Kinetic Theory of Gases — Introduction & Postulates

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Kinetic Theory of Gases — Introduction & Postulates

Kinetic Theory of Gases — Introduction & Postulates

Physics • Molecular Theory of Gases

The kinetic theory of gases explains the macroscopic behavior of gases (like pressure and temperature) in terms of the microscopic motion of molecules. It was mainly developed by Rudolf Clausius, James Clerk Maxwell, and Ludwig Boltzmann in the 1850s–1870s.

1. Historical Background

Before the kinetic theory, gases were described only by experimental laws (Boyle’s law, Charles’ law, etc.). Clausius and Maxwell introduced the idea that gas molecules are in constant random motion, and that temperature is a measure of their kinetic energy. Later, Boltzmann provided the statistical foundation for this theory.

DO YOU KNOW?
The term “kinetic” comes from the Greek word kinesis, meaning “motion.”

2. Main Postulates of Kinetic Theory

The theory is based on several assumptions about the behavior of gas molecules:

  • 1. Molecules are tiny and numerous: A gas contains a very large number of identical small molecules.
  • 2. Negligible volume: The actual volume of molecules is negligible compared to the total volume of the gas.
  • 3. Random motion: Molecules move in all directions randomly with different velocities.
  • 4. Elastic collisions: Collisions between molecules and with container walls are perfectly elastic (no energy loss).
  • 5. No long-range forces: Molecules exert negligible attractive or repulsive forces on each other except during collisions.
  • 6. Short collision time: The time of a collision is negligible compared to the time between successive collisions.
  • 7. Pressure arises from collisions: The pressure of a gas is due to molecules striking the container walls.
  • 8. Temperature link: The average kinetic energy of molecules is directly proportional to the absolute temperature: \(E_k = \tfrac{3}{2}kT\).

3. Significance

The kinetic theory provides a molecular explanation of gas laws, connects microscopic motion with macroscopic thermodynamics, and laid the foundation for statistical mechanics.

EXAMPLE
Using kinetic theory, we can explain Boyle’s law: At constant temperature, if volume decreases, molecular collisions with walls increase, raising pressure.

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