Basic Thermodynamics

Thermodynamics is a Greek work Thermo, which means heat and dynamic force. It is a branch of physics which deals with the temperature and their relationship between the work and energy. It defines the variables like Entropy, enthalpy and equilibrium. The thermodynamics deal with the system in equilibrium.

Classification of Thermodynamics:

Thermodynamics are classified into two types they are

  • Equilibrium Thermodynamics
  • Non-equilibrium thermodynamics

Equilibrium thermodynamics are further divided into three types they are

  • Classical Thermodynamics
  • Kinetic Theory
  • Statistical Thermodynamics

Classical Thermodynamics:

Classical thermodynamics is a science which deals with the large-scale and microscopic properties of matter. Common parameters are coefficient of expansion, specific heat capacities, compressibility, magnetic and dielectric coefficient, heat transformations etc., are established with the help of the classical thermodynamics. We cannot determine the actual magnitude with the help of the classical thermodynamics.

Kinetic Theory:

Kinetic energy is used to identify the numerical values of the individual quantities. It deals with the molecular models in which, an individual molecule monitor the laws of mechanism.


Statistical Thermodynamics:

Statistical thermodynamics ignores the detailed consideration of molecules as individuals and the statistical considerations are applied to find the distribution of a large number of molecules that make up a macroscopic piece of matter over their energy states.

Non-equilibrium thermodynamic is also known as irreversible thermodynamics.

Irreversible thermodynamics is also another branch of thermodynamics, which deals with the non-equilibrium irreversible processes.

Classical Thermodynamics:

Thermodynamic system:

A thermodynamic system is defined as the quantity of the matter or a region in the space upon which attention is concentrated in the analysis of a problem”.

The quantity of the matter can vary as solids, liquids or gases, electric field, magnetic field or even photons.

System are classified into three types they are

  • Open system
  • Closed system
  • Isolated system

Closed system:

In a closed system, there is no mass transfer but the energy transfer takes place.

Open system:

In an open system, there is both mass and energy transfer that takes place. The boundary line is observed in broken or dotted lines.


Isolated system:

In an isolated system, there is no mass and energy transfer across the boundary.


The separation between the system and the surroundings is known as the boundary. The boundary may be real or imaginary. The shape and size may increase or decrease.



The entire space around the system is known as the surroundings. By using different types of walls, the system and the surrounding are divided. The walls are classified as diathermal, rigid wall, and adiabatic walls.

Diathermal Wall: With the help of the diathermal wall the system is supposed to communicate thermally with its surroundings. If two systems are separated with the help of the diathermal wall then it is known as the thermal contact.

Rigid walls: The rigid walls restrict to bring changes in the volume of the system

Adiabatic Wall: It is the one that is impermeable to the thermal energy. With the help of the wall between the system and the surrounding, the thermal interaction is restricted cut off.


We can see two properties they are intensive property and extensive property

Intensive Property:

The system which is independent of the size is known intensive property. Pressure and temperature are the properties of the intensive system.

Extensive properties:

Extensive property depends upon the size of the system. The volume of the system is an example of the extensive property.

The ratio of mass to the extensive property or the property for unit mass or mole is known as specific property.

The ration of an extensive property to the number of moles of the substance, within a system or the property per mole of the substance is known as the molar property.

Energies associated with the thermodynamic processes:

Potential energy:

The energy possessed by a body by virtue of its position is known as the potential energy.

PE = mgh


m= mass of the body,

g = acceleration due to gravity (9.8 m/s^2) and

h = height from the ground

Kinetic Energy:

The energy possessed by a body by virtue of its motion is known as the kinetic energy.

KE = 1/2 mv^2


m = mass of the body and

v = velocity of moving particle

A thermodynamic system consists of a fluid; it may possess both the potential energy and kinetic energy. The potential energy plus the kinetic energy are expressed in macroscopic terms, and the quantities are measured directly. The thermodynamic system may possess constitute macroscopic form of mechanical energy. The potential energy and kinetic energy are in the form of inter-convertible. Matter is composed of molecules or atoms which have the capacity to rotate, translate and vibrate. With respect to the motion of electrons, intra-atomic interactions, spin of electrons are associated with the energy. Molecules which are in inter-molecular interaction are in electromagnetic nature mainly at short intermolecular separation distance. All the energy is in the microscopic form, and are not readily to estimate in terms of macroscopic measurable properties of matter. The microscopic form of energy is different from the kinetic energy and the potential energy of a system, or body and they are normally independent of the velocity or position of the body. Due to the macroscopic mode the energy possessed by the matter, the motion is referred as an internal energy. The microscopic transformation is observed in the thermodynamic system. In the thermodynamic system and its surroundings, the exchange may take place through the system boundary as either work or heat or both.

Laws of Thermodynamics:

Thermodynamic laws are divided into four types they are

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

 Zeroth law of thermodynamics


If the two systems are in thermal equilibrium with the third system then it known as the zeroth law of thermodynamics. And the two systems are thermal equilibrium with each other.

First law of thermodynamics:

According to the first law of thermodynamics the energy can be transformed, but the energy cannot be destroyed or created. The energy must pass in the form of heat, work, or matter into the system or outside of the system.

Second Law of Thermodynamics:

According to the natural thermodynamic process increase in the participating thermodynamics system, takes place with the sum of the entropies.

Third Law of Thermodynamics:

The third law of thermodynamics introduces the absolute entropy concept. When the total entropy of the pure elements approaches zero degrees, as the absolute temperature elements zero degrees.

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