Particle Model of Gases (Cambridge (CIE) IGCSE Physics): Revision Note

Motion of particles in a gas

• In the particle model, 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 when they collide:

  • With the walls of its container

  • With other molecules

• Pressure in a gas is caused by the collisions of particles with the walls of the container

• When the particles travel faster (e.g. at a higher temperature), they collide with walls more frequently

  • They also hit the walls with a greater force, because they are moving faster

  • This means the gas exerts a greater pressure

Random motion of gas molecules in a container. Collisions with container walls create pressure in the particle model

Pressure & force of particles in a gas

Extended tier only

• When a molecule bounces off a wall its momentum changes direction

• This change of momentum over time produces a force on the wall

• The pressure is defined as the force per unit area

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

• Where:

  • $p$ = pressure, measured in pascals (Pa)

  • $F$ = force, measured in newtons (N)

  • $A$ = area, measured in metres squared (m²)

• This equation and the particle model can be used to explain how particles exert pressure

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

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

  • Pressure is force per unit area (opens in a new tab), so the force of these collisions exerts a pressure

• When the particles move faster, they have more frequent collisions with the container walls and these exert a greater force
  

  • If the force exerted per unit area is greater and more frequent, the pressure exerted is also greater

Gas molecules bouncing off the walls of a container exert a force perpendicular to the surface