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Glossary

Second Law of Thermodynamics

In the 1850s Clausius and Kelvin independently formulated the second law of thermodynamics. Clausius invented the term entropy, S, to provide a measure of the "transformational content" (capacity for change). The term means transformation, from the Greek word τροπη.

The second law of thermodynamics states that, on average, the entropy of a closed system can only increase (in an irreversible reaction) or stay constant (in equilibrium):

dS / dt ≥ 0

where S is the entropy and t is time.

It is worth emphasizing that at any given instant the entropy of a closed system can actually decrease. The probability of such an event, however, is extremely small.

The second law of thermodynamics and entropy are related through the following mathematical relationship:

dS ≥ dQ / T

where dQ is the amount of heat absorbed, and T is the absolute temperature.

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