Temperature scales
How a thermometer works
- A thermometer needs something that changes with temperature.
- A liquid expands, a metal's resistance rises, a gas swells.
- Read that change and you read the temperature.
Kinds of thermometer
- Anything that changes repeatably with temperature works:
- expansion of a liquid, resistance of a metal, e.m.f. of a thermocouple, volume of a gas.
Practice
Select all the properties that can be used to measure temperature.
Any property that changes repeatably with temperature works. The fixed colour of the case does not.
The absolute scale
- The thermodynamic (absolute) scale uses the kelvin (K) and depends on no single substance.
- Absolute zero ($0\ \text{K}$) is the lowest possible temperature — least internal energy. Nothing is colder.
Practice
Absolute zero (0 K) is:
At 0 K a system has its least possible internal energy; nothing can be made colder.
Kelvin and Celsius
- $T/\text{K} = \theta/°\text{C} + 273.15$. So $0\ °\text{C} = 273\ \text{K}$.
- A kelvin and a Celsius degree are the same size, so a change of $1\ \text{K} = 1\ °\text{C}$.
- Always use kelvin in gas-law calculations.
Practice
What is $27\ °\text{C}$ in kelvin?
$T/\text{K} = \theta/°\text{C} + 273 = 27 + 273 = 300\ \text{K}$.
Practice
A temperature rise of 1 K is the same as a rise of 1 °C.
The two scales have the same size of degree — only their zero points differ (by 273.15).
Practice
In gas-law calculations, the temperature must be in:
Gas laws use absolute temperature — kelvin. Using °C gives wrong answers.
You've got it
Key idea
- a thermometer uses any property that changes repeatably with temperature
- absolute zero $= 0\ \text{K}$ is the lowest possible temperature
- $T/\text{K} = \theta/°\text{C} + 273$; use kelvin in gas laws