# Laws of Thermodynamics

Following are the four laws of thermodynamics:-

## Zeroth law of thermodynamics:-

This law states that when two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other.

## First law of thermodynamics:-

This law states that the heat and mechanical work are mutually convertible. According to this law, a definite amount of mechanical work is needed to produce a definite amount of heat and vice versa.

This law also states that the energy can neither be created nor be destroyed, through it can be transformed from one form to another. According to this law , the energy due to heat supplied(dQ) must be balanced by the external work done(dW)  plus the gain  in internal Energy(dE) due to rise in temperature.

dQ=dW+dE

## Second law of Thermodynamics:-

This law states that there is a definite limit to the amount of mechanical energy , which can be obtained from a given quantity of heat energy.

According to Clausius, this law may be stated as “ It is impossible for a self acting machine working in a cyclic process, to transfer heat from a body at a lower temperature to a body at a higher temperature without the aid of an external agency”.

The second law of thermodynamics has also been stated by Kelvin–plank as “ It is impossible to construct an engine working on a cyclic process, whose sole purpose is to convert heat energy into work”. According to this statement , the second law of thermodynamics is sometimes called law of degradation of energy.

## Third Law of Thermodynamics:-

The third law of thermodynamics refers to a state known as absolute zero. This is the bottom point on the Kelvin temperature scale. The Kelvin scale corresponding to absolute 0 is -273.150    Celsius or -459.7  Fahrenheit.

• Temperature of a system approaches to absolute Zero , its entropy approaches to zero.
• The entropy of a pure perfect crystal is zero.
• No object or system can have temperature of Zero Kelvin.
• This state is not physically possible, but it is a mathematical limit of the universe.