Absolute Zero (Edexcel IGCSE Physics)

Revision Note

Katie M

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Katie M

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Absolute zero

  • The amount of pressure that a gas exerts on its container is dependent 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

  • 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 they must no longer be moving, and hence not colliding with their container

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

Pressure Law, downloadable AS & A Level Physics revision notes

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 

  • 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 K above absolute zero

The Kelvin scale

  • The Kelvin temperature scale begins at absolute zero

    • 0 K is equal to -273 °C 

    • An increase of 1 K is the same change as an increase of 1 °C

  • It is not possible to have a temperature lower than 0 K

  • This means a temperature in Kelvin will never be a negative value

  • To convert between temperatures θ in the Celsius scale, and T in the Kelvin scale, use the following conversion:

θ / °C = T / K − 273

T / K = θ / °C + 273

K and Celsius conversion chart, downloadable AS & A Level Physics revision notes

Conversion chart relating the temperature on the Kelvin and Celsius scales

  • The divisions on both scales are equal. This means:

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

Worked Example

The temperature in a room is 300 K.

What is this temperature in Celsius?

Answer:

Step 1: Kelvin to Celsius equation

θ / °C = T / K − 273

Step 2: substitute in value of 300 K

300 K − 273 = 27 °C

Examiner Tips and Tricks

If you forget in the exam whether it’s +273 or −273, just remember that 0 °C = 273 K. This way, when you know that you need to +273 to a temperature in degrees to get a temperature in Kelvin. For example:  0 °C + 273 = 273 K.

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Katie M

Author: Katie M

Expertise: Physics

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.