Ncert Exercises & Solutions

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:

(a)	heat is completely converted to mechanical energy in such a process, which is not possible.
(b)	In this process, mechanical energy is completely converted to heat, 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.

Choose the correct alternatives:

1.	(a), (b)	2.	(a), (c)
3.	(b), (c)	4.	(c), (d)

(2)

Hint: Apply the concepts of the first and second laws of thermodynamics.

Step 1: Find the change in internal energy.

The given process is a cyclic process i.e. it returns to the original state 1. Hence, the change in internal energy, dU = 0.

Step 2: Find the work done by the gas.

\Rightarrow dQ = dU + dW = 0 + dW = dW

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

Step 3: Find if all the processes are possible.

When the gas expands adiabatically from 2 to 3, it is not possible to return to the same state without heat being supplied, hence, the process 3 to 1 cannot be adiabatic.

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