Laws of thermodynamics deals with

thermal equilibrium. First law deals with

heat conversion into work. Complete

conversion of heat into work is not

possible. All process in nature are

irreversible. Second law talks of

irreversibility. Third law gives irreversibility

is due to increase of entropy in a process.



Theoretical definition

Body ‘A’ and body ‘B’ are in thermal equilibrium.  body ‘B’ and body ‘C’ are in thermal equilibrium. Then body ‘A’ and body ‘C’ are also in thermal equilibrium.


 It explains the thermal equilibrium of two bodies in contact.
To measure human body temperature, the thermometer and the human body should acquire thermal equilibrium. Thus, there is no change in the thermometer reading over a period of time. Insert the thermometer in mouth. Take out after a short interval. It does not give the true temperature of the human body.
Take out the thermometer after 1 minute, take the reading. Reinsert and take out after two minutes, take the reading again. If the two readings are same, it is the true temperature. Thermal equilibrium reaches between the body and the thermometer. Thermal equilibrium is a pre-requisite for thermodynamics because thermodynamics operate under equilibrium conditions.


 All the three laws became well known all over the world without knowing the importance of thermal equilibrium. Since thermal equilibrium is a pre-requisite, it should come before the well known laws.  Its name is  Zeroth law. This law gives the idea for the measurement of temperature.

First Law of Thermodynamics

It is a law of conservation of energy. It states that one form of energy can change into another form of energy during a process. But that the total sum of energies of the system before and after the process remains constant. Further it does not give any information about the direction of the process. Hence it is  one of the limitations of the first law of thermodynamics. For example, it puts no restriction on the direction of the flow of heat. Whether heat flows from a cold body to a hot body or vice versa. Natural process does not bring out heat from  ice. Use external energy to achieve it.


1. It does not specify the feasibility of the process. It is silent about heat transfer.
2. This Law is silent about its % of conversion of energy from one form to another form. Work is not convertible into equivalent amount of heat. It is silent over this aspect.
3. Mathematically first law of thermodynamics is dU = dQ – dW. Thus first law gives the term of internal energy ’U’. This internal energy is a  derived thermodynamic property.
4. Thermodynamic system does work when an external source supplies heat at a higher temperature.
5. Thermodynamic system does work when external source receives heat at a lower temperature.
6. Work done in a closed process is=∫pdv
7. While work done in an open process is W = ∫-vdp
8. Internal energy ‘u’ is a point function and is independent of the path or process.
9. Heat is a form of energy in transition (motion). It is a function of temperature. Higher is the temperature, higher will be heat energy of the body and vice versa
10. Change of internal energy equals the difference between heat supplied to the system and the work done by the system.
11. The change of energy is zero in a cyclic process (starting and ending from the same point).
12. First  law for a cycle:
(i)  over the entire cycle, ∫δQ =∫δW
13. First law for a process
Q-W =dE
14. First law for an isolated system
Q=0 and W=0 and therefore dE=0

Second Law of Thermodynamics

It overcomes the limitations of the First Law.
The second law of thermodynamics tells about the feasibility of a process. It tells about its direction too.

 (a). Kelvin Planck Statement

There is no heat engine in the universe which can convert heat into an equivalent amount of work.  Engine rejects some heat.  This exits in steam turbine and internal combustion engine.  (Heat supplied—heat rejected) / heat supplied) becomes the efficiency of an heat engine. Thus it will be < 100 %. The change of heat into work and work  into heat are not like processes. Hence it deals with the concept ir-reversibility of energy conversion from one form into another form.

(b). Clausius Statement

Heat cannot flow from low temperature to high temperature without use of an external energy. This law gives about the direction of the process. The process in one way is possible but reverse is not possible in a natural way. Hence it deals with ir-reversibility of a process. in the forward and backward direction.


Thus both statements involve ir-reversibility in the energy conversion or direction of the process. This ir-reversibility is in terms of increase of entropy. Entropy term comes from the second law of thermodynamics. Increase of entropy is decrease in useful work in a certain process. Thus increase of entropy is ir-reversibility in a process. All the natural processes are irreversible. Hence the entropy of the universe is increasing. The second thus deals with ir-reversibility  of a process in terms of entropy. Entropy is a derived thermodynamic property.

Second law is a law of irreversibility.

Ir-reversibility is a loss of one kind

or the other. There are two types

of ir-reversibility.

(i)  Mechanical internal  ir-reversibility

(ii) thermal Internal ir-reversibility

Mechanical ir-reversibility is due to friction and turbulence.

It causes the followings:

(i) Pressure drop.

(ii) Internal friction causes rise of temperature

(iii) Increase of internal energy

(iv)  Volume increases

Thermal ir-reversibility

Chemical ir-reversibility is thermal ir-reversibility. It is due to change of energy on mixing of two fluids. It is because mixing is either exothermic or endothermic nature.

The ir-reversibility is reducible. It is not wipeable fully.

External Ir-reversibility

  1. These are external to the fluid.

  2. External friction between the moving parts

  3. friction in bearings

  4. friction between cylinder walls and the piston

  5. External ir-reversibility are between the system and the surroundings. These are due to friction between the moving /rotating parts and the atmosphere.

  6. External thermal ir-reversibility is due to temperature difference.


There are many reasons for a process to be irreversible.

  1. Friction

  2. Plastic deformation under an external force

  3. Free or unrestrained expansion

  4. Heat transfer due to finite temperature difference

  5. Momentum transfer

  6. Mixing of two substances

  7. Magnetization

  8. Polymerization

  9. Spontaneous chemical reaction like combustion

  10. Heat transfer due to temperature difference.

  11.  Work lost due to friction

  12.  Pressure drop due to friction

  13.  Internal /external leakage

  14.  Flow of fluid due to pressure difference

  15.  Flow of current due to voltage difference

  16.  Mass transfer due to concentration difference

  17.  Free expansion

  18.  Throttling expansion

  2. Heat transfer due to temperature difference

  3. Inelastic deformation

  4. Work lost due to friction

  5. Pressure drop due to friction

  6. Internal /external leakage

  7. Flow of fluid due to pressure difference

  8. Flow of current due to voltage difference

  9. Mass transfer due to concentration difference

  10. Free expansion

  11. Throttling expansion

  13. Availability is useful work from low grade energy.

  14. Unavailability is that part of low grade energy which is not convertible into useful work.

  15. Difference between total availability and actual availability is ir-reversibility.

  16. Every process is irreversible.

  17. Ir-reversibility is inherent to every process due to some loss of one kind or the other.

  18. Planck’s and Clausius statements of Second Law of Thermodynamic are the Laws of ir-reversibility.

  19. There is an internal as well as an external ir-reversibility.

  20. The internal & external ir-reversibility can be reduced.

  21. Ir-reversibility exists every moment in each and every process. It may be cooking, eating, fluid flow or heat transfer.


Entropy of a system decreases with the fall of temperature. It tends to be zero at absolute zero. It gives the idea about the measurement of entropy


It is a device to produce continuous supply of work without absorbing energy from the surroundings or from any source. It is impossible because of friction and has no practical value.


Perpetual motion machine of the second kind derives its energy from a source at a lower temperature. It is impossible because of  second law of thermodynamics. It states that heat cannot flow by itself from a body at lower temperature to a body at higher temperature.

Similar Posts