Question

Which statement is true for second law of thermodynamics ?

• A. Heat can flow spontaneously from a cold object to a hot object.
• B. You can not create a heat engine which extracts heat and converts it all to useful work.
• C. According to Plank-Kelvin's law under some circumstances a perfect heat engine is possible.
• D. None of these.

Answer 1

The correct option is B You can not create a heat engine which extracts heat and converts it all to useful work.

The second law of thermodynamics states , that it is impossible to construct a heat engine , operating in cycle , which extracts heat and can convert it all to useful work .In other words , it is impossible for a heat engine to convert heat completely in work .

The efficiency of a heat engine is ,

$\eta =1-\frac{Q_2}{Q_1}$

if $Q_2=0$ i.e. heat is not given to sink , it means total heat absorbed is converted into work , then

$\eta =1$ , which is practically not possible .