Kinetic theory of gases

The kinetic theory of gases is a simple classical model of the thermodynamic behavior of gases. It helps us understand how gases work by treating them as made up of tiny particles—like atoms or molecules—that are always moving around randomly. These particles are so small that we can’t see them, even with a microscope!

This theory explains how things like the volume, pressure, and temperature of a gas are related to each other. It does this by looking at how these tiny particles bump into each other and the walls of the container they’re in. Even though we can’t see these particles, their movement and collisions help us understand a lot about gases.

The basic version of this theory describes an ideal gas. In this model, the particles don’t lose any energy when they collide—they bounce off each other perfectly, like rubber balls. The particles are also assumed to be much smaller than the space between them, so they don’t get in each other’s way very much.

Because of these ideas, the kinetic theory of gases became important in the development of statistical mechanics. It was one of the first clear examples of using statistics to understand physics, and it helps explain the behavior of gases that are not very crowded. This theory gives us a strong foundation for studying how gases move and spread out.

History

See also: Heat § History, Atomism, and History of thermodynamics

Long ago, around 50 BCE, the Roman philosopher Lucretius thought that everything solid is made of tiny, fast-moving particles bumping into each other. This idea was not popular for many years, as most people followed the ideas of Aristotle.

Later, in the 1600s, scientists began to connect heat with movement. Francis Bacon said that heat is really the motion of tiny parts of matter. Galileo Galilei agreed, saying that what we feel as heat comes from particles moving inside objects.

In 1738, Daniel Bernoulli published a book called Hydrodynamica, where he suggested that gases are made of many tiny molecules moving in all directions. He said that when these molecules hit a surface, they create gas pressure, and the faster they move on average, the hotter the gas gets. This idea started the kinetic theory of gases. It took time for others to accept it, especially since the idea of energy conservation was not yet fully understood.

Later scientists like James Clerk Maxwell and Ludwig Boltzmann developed these ideas further, creating mathematical ways to describe how these particles move and spread out. Their work helped explain many things about gases and heat.

Assumptions

The kinetic theory of gases makes a few simple assumptions to help explain how gases behave. It assumes that gases are made of very small particles that are far apart from each other, so their own size doesn’t matter much compared to the space they’re in. These particles move quickly and bounce off each other and the walls of their container without losing any energy — these bounces are called elastic collisions.

We also assume there are so many particles that we can use statistics to predict their behavior. Between bounces, the particles don’t push or pull on each other at all. This makes it easier to use basic physics to describe how they move. Sometimes we also assume all particles have the same mass, although the theory can be adjusted for particles with different masses too.

Equilibrium properties

The kinetic theory of gases describes how gases behave using simple ideas about particles in motion. It helps explain many basic concepts in thermodynamics.

In this theory, gas pressure comes from tiny particles hitting the walls of their container. These particles are always moving randomly and bouncing off each other and the container walls. When they hit the walls, they push on them, creating what we feel as pressure.

The theory also links temperature to how fast these particles are moving. Higher temperatures mean the particles move faster, which affects both the pressure and energy of the gas. This connection helps us understand how gases behave under different conditions.

Transport properties

See also: Transport phenomena

The kinetic theory of gases studies not just gases in balance, but also gases that are not in balance. This helps us understand "transport properties" like viscosity, thermal conductivity, and mass diffusivity.

Basic kinetic theory works best for thin gases. For thicker gas mixtures, a method called Revised Enskog Theory was created in the 1980s by E. G. D. Cohen, J. M. Kincaid, and M. Lòpez de Haro, building on earlier work by H. van Beijeren and M. H. Ernst.