Temperature feels obvious — hot versus cold — but what is it actually measuring? It’s really a readout of how fast a substance’s particles are jiggling. Everyday life uses Celsius; physics uses the kelvin, a scale that starts at the coldest temperature there is. The good news: the two scales share the same-sized degree, so switching between them is just adding or subtracting 273.
📚 What you need to know
Temperature is a measure of the average kinetic energy of a substance’s particles — hotter means faster-moving particles.
Two scales: Celsius (°C, everyday) and the thermodynamic (Kelvin) scale (K, used throughout physics).
Absolute zero = 0 K = −273 °C: the coldest possible temperature, where particles have essentially zero kinetic energy. Nothing can be colder, so kelvin is never negative.
Convert with T/K = θ/°C + 273 and θ/°C = T/K − 273.
The degree is the same size on both scales, so a temperature changeΔT is the same number in K and °C.
Anchors: water freezes 0 °C = 273 K, boils 100 °C = 373 K; body 37 °C = 310 K; deep space ≈ 2.7 K.
What temperature actually measures
Back on the states-of-matter page we said particles are always moving. Temperature is simply how fast, on average, they’re moving — more precisely, a measure of their average kinetic energy. Heat something up and you’re handing its particles more kinetic energy, so they jiggle and fly around faster. Cool it down and they slow.
Temperature is the particles’ average kinetic energy. At absolute zero they’re still; the hotter it gets, the faster they move.
Don’t confuse temperature with “amount of heat”. A swimming pool at 30 °C stores far more energy than a mug of tea at 90 °C, because it has vastly more particles. Temperature only tells you how fast each particle moves on average — not how many there are. It’s a speed-of-jiggle reading, not an energy total.
Two scales: Celsius and Kelvin
We measure that “jiggle level” on a scale — and there are two you need.
Celsius (°C) is the everyday scale, pinned to water: 0 °C where it freezes, 100 °C where it boils. Great for weather and cooking. The Kelvin scale (K) is the one physics runs on. Its degree is exactly the same size as a Celsius degree, but its zero is placed at absolute zero — the genuine bottom of the temperature scale — instead of at water’s freezing point.
The same temperatures on both scales. The kelvin and Celsius degrees are the same size; the scales are just shifted by 273.
Why does physics prefer kelvin? Because so many laws are proportional to absolute temperature, and a proportional graph must pass through zero. Double the kelvin temperature and you genuinely double the average kinetic energy; double the Celsius temperature and you don’t. That only works when zero truly means “no energy” — which is what the kelvin scale gives you.
Absolute zero — the bottom of the scale
Absolute zero is the lowest temperature possible: 0 K, which is −273 °C (−273.15 °C to be exact). It’s defined as the temperature at which particles have the least possible kinetic energy — essentially zero. Because you can’t remove energy that isn’t there, nothing can be colder, and that’s why a kelvin temperature is never negative.
In the lab you can get achingly close to absolute zero — within a few billionths of a kelvin — but never quite touch it; there’s always a sliver more energy to coax out. Even the emptiest patch of deep space sits at about 2.7 K, gently warmed by the leftover glow of the Big Bang.
Converting between the scales
Because the two scales differ only by where zero sits, converting is just a shift of 273:
Celsius ↔ KelvinT / K = θ / °C + 273θ / °C = T / K − 273
And here’s the payoff that trips people up: since the degree is the same size on both scales, a temperature change is the same number either way. A rise of 60 °C is a rise of 60 K — you never add 273 to a change.
Identical tick spacing, zeros offset by 273: single temperatures shift by 273, but a ΔT reads the same on both scales.
Worked examples
WE 1
(a) Normal body temperature is 37 °C. Convert it to kelvin. (b) Liquid nitrogen boils at 77 K. Convert it to degrees Celsius.
(a) °C → K: add 273T = 37 + 273 = 310 K310 K(b) K → °C: subtract 273θ = 77 − 273 = −196 °C−196 °CA negative °C is fine — but notice both kelvin values (310, 77) stay positive. Kelvin never dips below zero.
WE 2
A copper block is heated from 20 °C to 80 °C. (a) What is the temperature change in °C? (b) In kelvin? (c) Check by converting both temperatures to kelvin first.
(a) Change in °C:ΔT = 80 − 20 = 60 °C(b) Same-sized degree, so the change is the same number:ΔT = 60 K(c) Convert first: 20 °C = 293 K, 80 °C = 353 KΔT = 353 − 293 = 60 K ✓This is why a temperature CHANGE never needs converting: 60 in °C is 60 in K. Keep it in mind for Q = mcΔT.
WE 3
Which is colder: 250 K or −30 °C?
Put both on the same scale. Convert −30 °C to kelvin (+273):−30 + 273 = 243 KCompare: 243 K versus 250 K−30 °C (243 K) is colderAlways convert to one scale before comparing — a minus sign can easily fool the eye.
🔧 Converting temperatures without slipping
Which way? Going to kelvin, +273. Going to Celsius, −273.
A single temperature: just add or subtract 273.
A temperature change (ΔT): don’t convert — the number is identical in K and °C.
Comparing or ordering: put everything on the same scale first.
Sanity check: kelvin can’t be negative — 0 K is the floor.
θ / °C everyday scale
+273 → ← −273
T / K physics scale
Quick recap: Temperature measures the average kinetic energy of particles — hotter means faster jiggling. Physics uses the kelvin, whose zero sits at absolute zero (0 K = −273 °C), the coldest possible temperature. Convert with T/K = θ/°C + 273. The degree is the same size on both scales, so a changeΔT is the same number in K and °C.
💡 Top tips
“Absolute temperature” means kelvin. Any law with “temperature proportional to…” (like KE ∝ T) needs kelvin, never °C.
Changes don’t convert.ΔT in °C = ΔT in K. Only single temperatures shift by 273.
Learn three anchors: 0 K = −273 °C, water freezes 273 K, water boils 373 K — place everything else from there.
Kelvin has no degree symbol. Write “300 K”, not “300 °K”.
Negative kelvin is a red flag. If a calculation gives it, you’ve slipped — nothing is colder than 0 K.
⚠ Common mistakes
Converting a temperature change by adding 273 — a change of 60 °C is 60 K, not 333 K
Writing “°K” — the kelvin has no degree symbol, just “K”
Using °C in a proportionality law (like KE ∝ T) instead of kelvin
Comparing temperatures on different scales without converting first
Getting the direction backwards — add 273 for kelvin, subtract for °C
You now know temperature is really a measure of how fast particles move, and that physics measures it from absolute zero on the kelvin scale. Next we make that link exact: there’s an equation tying the absolute temperature straight to the average kinetic energy of the particles — and it only ever works in kelvin. Coming up: temperature & kinetic energy.
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