IGCSE §5: Solids, Liquids and Gases · IGCSE
The Kelvin Scale and Absolute Zero
Revision notes on The Kelvin Scale and Absolute Zero for the OCR A-level Physics specification (H556). Free to read, with 4 practice questions in the app.
Temperature is a measure of the average kinetic energy of the particles in a substance. Hotter means the particles are moving faster on average.
Absolute zero — the lowest temperature possible, at which the particles have the minimum possible kinetic energy and effectively stop moving. It is −273 °C.
Nothing can be colder, because there is no less kinetic energy than none.
The Kelvin scale starts at absolute zero, so it has no negative values:
T (K) = θ (°C) + 273
and going the other way, θ (°C) = T (K) − 273.
A kelvin is the same size as a degree Celsius, so a change of 10 °C is a change of 10 K. Only the zero point differs. This is why ΔT can be given in either unit in E = mcΔT, while an absolute temperature in a gas law must always be in kelvin.
Why the gas laws need kelvin. Pressure is proportional to absolute temperature, not to Celsius temperature. Doubling the Celsius temperature from 10 °C to 20 °C does not double the pressure — in kelvin those are 283 K and 293 K, a rise of barely 4%. Using Celsius in a gas law is the most reliable way to get a gas question completely wrong, and it is worth converting as the very first line of working every time.
Useful values:
| °C | K | |
|---|---|---|
| Absolute zero | −273 | 0 |
| Water freezes | 0 | 273 |
| Room temperature | 20 | 293 |
| Water boils | 100 | 373 |
Example: a gas at 27 °C is heated to 327 °C. In kelvin these are 300 K and 600 K, so the absolute temperature has doubled and the pressure at constant volume doubles too. In Celsius it looks like a twelvefold increase, which would give a badly wrong answer.
4 questions on The Kelvin Scale and Absolute Zero
Reading is not revising. This topic has 4 questions in the app — multiple choice, calculations, and written answers marked against the mark scheme, point by point. Miss one and it comes back later with its options reshuffled, so you have to know it rather than remember where the answer sat.
- Every topic of Edexcel International GCSE Physics (4PH1)
- Flashcards and every equation, linked to the topic it comes from
- Full mock papers in the real paper structure
20 minutes a day is free. No card, nothing to cancel — and the paid version never renews itself either.
Start The Kelvin Scale and Absolute Zero