Volume & Pressure

• Pressure Changes in a Gas

  • 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

  A) Low

  B) High

  
• • Pressure and volume:
  • 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
• Volume and pressure:

  • 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
• Pressure & Volume

  • In a gas, molecules are far apart
  • This makes the gas easy to expand and compress
  • 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
• In summary:

  • When the volume decreases (compression), the pressure increases
  • When the volume increases (expansion), the pressure decreases
• Changes in Pressure & Volume

  pV = constant

  • 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)
• P1V1 = P2V2

  • 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)