## 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 **French engineer and physicist, Nicolas Léonard Sadi Carnot**. They

**advanced the study of the second law by forming a principle (also called**

**Carnot’s rule**) that specifies limits on the maximum efficiency that 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, which 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 a 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 an 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.