Question

Consider a cycle followed by an engine see (Figure).
$1$ to $2$ is isothermal
$2$ to $3$ is adiabatic
$3$ to $1$ is adiabatic.
Such a process does not exist because

• A. Heat is completely converted to mechanical energy in such a process , which is not possible
• B. Mechanical energy is completely converted to heat in this process, which is not possible
• C. Curves representing two adiabatic processes don't intersect
• D. Curves representing an adiabatic process and an isothermal process don't intersect

Answer 1

Answer: (A), (C)

Curves representing two adiabatic processes don't intersect
Heat is completely converted to mechanical energy in such a process , which is not possible

(a) The given process in a cyclic process i.e., it returns to the orignal state

1. Hence, change in internal energy dU = 0 .

⇒ $dQ=dU+dW=0+dW=dW$

Hence, total heat supplied is converted to work done by the gas (mechanical energy) which is not possible by second law of thermodynamics.

(c ) When the gas expands adiabatically form 2 to 3. It is not possible to return to the same state without being heat supplied, hence the process 3 to 1 cannot be adiabatic.