Kinetic Theory (Edexcel IGCSE Science)

Kinetic theory of gases

Spec point — 6.1 · explain how molecules in a gas have random motion and that they exert a force, and hence a pressure, on the walls of a container

Random motion

• Molecules in a gas are in constant random motion at high speeds

• Random motion means that the molecules are travelling in no specific path and undergo sudden changes in their motion if they collide:

  • with the walls of its container
  • with other molecules

• The random motion of tiny particles in a fluid is known as Brownian motion

Random motion of gas molecules in a container, caused by collisions

• Brownian motion provides evidence that air is made of small particles

• This is because when larger particles, such as smoke particles or pollen, are observed floating in the air:

  • the larger particles move with random motion
  • this is a result of the larger particles colliding with smaller particles that are invisible to the naked eye

Pressure

• A feature of gases is that they fill their container
• The pressure is defined as the force per unit area

$p=\frac{F}{A}$

• Where:

  • p = pressure in pascals Pa
  • F = force in newtons N
  • A = area in metres-squared m²

• As the gas particles move about randomly they collide with the walls of their containers

• These collisions produce a net force at right angles to the wall of the gas container (or any surface)

• Therefore, a gas at high pressure has more frequent collisions with the container walls and a greater force

  • Hence the higher the pressure, the higher the force exerted per unit area

Gas molecules colliding with the walls of a container, exerting a force over the area and hence generating pressure