Thermodynamic Equilibrium

Equilibrium: Equilibrium is that state of a system in which the state does not undergo any change in itself with passage of time without the aid of any external agent. Equilibrium state of a system can be examined by observing whether the change in state of the system occurs or not. If no change in state of system occurs then the system can be said in equilibrium.

Examples:
Let us consider a steel glass full of hot milk kept in open atmosphere. It is quite obvious that the heat from the milk shall be continuously transferred to atmosphere till the temperature of milk, glass and atmospheres are not alike. During the transfer of heat from milk the temperature of milk could be seen to decrease continually. Temperature attains some final value and does not change any more. This is the equilibrium state at which the properties stop showing any change in themselves.

Generally, ensuring the mechanical, thermal, chemical and electrical equilibrium of the system may ensure thermodynamic equilibrium of a system.

1. Mechanical Equilibrium: When there is no unbalanced force within the system and nor at its boundaries then the system is said to be in mechanical equilibrium.
   For a system to be in mechanical equilibrium there should be no pressure gradient within the system i.e., equality of pressure for the entire system.
2. Chemical Equilibrium: When there is no chemical reaction taking place in the system it is said to be in chemical equilibrium.
3. Thermal equilibrium: When there is no temperature gradient within the system, the system is said to be in thermal equilibrium.
4. Electrical Equilibrium: When there is no electrical potential gradient within a system, the system is said to be in electrical equilibrium.

When all the conditions of mechanical, chemical thermal, electrical equilibrium are satisfied, the system is said to be in thermodynamic equilibrium.

Differentiate amongst gauge pressure, atmospheric pressure and absolute pressure.

Pressure: While working in a system, the thermodynamic medium exerts a force on boundaries of the vessel in which it is contained. The vessel may be a container, or an engine cylinder with a piston etc. The exerted force F per unit area A on a surface, which is normal to the force, is called intensity of pressure or simply pressure p.

Thus

P = F/A= ρ.g.h
It is expressed in Pascal (1 Pa = 1 Nm²),
Standard atmosphere (1 atm =1.0132 bar),
The pressure is generally represented in following terms.

1. Atmospheric pressure
2. Gauge pressure
3. Vacuum (or vacuum pressure)
4. Absolute pressure

Atmospheric Pressure (Patm):
It is the pressure exerted by atmospheric air on any surface. It is measured by a barometer. Its standard values are;
1 Patm = 760 mm of Hg i.e. column or height of mercury
= ρ.g.h. = 13.6 × 10³ × 9.81 × 760/1000
= 101.325 kN/m² = 101.325 kPa
= 1.01325 bar
when the density of mercury is taken as 13.595 kg/m³ and acceleration due to gravity as 9.8066 m/s²

Gauge Pressure (Pgauge):
It is the pressure of a fluid contained in a closed vessel. It is always more than atmospheric pressure. It is measured by an instrument called pressure gauge (such as Bourdon’s pressure gauge). The gauge measures pressure of the fluid (liquid and gas) flowing through a pipe or duct, boiler etc. irrespective of prevailing atmospheric pressure.

Vacuum (Or Vacuum pressure) (Pvacc):
It is the pressure of a fluid, which is always less than atmospheric pressure. Pressure (i.e. vacuum) in a steam condenser is one such example. It is also measured by a pressure gauge but the gauge reads on negative side of atmospheric pressure on dial. The vacuum represents a difference between absolute and atmospheric pressures.

Absolute Pressure (Pabs):
It is that pressure of a fluid, which is measured with respect to absolute zero pressure as the reference. Absolute zero pressure can occur only if the molecular momentum is zero, and this condition arises when there is a perfect vacuum. Absolute pressure of a fluid may be more or less than atmospheric depending upon, whether the gauge pressure is expressed as absolute pressure or the vacuum pressure.

Inter–relation between different types of pressure representations. It is depicted in Fig, which can be expressed as follows.
Pabs = Patm + Pgauge
Pabs = Patm – Pvace