# Thermodynamics: Definition, System Boundaries, and Properties

## Definition of Thermodynamics:

Thermodynamics is a scientific field that deals with the transfer of heat and work, as well as the resulting changes in the properties of the substance being studied. The substance is usually isolated from its surroundings to determine its properties.

• System: A system refers to a collection of matter that can be identified within certain boundaries. It can either be open or closed, depending on whether or not mass transfer occurs across the boundary.
• Surrounding: The term surroundings typically refer to the particles of matter that exist outside of the system and can be affected by any changes that occur within the system. The surroundings themselves can also form another system.
• Boundry: A boundary is a surface, either physical or imaginary, that separates the system from its surroundings.

## Thermodynamic properties:

Thermodynamic properties refer to any quantity that is defined by the end states and the process involved. Examples of thermodynamic properties include Pressure, Volume, and Temperature of the working fluid in the system.

### 1. Pressure (P)

Pressure (P) is the force exerted per unit area of the surface within the system. It is often measured relative to atmospheric pressure and can be expressed in units such as kiloPascals, megaPascals, bars, or pounds per square inch.

Pabs = Patm + Pgauge

### 2. Specific Volume (V)

Specific Volume (V) refers to the volume of a unit mass, expressed in units of cubic meters per kilogram, and is the inverse of density.

### 3. Temperature (T)

Temperature (T) measures the degree of hotness or coldness of the system and is measured in Kelvin or Celsius scales. The absolute temperature of a body is defined relative to the temperature of ice.
The imperial unit of temperature is Fahrenheit where T°F = 1.8 × T°C + 32

### 4. Internal Energy (u)

Internal Energy (u) covers all forms of energy arising from the internal structure of the substance.

### 5. Enthalpy (h)

Enthalpy (h) is defined as h = u + PV and is a property of the system.

### 6. Entropy (s)

Entropy (s) is a measure of the microscopic disorder of the system and is an extensive equilibrium property.