The Gas Laws (Edexcel IGCSE Physics)

• Gas laws provide explanations for the relationships between:
  • Pressure and volume at a constant temperature
  • Pressure and (kelvin) temperature at a constant volume

Pressure & volume

• 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

Pressure increases when a gas is compressed

• 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:

By changing the pressure around the balloon, its change in volume can be seen

• For a fixed temperature, if the gas is compressed, the pressure will increase
  • The particles travel the same speed as before, but the distance they travel is reduced when the container is smaller
  • 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

• 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

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