Question

Which of the following processes net heat is absorbed by the gas? ($P$= pressure,$V$= volume, $T$= temperature,$\rho$= density)
(A)

(B)

(C)

(D)

Answer 1

The graph shows systems in different conditions with different parameters. The heat absorbed by the system means work is being done on the system. The net heat absorbed by the system is the difference between work done on the system and work done by the system and is given by the area enclosed in a curve.

Complete answer:
A thermodynamic process is a process in which a system goes from initial states to final states after some work is done. A thermodynamic process is state dependent and not path dependent.
In fig (A).

$\text{Work done }=\text{ heat absorbed by the system}$
As the volume is increasing, the work done is positive. Therefore, energy is absorbed by the system. So we can say that here, the system absorbs energy in the form of heat.
In fig (C).

According to the graph the system is in adiabatic conditions, this means that no heat or matter is exchanged with the surroundings. When system goes from A to B, work is being done on the system, when system goes from C to A work is being done by the system
$\text{The net heat absorbed}=\text{ work done on the system}-\text{ work done by the system}.$
The heat absorbed by the system is equal to the area of the closed figure.
So we can say that net heat is absorbed by the system.
In fig. (D).

According to the graph the system is in adiabatic conditions. As density increases, volume decreases. So the heat or energy absorbed by the system from A to B is used to increase the temperature and pressure. As the system goes from C to A, work is being done by it.
$\text{Net heat absorbed}=\text{ area of the closed figure}.$
Therefore we can say that net heat is absorbed by the system.

The net heat is absorbed by the systems in graph (A), (C), and (D).

Note:
In isothermal conditions when there is no change in volume the work done is zero. The heat absorbed can either be used to increase the temperature or volume. When volume is constant, temperature as well as pressure increases. When temperature is constant, volume increases and pressure decreases.