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Physics Topics cover a broad range of concepts that are essential to understanding the natural world.
What are the Conditions for Irreversible Process?
Let a thermodynamic system be in an initial state A. Owing to heat exchange and external work, the system attains its final state B. That is, the process is A → B. Now, we shall state the conditions under which the process may be called ‘reversible’.
A process A → B is reversible, if
- the process B → A occurs in nature and
- after the processes A → B → A, there is no net change in the surroundings.
The conditions
- and
- are called the conditions of reversibility. The process A → B is an irreversible process, if these conditions are not satisfied.
The concepts of reversibility and irreversibility are direct consequences of the second law of thermodynamics. The words self-acting machine’ in Clausius or Kelvin-Planck statements are closely related to the words no net change in the surroundings’ in the reversibility conditions. For example, we consider the process of heat flow from higher to lower temperature. The reverse process does occur in nature – a domestic refrigerator actually transfers heat from its cool container to comparatively hotter surroundings.
But it is not self-acting, because some work in the form of electrical energy must be supplied from the surroundings. As a result, the environment suffers a net change. So we may say that heat transfer from higher to Lower temperature is an irreversible process. This is because heat, on its own, cannot flow from lower to higher temperature (Clausius statement).
We now consider a process A → B on a pV diagram [Fig.].
Let the process occur along a particular path ACB. The opposite process B → A can take different paths from B to A. However, for reversibility, only the path BCA is important. Then for each elementary step (say, xy) the heat exchange and the work done in the forward process (x → y) are exactly equal and opposite to those in the reverse process (y → x). This is essentially the condition for reversibility, equivalent to the conditions discussed earlier.
A process A → B is reversible, if
- the process B → A occurs in nature and
- the heat exchange and the work done for each step in the forward process are exactly equal and opposite to those for the reverse process.
Conditions of reversibility:
i) A process is reversible if there is no dissipation of energy during this process. The origin of dissipative energy are friction, surface tension, etc., So, a process that occurs against friction, surface tension, etc., cannot be reversible.
ii) A process is reversible if it occurs infinitesimally slowly. Every real process in nature is irreversible. A reversible process is only an ideal process, never occuring in nature. Still, the concept is very useful to formulate important thermodynamic relations.
Example:
i) Let a gas be enclosed inside a cylinder-piston arrangement. Now the gas is allowed to get compressed very slowly through an isothermal process by applying force from its surroundings. So work is done on the gas by the piston. Now after this process, if the gas by itself expands very slowly pushing the piston up, it returns to the initial state. So work is done on the piston by the gas. This work done is nearly equal to the previous one. So apparently, the process is reversible. But actually, in each piston movement some heat is generated due to friction. This heat is absorbed by the surroundings which cannot be restored. So strictly speaking, the process is irreversible, but becomes nearly reversible only when the piston movements are very slow and heat generated due to friction becomes almost negligible.
ii) Let us take a container having 10 g of ice floating on 100 g of water at 0°C. If 80 cal of heat is supplied from the surroundings, 1 g of ice melts into 1 g of water. Now, if 80 cal of heat is taken away, we again get 10g of ice on 100 g of water. Here, the surroundings also come to its initial state. So the process of fusion of ice is apparently reversible. However, some heat exchange with the surroundings can never be avoided. That heat cannot be recovered in any manner. So the surroundings suffers a permanent change. For this reason, the processes like fusion and vaporisation are only approximately reversible.
iii) Free fall of a body due to gravity from a height h to the ground is an irreversible process because the body on its own cannot move up to the height h.
iv) When containers with two different gases are connected, the gases mix with each other. The gases cannot separate themselves on their own and so the process is irreversible.