Explain why a Carnot engine cannot have 100% efficiency?

Options

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

Answer

C

Explanation

The efficiency of Carnot engine is `eta=1-(T_2"/"1T_1)`
Where,`T_1` is the temperature of the source and `T_2` that of sink.
Since, ` (T_2)/(T_1)=(Q_2)/(Q_1)`
So, `et=1 -(Q_2)/(Q_1)`
To obtain 100% efficiency (i.e.,`eta=1),Q_2` must be zero i.e., 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 2nd law of thermodynamics. Hence, a negative temperature on the absolute scale is impossible. Hence, we cannot reach absolute zero temperature.