How Temperature and Pressure Are Linked (Absolute Temperature)

For a fixed mass of gas at constant volume, pressure is proportional to absolute temperature:

p ∝ T (constant volume)

Board note: Four of the five boards ask for this relationship qualitatively only — AQA (8463/8464), Edexcel (1PH0 14.13), OCR A (J249 P1.3b) and OCR B (J259 P6) all use the words “qualitative only” or equivalent: as temperature increases, pressure increases, because particles move faster and collide with the container walls more often and harder. On those four boards you never calculate with p ∝ T. Eduqas (3420U, Physics only, Higher tier) is the only board that treats it quantitatively — its Higher papers print pV/T = constant and T/K = θ/°C + 273 on the equation page. Edexcel also expects you to know absolute zero (14.14) and to convert between Kelvin and Celsius (14.15), but never inside a gas calculation.

Eduqas only: this means that if you double the absolute temperature (in Kelvin), you double the pressure — and for that calculation the temperature must be in Kelvin, not Celsius. The Kelvin scale and absolute zero below are examined by Edexcel and Eduqas; AQA, OCR A and OCR B do not examine them in this topic:

• Absolute zero (0 K = −273°C): The temperature at which particles have minimum kinetic energy and gas pressure would theoretically be zero.
• Conversion: T (K) = T (°C) + 273
• So 100°C = 373 K, and 0°C = 273 K

Why Kelvin? At −273°C, particles have the minimum possible kinetic energy — the least energy they can theoretically have. This is the lowest possible temperature — absolute zero. The Kelvin scale starts here, which is why pressure is directly proportional to Kelvin temperature.