Absolute Zero (Edexcel IGCSE Physics)

Spec point — 5.16 · understand why there is an absolute zero of temperature which is –273 °C

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

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

Spec point — 5.17 · describe the Kelvin scale of temperature and be able to convert between the Kelvin and Celsius scales

• 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

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

Exam Hint 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.