Question

Consider a cycle followed by an engine (Figure)

1 to 2 is isothermal
2 to 3 is adiabatic
3 to 1 is adiabatic

Such a process does not exist because ______.

1. heat is completely converted to mechanical energy in such a process, which is not possible.
2. mechanical energy is completely converted to heat in this process, which is not possible.
3. curves representing two adiabatic processes don’t intersect.
4. curves representing an adiabatic process and an isothermal process don’t intersect.

Answer 1

a and c

Explanation:

a. The given process is a cyclic process, i.e. it returns to the original state 1. And the change in internal energy in a cyclic process is always zero as for the cyclic process U_f = U_i. So, ∆U = U_f – U_i = 0

Hence, total heat is completely converted to mechanical energy. Such a process is not possible by the second law of thermodynamics.

c. Here, two curves are intersecting, when the gas expands adiabatically from 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. So, we conclude that such a process does not exist because curves representing two adiabatic processes do not intersect.