}{T_{1}})$ will be 100%, if T₂= 0 which is not possible to achieve.
Alternatively $η = \frac{W}{Q_{A}} = \frac{Q_{A} - Q_{R}}{Q_{A}}$
According to second Law of Thermodynamics, heat rejected (Q_R) cannot be zero.
Hence $η$ cannot be 100%.

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