# Entropy : What this actually mean ?? | Mechanical Engineering

### Entropy : What this actually mean ??

Let we discuss today on this topic —

Entropy is a function of a quantity of heat which shows the possibility of conversion of that heat into work. The increase in entropy is small when heat is added at a high temperature and is greater when heat addition is made at a lower temperature. Thus for maximum entropy, there is minimum availability for conversion into work and for minimum entropy there is maximum availability for conversion into work.”

In heat engine theory, the term entropy plays a vital role and leads to important results.

All heat is not equally valuable for converting into work. Heat that is supplied to a substance at high temperature has a greater possibility of conversion into work than heat supplied to a substance at a lower temperature.

Let us consider a system undergoing a reversible process from state 1 to state 2 along path L and then from state 2 to the original state 1 along path M. Applying the Clausius theorem to this reversible cyclic process, we have –

R  dQ / T = 0

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Let S1 = E n tropy at the initial state 1, and

S2 = E n tropy at the final state 2.

Then, the change in entropy of a system, as it undergoes a change from state 1 to 2, becomes

S2S1 = ∫21  (dQ / T)R The third law of thermodynamics states “When a system is at zero absolute temperature, the entropy of system is zero”.

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