(heat) from one to another. The systems are insulated from the rest of the surroundings also by similar adiabatic walls. The situation is shown schematically in Fig. .
(a). In this case, it is found that any possible pair of values ( P A , V A ) will be in equilibrium with any possible pair of values ( P B , V B ). Next, suppose that the adiabatic wall is replaced by a diathermic wall – a conducting wall that allows energy flow (heat) from one to another. It is then found that the macroscopic variables of the systems A and B change spontaneously until both the systems attain equilibrium states.
After that there is no change in their states. The situation is shown in Fig. . (b).
The pressure and volume variables of the two gases change to ( P B ′ , V B ′ ) and ( P A ′ , V A ′ ) such that the new states of A and B are in equilibrium with each other * . There is no more energy flow from one to another. We then say that the system A is in thermal equilibrium with the system B . What characterises the situation of thermal equilibrium between two systems ?
You can guess the answer from your experience. In thermal equilibrium, the temperatures of the two systems are equal. We shall see how does one arrive at the concept of temperature in thermodynamics? The Zeroth law of thermodynamics provides the clue.
. ZEROTH LAW OF THERMODYNAMICS Imagine two systems A and B , separated by an adiabatic wall, while each is in contact with a third system C , via a conducting wall [Fig. . (a)].
The states of the systems (i.e., their macroscopic variables) will change until both A and B come to thermal equilibrium with C . After this is achieved, suppose that the adiabatic wall between A and B is replaced by a conducting wall and C is insulated from A and B by an adiabatic wall [Fig. . (b)].
It is found that the states of