Temperature
Temperature indicates how hot or cold an object is. In physics, temperature is understood as a measure of the average kinetic energy of the particles in a substance. In everyday life, temperature is measured in degrees Celsius, but there are also other scales such as Kelvin and Fahrenheit.
SI unit of temperature
The fundamental SI unit of temperature is the kelvin, with the symbol \( \large K \).
$$ \large 0\ \text{°C} = 273.15\ \text{K} $$
The Kelvin scale starts at absolute zero, where all molecular motion ceases. The temperature 0 K corresponds to \( \large -273.15\ \text{°C} \).
Derived scales
In practice, three temperature scales are most commonly used:
| Scale | Symbol | Relation | Note |
|---|---|---|---|
| Celsius | °C | \( \large T_{C} = T_{K} - 273.15 \) | Used in everyday life |
| Kelvin | K | \( \large T_{K} = T_{C} + 273.15 \) | SI unit (absolute temperature) |
| Fahrenheit | °F | \( \large T_{F} = \frac{9}{5}T_{C} + 32 \) | Used in English-speaking countries |
Examples
- Water freezes at \( \large 0\ \text{°C} = 273.15\ \text{K} = 32\ \text{°F} \).
- Water boils at \( \large 100\ \text{°C} = 373.15\ \text{K} = 212\ \text{°F} \).
- Room temperature is about \( \large 20\ \text{°C} = 293.15\ \text{K} = 68\ \text{°F} \).
Temperature scales
Here you can see different temperature scales and their values at the melting and boiling points of water:
| Scale | Melting point | Boiling point |
|---|---|---|
| Celsius | 0° | 100° |
| Kelvin | 273.15 | 373.15 |
| Fahrenheit | 32° | 212° |
| Rømer | 7.5° | 60° |
| Réaumur | 0° | 80° |
| Rankine | 491.67° | 671.67° |
| Delisle | 150° | 0° |
| Newton | 0° | 33° |
The historical scales are no longer used, but they show how different scientists attempted to describe temperature before the modern SI definition.