Basic Concepts of Thermodynamics

Systems and Surroundings

• A system is defined as a quantity of matter or a region in space upon which the study is focused or concentrated.
• Every thing external to the system is called surroundings.
• System and surrounding together called universe.
• Boundary is the separation between the system and surroundings and it can be real or imaginary. It can be fixed or movable.

Types of system	Mass transfer	Energy transfer	Example
Closed	No	Yes	Piston cylinder
Open	Yes	Yes	Turbine, pumps, compressors
Isolated	No	No	Universe, hot tea in a perfectly  insulated thermos

Thermodynamic Equilibrium

A system is said to be in thermodynamic equilibrium if it is in-

• Thermal Equilibrium 
• Mechanical Equilibrium
• Chemical Equilibrium

Properties of system

Properties are point function and are exact differentials e.g. Temperature, enthalpy, entropy.

Intensive properties

Those properties are independent of mass. e.g. pressure, temperature, density and specific volume.

Extensive properties

Those properties are dependent of mass. e.g. volume, energy, enthalpy

• All Specific properties are intensive properties. 
• Specific property = extensive property / mass 
• Ratio of two extensive properties is intensive.

Process

Change of state is known as process

Reversible process

A process is called reversible if it reversed in direction follows the same path without leaving any effect on system and surroundings.

A friction-less Quasi static process is reversible process.

Irreversible process

The process which is not reversible is known as irreversible. All actual process are irreversible process.

Gib's Phase Rule

P + F = C + 2

where,

P = Number of phases 

F = Degree of freedom i.e. Minimum number of independent intensive variables to represent the state

C = Number of components 

Zeroth Law of Thermodynamics

When a body A is in thermal equilibrium with a body B and also separately with body C then body B and body C will be in thermal equilibrium each other.

Zeroth law is the basis of temperature measurement.

Conversion of Temperature Unit

° C = 5/9 (° F - 32) = K - 273.15

Here,

C = Temperature in degree Celsius     

F = Temperature in degree Fahrenheit            

K = Temperature in Kelvin                  

Note:

• Triple point is the point where all phases are co-exist. For water it is 273.16 K = 0.01° C.
• Absolute zero temperature is 0 K.
• Celsius and Fahrenheit are the same at -40°.
• Water boils at 100°C or 212°F.
• Water freezes at 0°C or 32°F.