Laws of Thermodynamics

There are four laws of thermodynamics, they are

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

Zeroth law of thermodynamics:

This law states “Consider three bodies namely A, B, C, if A and B are individually in thermal equilibrium with a third body C then, the bodies A and B also will be in thermal equilibrium with each other. If two bodies are in thermal equilibrium then they will have same temperature, here all bodies are in thermal equilibrium with each other, hence the value of temperature for three bodies is same.

First law of thermodynamics:

This law also known as Law of conservation of energy, it states ” the energy is always conserved i.e the energy can be converted from one form of energy to another form of energy but the energy cannot be created or destroyed. Example : heat is converted into work. The amount of energy contained in the system is always constant.”, thus the first law defines “In a cyclic process, the net heat transfer is equal to net work transfer“.

Disadvantages of First law of thermodynamics:

  • The first law simply says “All the amount of  heat is converted into work”. But it is impossible to convert all the heat into work, there will be always some losses to surroundings, only particular quantity of heat is converted into work, remaining heat is wasted to sink.
  • It doesn’t define the direction of energy transfer i.e whether the energy is conserved from hot body to cold body or cold body to hot body.
  • Due to the above reasons, the Second law of thermodynamics was developed.

Second law of thermodynamics:

There are two sub laws in second law to overcome all the disadvantage of first law, they are

  • Kelvin-Plank statement
  • Clausius statement

Kelvin-Plank statement:

It states” All the heat energy given to an engine cannot be converted into useful work, some amount of heat energy will be rejected to the surroundings or sink.”

Clausius statement:

It states”Heat cannot flow from cold reservoir to hot reservoir without any external work, but heat can flow from hot reservoir to cold reservoir without any external work”.

Third law of thermodynamics:

This law states “The entropy of a system approaches a constant value as temperature approaches zero”. The entropy of a system at absolute temperature is zero.




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