Particle change and pressure

• Motion of molecules in a gas

  They are in constant random motion
• Factor affecting average kinetic energy of gas molecules

  • The temperature of the substance
  • The higher the temperature, the higher the average kinetic energy of the molecules
• Increasing temperature of a gas held at constant volume

  Pressure of the gas will increase
• Why pressure increases as temperature increases (at constant volume)

  1. Kinetic energy of molecules increases

  2. Collisions between molecules becomes more frequent

  3. Greater rate of change of momentum

  4. Greater force and therefore pressure
• What happens if gas A is at a low pressure, and gas B is at a high pressure

  • There are more collisions per second in gas B than in gas A
  • The rate of collisions is higher in B
• Force exerted by pressure of gas on container walls

  • The net force acts at right-angles to the container's surface
  • The force increases as pressure increases
• How increasing volume of a gas results in decreased pressure

  1. Molecules become more spread out and so time between collisions increases

  2. This reduces the rate of collisions

  3. Rate of change of momentum decreases, and so force exerted on container decreases, resulting in a lower pressure
• For a fixed mass of gas at constant temperature, the product of pressure and volume is constant
• Unit for pressure

  Pascal (Pa)
• What increases when you do work on a gas

  • The internal energy of the gas
  • This can also lead to an increase of temperature
• Why temperature of air inside a bike pump increases when pumped

  1. Work is done on a gas when it is compressed

  2. Doing work on a gas increases its internal energy, so also increases the average kinetic energy of the molecules

  3. Temperature increases with an increase of average kinetic energy