Pressure & Volume (OCR Gateway 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

• 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:

• 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

Pressure on a Surface

• As the gas particles move about randomly they collide with the walls of their containers
• These collisions produce a net force at right angles to the wall of the gas container (or any surface)
• Therefore, a gas at high pressure has more frequent collisions with the container walls and a greater force
  • Hence the higher the pressure, the higher the force exerted per unit area

Gas molecules bouncing off the walls of a container

• It is possible for someone to experience this force by closing their mouth and forcing air into their cheeks
• The strain on the cheeks is due to the force of the gas particles pushing at right angles to the cheeks

• In a gas, the molecules 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, since the size of the container has decreased but the number of molecules has remained the same
  • This allows more frequent collisions of the molecules on the container wall
  • This means they hit the walls with a greater force and therefore increases the pressure

• When a gas expands, the volume is increased
  • This causes a decrease in pressure

Decreasing the volume increases the pressure of molecules at the same temperature

• Therefore, in summary:
  • When the volume decreases (compression), the pressure increases
  • When the volume increases (expansion), the pressure decreases

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

Calculating Changes in Pressure & Volume

• 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)

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

ANSWER:  B

• • A is true because the particles only spread about, but there is no chemical change
  • C is true because the particles have a larger volume, which means the collide less frequently with any surfaces
    • the pressure therefore decreases

  • D is true because the deodorant is able to spread out as it leaves the can
  • B is not true because as gases expand their temperatures decrease