Absolute Zero & Temperature (WJEC GCSE Physics)

The Kelvin Temperature Scale

• The Kelvin temperature scale begins at absolute zero
• 0 K is equal to -273 °C 
• Because it is not possible to have a temperature lower than 0 K, a temperature in kelvin will never be a negative value

• To convert between temperatures in Celsius, and temperatures in kelvin:

• The divisions on both the Kelvin scale and the Celsius scale are equal. This means:

A change in a temperature of 1 K is equal to a change in temperature of 1 °C

Comparison of the Kelvin Scale and the Celsius Scale

Absolute zero is the lowest possible temperature, a change in temperature in K is equal to the same change in temperature in °C

Absolute Zero

• The temperature of a gas is related to the average speed of the molecules:
  • The hotter the gas, the faster the molecules move 
  • Faster moving molecules collide with the surface of the walls more frequently and with more force
  • This increases the pressure

Gas Molecules in Random Thermal Motion Inside a Sealed Box

Gas molecules hit the sides of the container which creates pressure. A higher temperature increases the average speed of the molecules, increasing the pressure

• As the temperature of the gas decreases, the pressure exerted on the surfaces of the container also decreases
• In 1848, Mathematician and Physicist, Lord Kelvin, recognised that there must be a temperature at which the particles in a gas exert no pressure
  • At this temperature the particles must no longer be moving, and hence not colliding with their container

• This temperature is called absolute zero and is equal to -273 °C

At absolute zero, or -273 °C, particles will have no net movement. It is therefore not possible to have a lower temperature

• Absolute zero is defined as:

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

• So for a system at absolute zero, it is not possible to remove any more energy from it
• This means that absolute zero is the lowest temperature possible

• Even in space, the temperature is roughly 2.7 °C above absolute zero