Kinetic Theory of Gases (OCR A Level Physics)

Revision Note

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Katie M


Katie M



Model of the Kinetic Theory of Gases

  • Gases consist of atoms or molecules randomly moving around at high speeds
  • The kinetic theory of gases models the thermodynamic behaviour of gases by linking:
    • The microscopic properties of particles i.e. mass and speed
    • The macroscopic properties of particles i.e. pressure and volume
  • The theory is based on a set of the following assumptions:
    • Molecules of a gas behave as identical (or all have the same mass)
    • Molecules of gas are hard, perfectly elastic spheres
    • The volume of the molecules is negligible compared to the volume of the container
    • The time of a collision is negligible compared to the time between collisions
    • There are no intermolecular forces between the molecules (except during impact)
    • The molecules move in continuous random motion
    • Newton's laws apply
    • There are a very large number of molecules

  • The number of molecules of gas in a container is very large, therefore the average behaviour (eg. speed) is usually considered

Exam Tip

Make sure to memorise all the assumptions for your exams, as it is a common exam question to be asked to recall them.

Pressure in the Model of Kinetic Theory of Gases

  • A gas is made of a large number of particles
    • Gas particles have mass and move randomly at high speeds
  • Pressure in a gas is due to the collisions of the gas particles with the walls of the container that holds the gas
  • When a gas particle hits a wall of the container, it undergoes a change in momentum due to the force exerted by the wall on the particle (as stated by Newton's Second Law)
    • Final momentum = –mv
    • Initial momentum = mv


  • Therefore, the change in momentum Δp can be written as:

Δp = final momentum – initial momentum

Δp = –mvmv = –2mv

bold minus bold italic F bold equals fraction numerator bold increment bold p over denominator bold increment bold t end fraction bold equals bold minus fraction numerator bold 2 bold m bold v over denominator bold increment bold t end fraction

  • According to Newton's Third Law, there is an equal and opposite force exerted by the particle on the wall (i.e. Ffraction numerator 2 m v over denominator straight capital delta t end fraction)

Gas Pressure, downloadable IB Physics revision notes

A particle hitting a wall of the container in which the gas is held experiences a force from the wall and a change in momentum. The particle exerts an equal and opposite force on the wall

  • Since there is a large number of particles, their collisions with the walls of the container give rise to gas pressure, which is calculated as follows:

space p equals F over A

  • Where:
    • p = pressure in pascals (Pa)
    • F = force in newtons (N)
    • A = area in metres squared (m2)

Exam Tip

Momentum is a Mechanics topic that should have been covered in a previous unit. The above derivation of change in momentum and resultant force should have already been studied - if you're not comfortable with it then make sure you go back to revise this!

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Katie M

Author: Katie M

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

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