Thermodynamics Basics

Four laws of thermodynamics:

  • Zeroth law of thermodynamics
  • First law of thermodynamics
  • Second law of thermodynamics
  • Third law of thermodynamics

Thermodynamic system:

A definite area where some thermodynamic process takes place


Anything external to system


A surface which separates the system from its surroundings. It may be real or imaginary, fixed or moving.

Closed system:

Only energy transfer takes place

Open system:

Mass and energy transfer takes place

Isolated system:

No interaction with surroundings

Example for isolated system:


Extensive property:

Property which depends on mass and size

Intensive property:

Property which is independent of mass

Examples for extensive property:

Entropy, Enthalpy, Internal Energy, length etc.

Examples for intensive property:

Specific volume, specific entropy, specific enthalpy, specific internal energy, temperature, specific heat etc.

Thermodynamic equilibrium:

A system is said to be in thermodynamic equilibrium if it satisfies the conditions of Mechanical equilibrium, Chemical equilibrium, Thermal equilibrium.

Quasi static process:

A process is said to be quasi static when it is carried out in such way that, at every instant the system’s deviation from the thermodynamic equilibrium is very small.

Thermal equilibrium:

Temperature remains constant

Mechanical equilibrium:

No unbalanced forces between system and its surroundings

Chemical equilibrium:

No phase change or chemical reaction

Point function:

If anything which can be represented in graph by means of point in the sense, then it is point function.

Path function:

If anything which cannot be be represented in graph by means of point, but can be represented by means of area means then it is path function.

Examples of point function:

Pressure, volume, temperature, internal energy

Examples of path function:

Work, Heat

Reversible process

Initial states of the system can be achieved by reversing the process

Irreversible process:

The initial state of the system cannot be restored without any changes in system and its surroundings


The sum of internal energy and flow energy is called as enthalpy.


Entropy is function of quantity of heat, the property of the system which decides the percentage of heat that can be converted into work.

Difference between entropy and enthalpy:

Entropy is property where as enthalpy is energy


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