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Q.
Even Carnot engine cannot give $100\%$ efficiency because we cannot:
AIEEEAIEEE 2002Thermodynamics
Solution:
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.