Absolute Zero (SQA National 5 Physics): Revision Note

Absolute zero

• Absolute zero is a theoretical concept of the lowest possible temperature

• Absolute zero is defined as:

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

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

• 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

Converting between Kelvin and Celsius

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

The theory of absolute zero

• The temperature of a gas is related to the mean kinetic energy 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

• 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

• For a substance at absolute zero, it is not possible to remove any more energy from it

• This means that absolute zero is the lowest temperature possible

• However, this is a theoretical concept

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