The Gas Laws (CIE IGCSE Physics: Co-ordinated Sciences (Double Award))

Extended tier only

Pressure & volume (constant temperature)

• If the temperature of a gas remains constant, the pressure of the gas changes when it is:
  • Compressed – decreases the volume which increases the pressure
  • Expanded – increases the volume which decreases the pressure

At constant temperature, changing the volume changes the pressure

• Similarly, a change in pressure can cause a change in volume
• A vacuum pump can be used to remove the air from a sealed container
• The diagram below shows the change in volume to a tied up balloon when the pressure of the air around it decreases:

At constant temperature, changing the pressure changes the volume

• When a gas is compressed, the molecules will hit the walls of the container more frequently
  • This creates a larger overall net force on the walls which increases the pressure

Pressure & temperature (constant volume)

• Increasing temperature increases the pressure of a gas which is kept at a constant volume
• The average speed of molecules increases when the temperature increases (and vice versa)
• As the gas heats up, the molecules will travel at a higher speed
  • They collide with the walls more often and with greater force, increasing the 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 as the temperature increases
  • Diagram B shows that since the temperature is directly proportional to the pressure (at constant volume), the graph is a straight line

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