The Kelvin Scale
The Kelvin Scale
- Celsius is built around water. Physics needs a scale built around nothing at all — one that starts where molecular energy runs out.
Absolute zero
- Cool a gas and its pressure falls. Follow the straight line down and it reaches zero pressure at −273.15 °C — the same value for every gas. Below this, molecules would need less than their minimum possible energy, so no lower temperature exists.
- This point is : the temperature at which the energy of the particles is at its minimum. It is , equal to −273.15 °C (9702/41/O/N/25 Q4(a)).
- “Minimum”, not “zero”: quantum physics leaves the particles a small leftover energy even at 0 K, so mark schemes accept minimum internal energy and can reject zero energy.
The thermodynamic scale
Symbols
- = thermodynamic (absolute) temperature (K (kelvin))
- = the same temperature on the Celsius scale (°C)
- The starts at absolute zero and climbs in steps the same size as Celsius degrees. A change of 1 K is a change of 1 °C — only the zero point moves.
- Its selling point for examiners: it does not depend on the property of any particular substance. A mercury thermometer assumes mercury expands evenly; a resistance thermometer assumes resistance changes evenly. The thermodynamic scale is defined through energy itself, so no substance can bias it (9702/41/M/J/24 Q2(a)(ii)).
- Precision habit: use 273.15 when the question data has enough figures to care; 273 is accepted for ordinary 2–3 significant-figure work.
Worked example
Smallest case: one conversion
Liquid nitrogen boils at 77 K. What is this in °C?
- (2 s.f. data, so −196 °C is fine).
Answer
Common mistake
Temperature differences never need converting: and are the same statement. Adding 273 to a temperature difference is a classic way to wreck an otherwise correct calculation.