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 thinking of them as tiny particles, too small to see, that are always moving around in random directions. These particles are actually the atoms or molecules that make up the gas.

The temperature of the ideal gas is proportional to the average kinetic energy of its particles. The size of helium atoms relative to their spacing is shown to scale under 1,950 atmospheres of pressure. The atoms have an average speed relative to their size slowed down here two trillion fold from that at room temperature.

This theory explains how the movement of these tiny particles creates things we can measure, like how much space the gas takes up, how much pressure it creates, and how hot or cold it is. It shows how the particles bounce off each other and the sides of the container they are in, which helps us understand many properties of gases.

The basic idea of the theory works best for an ideal gas, where the particles bounce off each other without losing any energy and are far apart from one another. This theory was very important in the history of science because it was one of the first clear uses of the ideas of statistical mechanics and it helps explain the behavior of gases that are not very crowded.

History

Kinetic theory of matter

Francis Bacon

Antiquity

Around 50 BCE, the Roman philosopher Lucretius thought that things that look still are made of tiny particles moving and bumping into each other. This idea was not accepted for a long time because other ideas were more popular.

Modern era

John Locke

"Heat is motion"

In 1620, the English philosopher Francis Bacon said that heat is the movement of tiny particles, not the whole object moving. In 1623, Galileo Galilei agreed, saying heat and pressure happen because of particles moving.

In 1665, Robert Hooke shared this idea, and in 1675, Robert Boyle said hitting a nail with a hammer turns the force into movement of the nail’s tiny parts, which is heat. Boyle thought all traits of objects, like color and taste, come from how their tiny parts are arranged and move.

Catherine the Great visiting Mikhail Lomonosov

In 1744, Mikhail Lomonosov said that just like we can see leaves move in the wind even though we can’t see the wind, the tiny parts in warm objects move too fast for us to see. He also said this movement helps mix substances.

In 1738, Daniel Bernoulli wrote a book called Hydrodynamica, starting the kinetic theory of gases. He thought gases are made of many molecules moving in all directions. When these molecules hit a surface, they make gas pressure, and how fast they move on average decides the gas’s temperature.

Other scientists like Mikhail Lomonosov, Georges-Louis Le Sage, John Herapath, and John James Waterston also worked on this theory, but their ideas were not accepted at first.

In 1856, August Krönig made a simple model of how gas particles move, and in 1857, Rudolf Clausius made a more detailed model that included how particles spin and vibrate. In 1859, James Clerk Maxwell found a way to describe how fast gas particles move, and in 1871, Ludwig Boltzmann expanded this idea.

At the start of the 20th century, many scientists still thought atoms were make-believe. This changed when Albert Einstein and Marian Smoluchowski used the kinetic theory to show how tiny particles move in water, proving atoms are real.

Assumptions

The kinetic theory of gases is based on a few simple ideas. It says that gases are made of very tiny particles that are far apart from each other. Because of this, the size of the particles doesn’t matter much. There are so many particles that we can use special math to understand how they behave on average.

These particles are always moving and bumping into each other and the walls of their container. The bumps are perfectly elastic, meaning they don’t lose energy. Between bumps, the particles don’t push or pull on each other. This makes it easier to describe their movement using basic physics rules.

Equilibrium properties

The kinetic theory of gases explains how gases act based on the movement of their tiny particles. It helps us learn about temperature and pressure.

One important idea is that gas particles are always moving and bumping into each other and the walls of their container. When these particles hit the walls, they create pressure. Scientists can figure out this pressure by looking at how often particles crash into the walls and how fast they are moving.

The theory also links temperature to the energy of these moving particles. When the temperature is higher, the particles move faster, and this movement is connected to the heat we feel. By watching these movements, scientists can explain many things about gases in a simple way.

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