Pressure & Volume (AQA GCSE Physics)

• 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

• The pressure produces a net force at right angles to the wall of the gas container (or any surface)

Gas molecules bouncing off the walls of a container

• Therefore, if the 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

• In a gas, the molecles are widely spread
• This makes the gas easy to expand and compress
• Changing the pressure acting on the gas will compress it or allow it to expand if the temperature is kept constant
• When a gas is compressed, the volume is decreased
  • The density of the gas increases, allowing more frequent collisions of the molecules on the container wall
  • This increases the pressure

• For a fixed mass of a gas held at a constant temperature:

pV = constant

• Where:
  • p = pressure in pascals (Pa)
  • V = volume in metres cubed (m³)

• This means that the pressure and volume are inversely proportional to each other
  • When the volume decreases (compression), the pressure increases
  • When the volume increases (expansion), the pressure decreases

• This is because when the volume decreases, the same number of particles collide with the walls of a container but more frequently as there is less space
  • However, the particles still collide with the same amount of force meaning greater force per unit area (pressure)

• The key assumption is that the temperature and the mass (and number) of the particles remains the same

• This equation can also be rewritten for comparing the pressure and volume before and after a change in a gas:

P₁V₁ = P₂V₂

• Where:
  • P₁ = initial pressure in pascals (Pa)
  • V₁ = initial volume in metres cubed (m³)
  • P₂ = final pressure in pascals (Pa)
  • V₂ = final volume in metres cubed (m³)

• This equation is sometimes referred to as Boyle's Law

Initial pressure and volume, P₁ and V1, and final pressure and volume, P₂ and V₂