Entropy

By Victor M Fontane

Entropy is one of the most profound concepts in physics. It describes the degree of disorder or dispersion of energy in a system and lies at the heart of the second law of thermodynamics: in every natural process, the total entropy of the universe tends to increase over time. Simply put, everything tends to move from order to disorder, and this tendency is irreversible.

In a closed system, such as a gas in a container, the molecules tend to move in an increasingly random manner until they reach equilibrium. This evolution toward uniformity reflects an increase in entropy. A solid, for example, has low entropy because its atoms are organized and fixed; in contrast, a gas has high entropy because the particles move freely and chaotically.

The Clausius equation, ΔS = Q_rev / T, expresses this concept quantitatively: the change in entropy (ΔS) depends on the heat exchanged (Q_rev) divided by the temperature (T). When heat flows in a reversible process, it's possible to calculate how much energy is lost. But in reality, natural processes are rarely reversible: there is always a loss of useful energy in the form of heat, and this causes entropy to increase.

This principle explains why physical phenomena progress in only one direction: ice melts, heat flows from hot to cold, stars consume their fuel. Entropy gives meaning to the passage of time, marking the arrow that distinguishes "before" from "after.”