Celsius is what you’ll see on a weather app, but physics runs on a different scale entirely — one that starts at the coldest temperature the universe can possibly reach, and never dips below it.
📘 What you need to know
The Kelvin (thermodynamic) scale starts at absolute zero — the lowest possible temperature
Absolute zero is equal to 0 K, or −273 °C
A temperature in kelvin can never be negative
Absolute zero is defined as the temperature at which molecules have zero kinetic energy
A change of 1 K is exactly equal to a change of 1 °C — the scales share the same size of division
Absolute Zero and the Kelvin Scale
On the thermodynamic, or Kelvin, scale, absolute zero is the coldest temperature that can possibly exist. Since molecules at absolute zero have no kinetic energy left to lose, it’s simply not possible to cool a system any further once it gets there — there’s no more energy left to remove.
Even the depths of space aren’t quite at absolute zero — the background temperature of the universe sits at around 2.7 K, just a few degrees above it.
Converting Between Celsius and Kelvin
Kelvin to Celsiusθ⁄°C = T⁄K − 273
Celsius to KelvinT⁄K = θ⁄°C + 273
Kelvin and Celsius share the same size of division — only the zero point is shifted by 273
Quick recap: T/K = θ/°C + 273. Kelvin never goes negative, and a 1 K change always matches a 1 °C change.
WE 1
Average human body temperature is 37 °C. Express this temperature in kelvin.
A cold-storage sensor reads 194 K. Convert this to degrees Celsius, and comment on what this temperature is close to in everyday terms.
Step 1 — Recall the conversion
θ/°C = T/K − 273
Step 2 — Substituteθ = 194 − 273= −79 °CThis is close to the sublimation temperature of solid carbon dioxide (dry ice), around −78.5 °C.
💡 Top tips
If you’re unsure whether to add or subtract 273, check the data booklet — it’s given there as T/K = θ/°C + 273
A change in temperature, ΔT, can usually stay in °C without converting — it’s only absolute temperatures that must be in kelvin
If a calculated kelvin value comes out negative, that’s a strong signal a mistake has been made somewhere
Equations involving absolute temperature (like those linking temperature to particle kinetic energy) always need kelvin, never Celsius
⚠ Common mistakes
Adding 273 when converting from kelvin to Celsius, instead of subtracting it (or vice versa)
Forgetting to convert to kelvin before substituting into an equation that requires an absolute temperature
Assuming a temperature change in °C needs converting to K before use — the size of the change is identical in both scales
Treating 0 °C as though it were the coldest possible temperature, rather than 0 K
Up next: Temperature & Kinetic Energy — where we connect this temperature scale to the actual motion of particles.
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