## Various Statements of the Law

The **second law of thermodynamics** may be expressed in many specific ways. Each statement expresses the same law. Listed below are three that are often encountered.

Before these statements, we have to remind the work of **a French engineer and physicist, Nicolas Léonard Sadi Carnot advanced the study of the second law by forming a principle (also called Carnot’s rule) that specifies limits on the maximum efficiency any heat engine can obtain.**

## Entropy and the Second Law

One consequence of the **second law of thermodynamics** is the development of the **physical property** of matter, that is known as the** entropy (S)**. The change in this property is used to determine the **direction** in which a given process will proceed. **Entropy** quantifies the **energy** of a substance that is **no longer available to perform useful work**. This relates to the** second law** since the second law predicts that not all heat provided to a cycle can be transformed into an equal amount of work, some heat rejection must take place.

See also: Entropy

According to Clausius, the **entropy** was defined via the **change in entropy S** of a system. The change in entropy S, when an amount of heat Q is added to it by a reversible process at constant temperature, is given by:

Here **Q** is the energy transferred as **heat** to or from the system during the process, and **T** is the** temperature** of the system in kelvins during the process. The SI unit for entropy is **J/K**.

The **second law of thermodynamics** can also be expressed as **∆S≥0** for a closed cycle.

In words:

*The entropy of any isolated system never decreases. In a natural thermodynamic process, the sum of the entropies of the interacting thermodynamic systems increases. *

**∆S≥0**

Because entropy tells so much about the usefulness of an amount of heat transferred in performing work, the steam tables include values of specific entropy (s = S/m) as part of the information tabulated.

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