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Learn9702Paper 4 · A Level Structured QuestionsTemperatureThermal equilibrium and temperature scales
Temperature · Subtopic 1 of 2

Thermal Equilibrium and Temperature Scales

Thermal energy flows from hotter to colder regions until they reach the same temperature: thermal equilibrium. Any property that changes with temperature (length of mercury, resistance, thermocouple e.m.f.) can make a thermometer, but different properties don't agree between fixed points. The thermodynamic scale doesn't depend on any substance: its zero is absolute zero, and T/K=θ/°C+273.15T/\text{K}=\theta/°\text{C}+273.15.

Cambridge 9702 Paper 4Edexcel YPH11 WPH15AQA 7408 Paper 2≈ 30 minRevision 6 · updated 2026-10-27
Section 1 of 14

Learning objectives

  • Describe thermal equilibrium and the direction of energy transfer.
  • Explain how a thermometric property is used to measure temperature.
  • Convert between kelvin and degrees Celsius.
  • Explain absolute zero and why the thermodynamic scale is substance-independent.
Pause and think

A metal bench and a wooden bench sit outside all night. In the morning the metal feels much colder. Are they at different temperatures?

Section 2 of 14

Core idea

Start hereIs 40 °C twice as hot as 20 °C?

No. The Celsius zero is just where water freezes; molecules still move vigorously there. A true scale starts at the point of minimum molecular energy: absolute zero.

The big idea

Heat flows from hot to cold until thermal equilibrium. T/K=θ/∘C+273.15T/\text{K}=\theta/^\circ\text{C}+273.15. Thermometers use properties that vary with temperature.

1. Absolute zero

Pressure of a fixed volume of gas against temperature
−300−200−100100204060801001201401600temperature (°C)pressure (kPa)extrapolates to −273 °C
All gases extrapolate to zero pressure at the same temperature: absolute zero. This defines the Kelvin scale independently of any substance.
  • Absolute zero: minimum internal energy.
  • A change of 1 K equals a change of 1 °C.

2. Thermometers

  • Properties: liquid expansion, resistance, thermocouple e.m.f.
  • Thermocouple: small, fast, wide range.
  • Thermistor: sensitive over a narrow range.
Linear calibration
  1. θ=Xθ−X0X100−X0×100\theta=\frac{X_\theta-X_0}{X_{100}-X_0}\times100
    Fraction of the way from the 0 °C to the 100 °C value of property XX.
  2. assumes X varies linearly\text{assumes }X\text{ varies linearly}
    Different thermometers can disagree between fixed points.
Worked exampleResistance thermometer

100 Ω at 0 °C, 139 Ω at 100 °C; reads 120 Ω.

Goal
Temperature in °C and K.
Strategy
Linear calibration.
  1. θ=120−100139−100×100=51.3∘C\theta=\frac{120-100}{139-100}\times100=51.3^\circ\text{C}
    Fraction 20/39.
  2. T=51.3+273.15=324.4 KT=51.3+273.15=324.4\text{ K}
    Convert.
Answer51.351.3 °C; 324.4324.4 K.
✓ Check120 Ω is just over half way between 100 and 139 ✓.
Common misconceptionRatios in Celsius 20 °C → 40 °C is 293 K → 313 K: only ~7% more mean KE.

Summary

  • T=θ+273.15T=\theta+273.15.
  • Calibrate thermometers.
Key words
Thermal equilibrium
No net heat flow.
HistoryKelvin's absolute scale. In 18481848 William Thomson (later Lord Kelvin) proposed a temperature scale based on the efficiency of ideal heat engines, so it couldn't depend on any particular substance. Its zero, absolute zero, lies at −273.15-273.15 °C.
Fun factThe coldest place. Physicists have cooled atoms to below 11 nanokelvin, a billionth of a degree above absolute zero. That's far colder than deep space, which sits at about 2.72.7 K thanks to the cosmic microwave background.
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