question_answer

Even Carnot engine cannot give 100% efficiency because we cannot             [AIEEE 2002]

A)  prevent radiation           

B)  find ideal sources

C)  reach absolute zero temperature

D)  eliminate friction

Correct Answer: C

Solution :

[c] The efficiency of Carnot engine is

where,  is the temperature of the source and   that of sink.

Since,

So,

To obtain 100% efficiency (i.e., ),  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.