Question

Identify different thermodynamic systems and study their equilibrium. Classify them in to one of the categories we just discussed.

Answer 1

Different thermodynamic systems can be identified according to the exchange of matter and energy with the surroundings.

System	Classification	Equilibrium condition
Water boiling in an open vessel	Open system	It is generally not in equilibrium because heat and matter continuously pass between the system and surroundings.
Gas enclosed in a sealed cylinder with a movable piston	Closed system	It is in equilibrium when its temperature, pressure and other macroscopic properties remain constant with time.
Water in a tightly closed bottle	Closed system	It reaches equilibrium when there is no net change in temperature, pressure or phase distribution.
An ideal thermos flask containing hot water	Isolated system	It is ideally isolated from the surroundings and approaches internal thermal equilibrium.
Human body	Open system	It continuously exchanges matter and energy with the surroundings, so it is not a thermodynamic equilibrium system in the strict sense.

A thermodynamic system is said to be in thermodynamic equilibrium when it is simultaneously in:

• Thermal equilibrium: Temperature is uniform and there is no net heat flow.
• Mechanical equilibrium: Pressure is balanced and there is no unbalanced force.
• Chemical equilibrium: There is no net chemical reaction or change in composition.

Thus, a system is in complete thermodynamic equilibrium only when all these equilibrium conditions are satisfied.