Kinetic Theory (Edexcel GCSE Physics)

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The Pressure of a Gas

Motion of Particles in a Gas

  • 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 particles, IGCSE & GCSE Chemistry revision notes

Random motion of gas molecules in a container

Pressure

  • Molecules of gas in a container will collide with the container walls
  • Pressure is defined as the force exerted per unit area 

Pressure equation, downloadable AS & A Level Physics revision notes

  • Pressure is measured in the units Pascals (Pa)
  • The area should always be the cross-sectional area of the object
    • This means the area where the force is at right angles to it

  • This equation can be rearranged with the help of a formula triangle:

Pressure triangle (2), IGCSE & GCSE Physics revision notes

Pressure, force, area formula triangle

  • Imagine molecules of gas that are free to move around in a box
  • The molecules in the gas move around randomly at high speeds, colliding with surfaces and exerting pressure upon them
  • The temperature of a gas is related to the average speed of the molecules:
    • The hotter the gas, the faster the molecules move and vice versa
    • Hence, the molecules collide with the surface of the walls more frequently and with more force
    • This increases the pressure

Gas molecules in a box_2, downloadable AS & A Level Physics revision notes

Gas molecules hit the sides of the container which creates pressure

Temperature & Pressure

  • The motion of molecules in a gas changes according to the temperature
  • As the temperature of a gas increases, the average speed of the molecules also increases
  • Since the average kinetic energy depends on their speed, the kinetic energy of the molecules also increases if its volume remains constant
    • The hotter the gas, the higher the average kinetic energy
    • The cooler the gas, the lower the average kinetic energy

  • If the gas is heated up, the molecules will travel at a higher speed
    • This means they will collide with the walls more often
    • This creates an increase in pressure

  • Therefore, at a constant volume, an increase in temperature increases the pressure of a gas and vice versa
  • Diagram A shows molecules in the same volume collide with the walls of the container more with an increase in temperature
  • Diagram B shows that since the temperature is proportional to the pressure, the graph against each is a straight line

States of Matter Temperature and Pressure, downloadable IB Chemistry revision notes

At constant volume, an increase in the temperature of the gas increases the pressure due to more collisions on the container walls

Examiner Tip

You are required to be able to describe the link between temperature and pressure qualitatively. This means that the correct use of terms such as 'collision', 'kinetic energy' and 'frequency', will be really important.

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Ashika

Author: Ashika

Expertise: Physics Project Lead

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.