14.2 Temperature scales
- Syllabus
- 9702–2028–2029
- Topic
- 14.2
- Level
- A2
Learning objectives
14.2.1A physical property that varies with temperature may be used for the• understand that a physical property that varies with temperature may be used for the measurement of temperature and state examples of such properties• Including the density of a liquid, volume of a gas at constant pressure, resistance of a metal, e.m.f. of a thermocouple14.2.2The scale of thermodynamic temperature does not depend on the property of• understand that the scale of thermodynamic temperature does not depend on the property of any particular substance14.2.3Convert temperatures between kelvin and degrees Celsius and recall that T /• convert temperatures between kelvin and degrees Celsius and recall that T / K = θ / °C + 273.1514.2.4The lowest possible temperature is zero kelvin on the thermodynamic• understand that the lowest possible temperature is zero kelvin on the thermodynamic temperature scale and that this is known as absolute zero
Calibrate a temperature-dependent physical property as a thermometer
A physical property that changes with temperature can be used as a thermometric property: calibrate measured property values against known temperatures, then infer an unknown temperature from its measured value after thermal equilibrium is reached.
| Official thermometric property | Typical instrument/condition |
|---|---|
| density of a liquid | density-based liquid thermometer; property must give a unique reading |
| volume of a gas at constant pressure | constant-pressure gas thermometer |
| resistance of a metal | metal/platinum resistance thermometer |
| e.m.f. of a thermocouple | thermocouple junction pair |
| Useful characteristic | Why it matters |
|---|---|
| monotonic, ideally near-linear | one property value maps unambiguously to temperature |
| reproducible and stable | calibration remains valid |
| sensitive over required range | small temperature changes are resolvable |
| low thermal mass / fast response when needed | reaches equilibrium without strongly disturbing or lagging the object |
A thermocouple's small sensing junction suits rapidly changing temperature. A bulky gas thermometer can be accurate for calibration but responds slowly and may disturb a small object. Water density is unsuitable over ranges where its variation is non-monotonic or one density corresponds to more than one temperature.
Do not replace the official liquid-density example with liquid volume, and do not omit 'constant pressure' for gas volume or 'metal' for resistance. Variation alone is insufficient without calibration, unique response and an appropriate range/time response.
Thermodynamic temperature is defined independently of any particular thermometric substance
The thermodynamic temperature scale is based on universal physical principles rather than a chosen material property; the kelvin is the SI unit.
A practical thermometer is calibrated to approximate this scale, but its raw property may be nonlinear or limited in range.
A gas, resistance and radiation thermometer can agree after calibration even though their measured properties differ.
The Celsius scale and a material’s expansion are convenient representations, not the fundamental definition of temperature.
Convert between kelvin and Celsius using T(K)=θ(°C)+273.15
Thermodynamic temperature T in kelvin relates to Celsius temperature θ by T=θ+273.15.
Kelvin is an absolute scale with the same degree size as Celsius but a different zero. Use kelvin in gas and thermodynamic equations unless instructed otherwise.
25 °C is 298.15 K; 0 °C is 273.15 K, not 0 K.
A temperature difference of 1 °C equals 1 K, but an absolute temperature of 1 °C is not 1 K.
Absolute zero is 0 K, the lower limit of the thermodynamic temperature scale
Absolute zero is zero kelvin, the lowest limit of thermodynamic temperature; it corresponds to −273.15 °C.
It is a limiting state, not simply “no motion” in every quantum description. Use it as the zero of the absolute scale.
Cooling from 300 K to 150 K halves the absolute temperature even though Celsius readings do not behave as a ratio scale.
Negative Celsius temperatures can be physically valid, but temperatures below 0 K are not reached in the ordinary thermodynamic scale.