What is temperature?
Temperature is a number that tells how hot or cold something is.
Everything is made of tiny particles (atoms and molecules). They are always moving: in a gas they fly about, in a liquid they slide past each other, in a solid they shake in place. The faster they move, the more kinetic energy (energy of motion) they have.
Temperature measures the average kinetic energy of the particles. Hot tea has faster particles than cold water.
Heat and temperature are not the same
Heat is energy that moves from a hotter object to a colder one. It is measured in joules (J). Temperature is how hot something is, measured in °C or K. A bathtub of warm water holds much more heat energy than a cup of boiling water, even though the cup has the higher temperature.
Thermometers
A thermometer measures temperature. Most use something that changes in a regular way when it gets hotter.
- Liquid-in-glass thermometer: alcohol (coloured red) or mercury in a thin glass tube. When it warms, the liquid expands and rises up the tube.
- Clinical thermometer: reads 35–42 °C. It has a kink so the reading stays after you take it out of your mouth.
- Digital thermometer: a sensor whose electrical resistance changes with temperature.
- Infrared thermometer: measures the invisible heat rays a body gives off, without touching it.
To make a scale, we mark two fixed points: melting ice and boiling water (at normal air pressure), and divide the gap evenly.
Temperature scales: Celsius, kelvin and Fahrenheit
| Celsius (°C) | Kelvin (K) | Fahrenheit (°F) | |
|---|---|---|---|
| Absolute zero | −273 | 0 | −460 |
| Ice melts | 0 | 273 | 32 |
| Human body | 37 | 310 | 98.6 |
| Water boils | 100 | 373 | 212 |
Converting
- K = °C + 273 (more exactly 273.15)
- °C = K − 273
- °F = (9/5) × °C + 32 and °C = (5/9) × (°F − 32)
A change of 1 °C is the same size as a change of 1 K. A change of 1 °C equals a change of 1.8 °F. Kelvin is the SI unit, and we write "K", not "°K".
Absolute zero and thermal equilibrium
Absolute zero
If you keep cooling something, its particles move slower and slower. At −273.15 °C (0 K) they have the least possible energy. This is absolute zero. Nothing can be colder, so the kelvin scale has no negative numbers.
Thermal equilibrium
When a hot object touches a cold one, fast particles bump into slow ones and pass on energy. Heat flows from higher to lower temperature, never the other way by itself. It stops when both are at the same temperature: thermal equilibrium. This is why a thermometer must be left in place for a while: it has to reach the same temperature as the thing it measures.
The internal energy of an object can be raised in two ways: by heating it (heat transfer) or by doing work on it (rub your hands: they get warm).
Try it: feel the difference
Fill three bowls: cold, room-temperature and warm water. Put your left hand in cold and your right hand in warm for one minute. Then put both hands in the middle bowl. One hand says "warm", the other says "cold"! Our skin feels heat flow, not temperature. That is why we need a thermometer.
Key formulas and definitions
- K = °C + 273
- °C = K − 273
- °F = (9/5) × °C + 32
- °C = (5/9) × (°F − 32)
- Absolute zero = 0 K = −273 °C
Worked examples
1. Convert 27 °C to kelvin.
K = 27 + 273 = 300 K.
2. Liquid nitrogen boils at 77 K. What is this in °C?
°C = 77 − 273 = −196 °C.
3. Convert 37 °C (body temperature) to °F.
°F = 9/5 × 37 + 32 = 66.6 + 32 = 98.6 °F.
4. A US weather report says 86 °F. Is it hot? Convert to °C.
°C = 5/9 × (86 − 32) = 5/9 × 54 = 30 °C. Yes, a warm day.
5. The temperature rises from 15 °C to 40 °C. What is the rise in kelvin?
Rise = 40 − 15 = 25 °C. A 1 °C step equals a 1 K step, so the rise is 25 K.
6. At what temperature do the Celsius and Fahrenheit scales show the same number?
Set x = 9/5 x + 32 → −4/5 x = 32 → x = −40. So −40 °C = −40 °F.
Common mistakes
- Thinking heat and temperature are the same. Heat is energy that flows (J); temperature is how hot (°C or K).
- Writing "°K". The kelvin has no degree sign: 300 K.
- Converting °F to °C by subtracting 32 only, and forgetting to multiply by 5/9 (or doing them in the wrong order).
- Believing "cold" flows into things. Only heat flows, always from hot to cold.