We were discussing the first law, second law and also Zeroth law of thermodynamics in our previous post. Let us see here the third law of thermodynamics in this post.

Before understanding the third law of thermodynamics, we must have to understand here the concept of entropy and absolute temperature also. Therefore, let us first start to understand here the concept of entropy.

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**Entropy**

In order to understand third law of thermodynamics, we must have to understand the entropy and absolute temperature.

The concept of entropy is quite simple. It is observed that all heat energy supplied to the system will not be equally important or valuable during conversion in to the work energy. Heat energy, which will be supplied at higher temperature to the system, will have more possibilities for conversion in to the work energy. While in other hand heat energy, which will be supplied at lower temperature to the system, will have least possibilities for conversion in to the work energy.

Entropy is basically defined as the function of a heat quantity that indicates the possibility of conversion of that heat quantity in to work energy. Increment in entropy will be small when heat energy will be supplied to the system at higher temperature while in other hand increment in entropy will be higher when heat energy will be supplied to the system at lower temperature.

Therefore, it could be expressed that for the maximum entropy there will be least availability of conversion in to work energy and for minimum entropy there will be much availability of conversion in to work energy.

In simple way, we could express the entropy as follows

When heat energy will be supplied to a thermodynamic system by a reversible process, the change in entropy in the thermodynamic system will be expressed as

∆S = Q/T, Temperature is constant

dS = dQ/T, Temperature is not constant

If system which is reversible from a state a to b, we will have

Entropy is basically dependent on initial and final state of thermodynamic system and it does not depend over the path followed during the process.

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**Absolute temperature**

Temperature measured with respect to absolute zero will be termed as absolute temperature. Kelvin is used as unit of measurement for the absolute temperature and indicated by K. Absolute zero shows 0K. Absolute temperature is also known as thermodynamic temperature

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**Third law of thermodynamics**

Third law of thermodynamics is a basic law of nature and it could not be proved but it is always observed that it could not be violated and always followed by nature.

Thermodynamics third law is based on study of entropies of a perfect crystalline solid at absolute zero temperature.

According to the third law of thermodynamics, “At absolute zero temperature, the entropy of a perfect crystalline solid will be zero”.

It is not possible to reduce the entropy of system by any way to its zero point entropy value within a finite number of steps.

It can also be expressed as; it is not possible to reduce the temperature of a system to absolute zero within a finite number of operations.

In a simple way we can express the third law of thermodynamics as follows

Entropy of a perfect crystalline solid will approach to zero, if absolute temperature is approaching to zero. Third law of thermodynamics has provided one absolute datum point in order to measure the entropy.

We will start our discussion with another topic in our next post.

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