What Is Entropy in Physics?

Cory Carnley

April 12, 2023

Entropy in Physics is the amount of thermal energy per unit temperature unavailable for valuable work. This is the underlying principle of thermodynamics.

Entropy is a crucial microscopic concept for describing the behaviour of systems of molecules. It is also an important concept in information theory.


Entropy in Physics is a measure of disorder or randomness in a thermodynamic system. It is used in the thermodynamics of physics to describe how energy is dispersed at a given temperature.

It is an extensive property, meaning it scales with the size or extent of a system. It is also an intensive property, meaning that it can be expressed relative to the mass of the substance.

The entropy in Physics of a pure crystal with only one state with minimum energy is zero at zero temperature. This makes it possible to establish an absolute scale for entropy that can be used to determine the degree of randomness or disorder in a system.

The second law of thermodynamics states that all closed systems tend to increase in entropy toward a maximum value. This is because heat cannot be converted to work spontaneously. Therefore, the entropy of closed systems increases over time until no free energy is available for doing valuable work.

Molecular entropy

Entropy is a fundamental concept in the molecular world. It is a fundamental property of matter, and it has been studied and developed since 1850 by German physicist Rudolf Clausius.

In the molecular world, entropy increases with temperature (more kinetic energy is available), the number of molecules in the system, and the volume. For example, when the volume of a gas is doubled, each molecule now has twice the space to wander in so that it will move around more than before.

This is an easy-to-understand idea and a great way to help children learn about physics. You can even visualize it by making a simple applet in which we partition the molecules into two “red” and two “green” parts.

Quantum entropy

Entropy is the measure of randomness or disorder. It’s an essential concept because it helps us understand how disorganized a system is. For example, a gas spread out over a volume has high entropy; a gas in one corner of a box has low entropy. In the quantum world, entropy is even more critical. It’s an exact way of measuring a system’s disorder degree.

When the entropy of a system increases, it means that the particles in the system are becoming more disorganized. This is not necessarily the same as an increase in an entanglement because entanglement is the presence of a hidden order that allows two particles to connect.

In addition to being a good tool for describing the degree of disorder in a system, entropy also plays a vital role in information theory. As discussed in our previous post, entropy can help determine how many bits of information a message needs to be transmitted successfully.

Statistical entropy

Entropy measures randomness, disorder, disorganization and unpredictability in the statistical world. It has been used as a quantitative tool in many scientific fields, such as cosmology, logic, biology and informatics.

Moreover, it is also a powerful tool for analysing other kinds of systems and system states. It is a probabilistic measure comparable to the Boltzmann entropy and the Gibbs entropy.

If a large sample of atoms is placed in a corner, it will be doubtful for their order to decrease. Instead, they will quickly disperse and become uniformly distributed, with entropy increasing.

This general property of irreversible processes makes entropy a valuable measurement of how much energy is unavailable to do work. It is, therefore, the basis of the second law of thermodynamics.

In physics, this concept is a critical component of the Second Law of Thermodynamics and Nernst’s Theorem. It is based on the fact that a system cannot transfer heat from hot to cold without increasing its entropy.