Question

What is the change in entropy in an adiabatic process? Does this imply that adiabatic processes are reversible? Can a process be precisely adiabatic for a macroscopic system?

Short answer

The change in entropy in an adiabatic system is zero, which implies it is a reversible process. But the change in entropy can be increased in some cases, and it is irreversible. A process can’t be precisely adiabatic for a microsystem.

Step-by-step solution

Find how much the entropy changes in an adiabatic process.

\(\)The adiabatic process is a process in which no heat is transferred between the system and surroundings. The entropy of a process is the ratio of the amount of heat transfer to the absolute temperature at which the process takes place. It is expressed as follows;

\(\)\(\Delta S = \frac{Q}{T}\)

Here,\(\Delta S\)is the change in entropy,\(Q\)is the amount of heat transferred during the process and\(T\)is the absolute temperature. In an adiabatic process,

\[\begin{array}{c}Q = 0\\\Delta S = 0\end{array}\]

Therefore, the change in entropy in an adiabatic process is zero. That means entropy remains constant.

Describe whether this process is reversible or not.

In a reversible process, the change in entropy is zero because the state of the system remains unchanged. Since thechange in entropy is zero here, you can say that this process is reversible. This one is an ideal case in which no heat transfer occurs. But you can’t generalise that all adiabatic processes are reversible because some amount of heat transfer may occur and change in entropy will increase. The increase in entropy change is associated with an irreversible process. So, the adiabatic process can be reversible or irreversible.

A process can’t be precisely adiabatic for a macroscopic system. Because some kind of heat transfer may take place between the system and the environment.

So, change in entropy is zero in an adiabatic reversible process, and change in entropy increases in adiabatic irreversible process.