Entropy

When work is in transit, orderly motion of molecules is involved. Work is thus a high grade energy.

But when work is converted into heat, the order which was seen earlier in the molecules, disappears. Just like that. The molecules now move in a random and completely disorderly fashion. Thus, heat is a low grade energy.

When heat flow or transfer occurs, the molecular chaos increases. But this is not seen in the case of work flow.

The tendency of the universe (which is an isolated system) is always to reach a state of greater disorder, or in other words entropy. (Second law)

The entropy of a system is a measure of the degree of molecular disorder existing in the system

The above also indicates that even though energy is conserved quantity-wise (from first law ), energy gets degraded quality-wise (second law). This is the law of degradation of energy.

(In case of a perfectly crystalline solid at absolute zero temperature, its entropy is zero - Third law)



Source : "Engineering Thermodynamics" - P K Nag
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