Temperature scales
| English | Chinese | Pinyin |
|---|---|---|
| thermometer | 温度计 | wēn dù jì |
| thermocouple | 热电偶 | rè diàn ǒu |
| thermodynamic | 热力学 | rè lì xué |
| kelvin | 开尔文 | kāi'ěrwén |
| absolute zero | 绝对零度 | jué duì líng dù |
| Celsius | 摄氏度 | shè shì dù |
How a thermometer 温度计 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.
Temperature scale lab
Kelvin index = Celsius index + 2.73
Slide Celsius temperature and see the Kelvin scale shift by 273.
Kinds of thermometer
- Anything that changes repeatably with temperature works:
- expansion of a liquid, resistance of a metal, e.m.f. of a thermocouple 热电偶, volume of a gas.
A thermometer measures temperature on a defined scale
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 热力学 (absolute) scale uses the kelvin 开尔文 (K) and depends on no single substance.
- Absolute zero 绝对零度 ($0\ \text{K}$) is the lowest possible temperature — least internal energy. Nothing is colder.
Absolute zero (0 K) is:
At 0 K a system has its least possible internal energy; nothing can be made colder.
Kelvin and Celsius 摄氏度
- $T/\text{K} = \theta/°\text{C} + 273.15$. 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.
Extrapolating the pressure-temperature line back to zero pressure gives absolute zero, about -273 degC
What is $27\ °\text{C}$ in kelvin?
$T/\text{K} = \theta/°\text{C} + 273 = 27 + 273 = 300\ \text{K}$.
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).
In gas-law calculations, the temperature must be in:
Gas laws use absolute temperature — kelvin. Using °C gives wrong answers.
Why the thermodynamic scale is special
- It does not depend on the property of any particular substance — it is defined from the behaviour of an ideal gas, with absolute zero and the triple point of water as its fixed points.
- A liquid-in-glass thermometer assumes its liquid expands uniformly with temperature. That is only approximately true, so it agrees with the thermodynamic scale exactly only at the points where it was calibrated.
- That is the exam's "explain why a liquid-in-glass thermometer does not measure thermodynamic temperature".
Choosing a thermometer
- Platinum resistance: resistance varies linearly and repeatably with temperature over a wide range — very accurate. But it has a large thermal capacity, so it is slow and cannot follow a rapidly changing temperature.
- Thermocouple: two metal junctions give an e.m.f.; tiny, fast, wide range, measures at a point — ideal for a changing temperature.
- Liquid in glass: cheap and simple; limited range, slow, and reads only where the bulb is.
Match each thermometer to its key feature.
A large thermal capacity makes the resistance thermometer accurate but slow; a thermocouple's tiny junction responds fast; the absolute scale is defined from ideal-gas behaviour, not a material.
Worked example: finding absolute zero, then converting
A fixed volume of gas has pressure $100\ \text{kPa}$ at $0\ °\text{C}$ and $136.6\ \text{kPa}$ at $100\ °\text{C}$. Then: a gas is heated from $27\ °\text{C}$ to $742\ °\text{C}$.
- Gradient: $\dfrac{136.6 - 100}{100} = 0.366\ \dfrac{\text{kPa}}{°\text{C}}$; pressure is a straight line in temperature.
- Extrapolate to zero pressure: $\theta = -\dfrac{100}{0.366} = -273\ °\text{C}$ — absolute zero.
- Convert: $27\ °\text{C} = 300\ \text{K}$; $742\ °\text{C} = 1015\ \text{K}$.
- Temperature change: $742 - 27 = 715\ °\text{C} = 715\ \text{K}$ — a difference is the same number on both scales.
- Check: absolute zero is where the pressure of an ideal gas would fall to zero; the gas would liquefy long before, which is why the point is found by extrapolation, not by cooling.
A gas is heated from $20\ °\text{C}$ to $95\ °\text{C}$. What is the temperature change, in K?
A change is the same number on both scales: $95 - 20 = 75\ °\text{C} = 75\ \text{K}$. Adding 273 would be wrong here.
Add $273$ to convert a Celsius temperature to kelvin, but a temperature difference is the same number on both scales — do not add $273$ to a $\Delta T$. A kelvin temperature can never be negative. And absolute zero is the temperature of minimum internal energy, not zero energy — "the molecules stop moving" is not an accepted answer.
Absolute zero is the temperature at which a substance has its ____ internal energy.
Not zero energy — the minimum possible. That is why "the molecules stop moving" is not accepted.
You've got it
- a thermometer uses any property that changes repeatably with temperature; the thermodynamic scale depends on no substance
- absolute zero $= 0\ \text{K} = -273.15\ °\text{C}$, the lowest possible temperature (minimum internal energy)
- $T/\text{K} = \theta/°\text{C} + 273$; a difference is the same in K and °C; use kelvin in gas laws