What is relative humidity [RH]? Role of Entropy.

What is relative humidity [RH]?

The usual definition is RH = Partial pressure of moisture in air / Saturated vapor pressure x 100. This definition often confuses people. The definition has a hidden meaning.

RH of air is a thermodynamic controlled property of air

Explanation

How do thermodynamics work in psychrometry? Role of entropy

Thermodynamics is the study of the relationships between heat, work, and energy. There are four fundamental laws of thermodynamics, which govern the behavior of energy and matter and are crucial to understanding the behavior of air as it reaches saturation and the relative humidity becomes 100%.

The first law of thermodynamics, also known as the law of conservation of energy, states that energy cannot be created or destroyed, only converted from one form to another. This law is important in understanding the energy exchange that occurs in the atmosphere as air reaches saturation.

The second law of thermodynamics states that the total entropy of an isolated system can never decrease over time. Entropy is a measure of disorder or randomness in a system. When the air becomes saturated with moisture, the entropy reaches its maximum value, and any further addition of moisture will result in the condensation of water vapor into liquid water, releasing latent heat and increasing the entropy of the system.

Additionally, the Gibbs free energy[G] equation, a fundamental concept in thermodynamics, describes the relationship between enthalpy changes, [H] entropy, [S] and the temperature, [T] of a system, dG = dH - TdS.

As the relative humidity of air reaches 100%, which happens at the dew point temperature, the entropy of the wet air reaches maximum because the specific volume of air increases with its moisture content. Correspondingly, the Gibbs free energy of the system reaches its minimum value, moving it into a state of equilibrium. Any further addition of moisture would result in the release of latent heat and the condensation of water vapor into liquid water, to maintain the equilibrium and minimize the free energy of the system.

Further explanation:

Role of entropy:

Relative humidity refers to how close the air is to being saturated with moisture. When air is saturated, both its liquid and vapor phases are in thermodynamic equilibrium as explained, with identical state properties, maximum entropy, and minimum Gibbs free energy. This principle applies to wet air as well. Relative humidity (RH) is a measure of how far the air composition is from minimum free energy and maximum entropy. For example, saturated air has the maximum entropy when the partial pressure of moisture equals the vapor pressure, resulting in 100% relative humidity. As relative humidity decreases, the air becomes less saturated and its entropy decreases.