Temperature & Pressure (Cambridge O Level Physics)

• Molecules in a gas move around in constant random motion at high speeds
• Random motion means that molecules
  • Travel in no specific direction
  • Undergo sudden changes in direction if they collide with either the walls of the container, or with other molecules

Random Motion of Gas Molecules

Gas molecules in a container move around randomly at high speeds

• The motion of molecules in a gas depends on the temperature of the gas

• More specifically, the temperature of a gas is related to the average kinetic energy of the molecules
  • The hotter the gas, the higher the average kinetic energy
  • The cooler the gas, the lower the average kinetic energy

• Since the kinetic energy is related to speed, the speed of the molecules also changes with temperature (as long as the volume of the container is constant)
  • The hotter the gas, the faster the gas molecules move
  • The cooler the gas, the slower the gas molecules move

Absolute Zero

• In 1848, the physicist Lord Kelvin recognised that there must be a temperature at which the particles in a gas must no longer be moving, or exerting pressure on their surroundings
• This temperature is called absolute zero and is equal to −273 °C

• Absolute zero is defined as:

The temperature at which the molecules in a substance have zero kinetic energy 

• This means for a system at absolute zero, it is not possible to remove any more energy from it
• Even in space, the temperature is roughly 2.7°C above absolute zero

Where does absolute zero come from?

At absolute zero (−273°C) particles will have no net movement. It is therefore not possible to have a lower temperature than this

• A feature of gases is that they fill the space of any container that they occupy
• As the gas particles move about randomly they collide with the walls of the container and exert a pressure
• These collisions produce a net force at right angles to the wall of the container (or any surface)
• This pressure is defined as the force per unit area

• Where:
  • P = pressure exerted by the gas (Pa)
  • F = force exerted by the gas (N)
  • A = area the force acts over (m²)

• This equation means that
  • The higher the gas pressure, the more frequently the particles collide with the container walls and the greater the force exerted per unit area
  • The lower the gas pressure, the less frequently the particles collide with the container walls and the smaller the force exerted per unit area

Pressure & Force in a Gas

Gas molecules bouncing off the walls of a container

• The pressure of a gas also depends on the temperature of the gas
  • This is because particles move with more energy as their temperature increases
  • As the temperature of the gas decreases, the pressure on the container also decreases

Gas Molecules in a Container

Gas molecules hit the sides of the container and exert a force, which creates pressure