Question

Even Carnot engine cannot give $100\%$ efficiency because we cannot:

• A. prevent radiation
• B. find ideal sources
• C. reach absolute zero temperature
• D. eliminate friction

Answer 1

Answer: (C)

The efficiency of Carnot engine is,
$\eta =1-\frac{T_2}{T_1}$
where, $T_1$ is the temperature of the source and $T_2$ that of sink.
Since,$\frac{T_2}{T_1}=\frac{Q_2}{Q_1}$
So,$\eta =1-\frac{Q_2}{Q_1}$
To obtain $100\%$ efficiency (i.e., $\eta =1$ ), $Q_2$ must be zero that is, if a sink at absolute zero would be available, all the heat taken from the source would have been converted into work. The temperature of sink means a negative temperature on the absolute scale at which the efficiency of engine is greater than unity. This would be a violation of the 2 nd law of thermodynamics. Hence, a negative temperature on the absolute scale is impossible. Hence, we cannot reach absolute zero temperature.