*Answer briefly on the following (4 x 5)*

**Zeroth law of thermodynamics**

Zeroth law of thermodynamics states that if the bodies A and B are in thermal equilibrium with a third body C separately then the two bodies A and B shall also be in thermal equilibrium with each other.

**Condensation**

Condensation is the change of the physical state of matter from the gas phase into the liquid phase and is the reverse of evaporation. It can also be defined as the change in the state of water vapour to liquid water when in contact with any surface.

**Clausius inequality**

The following equation is known as the inequality of Clausius. It provides the criterion of the reversibility of a cycle.

∮(dQ/T) ≤ 0

- If ∮(dQ/T) = 0 then the cycle is reversible
- If ∮(dQ/T) < 0 then the cycle is irreversible and possible
- If ∮(dQ/T) > 0 the cycle is irreversible and possible

**Kirchhoff’s law**

Kirchhoff's Law: The law states that at any temperature the ratio of total emissive power E to the total absorptivity a is a constant foi~ all substances that are in thermal equilibrium, with their environment.

E

_{b}= E/εwhere E is emissive power and ε is emissivity. E

_{b}is energy fall on the surface.Thus, Kirchhoff's law also states that the emissivity of a body is equal to its absorptivity when the body remains in thermal equilibrium with its surroundings.

**Psychrometry**

- The art of measuring the moisture content of air is termed “psychrometry”.
- Psychrometry is a branch of engineering science, which deals with the study of moist air i.e. dry air mixed with water vapour or humidity. It also includes the study of the behaviour of dry air and water vapour mixture under various sets of conditions.
- Though the earth’s atmosphere is a mixture of gases including nitrogen (N₂), oxygen (O₂), argon (Ar) and carbon dioxide (CO₂), yet for the purpose of psychrometry, it is considered to be a mixture of dry air and water vapour only.

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