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B.1.3—Temperature scales

Syllabus
First assessment 2025
Objective
—
Level
HL

Use Kelvin and Celsius Scales

Two temperature scales

Celsius is convenient for everyday temperature differences. Kelvin is the absolute thermodynamic scale used when temperature is linked to particle energy or radiation.

Convert between them

TK=θ∘C+273.15T_{\mathrm K}=\theta_{\circ\mathrm C}+273.15

So 0 °C = 273.15 K and 100 °C = 373.15 K. Kelvin is written without a degree symbol.

Choose the scale

Use Celsius when a question asks for a familiar temperature or a change described on the Celsius scale. Use Kelvin in equations such as Ek=32kBTE_k=\frac32k_BT, L=σAT4L=\sigma AT^4 and λmax⁡T=2.9×10−3 m K\lambda_{\max}T=2.9\times10^{-3}\,\mathrm{m\,K}.

Common trap

Never substitute a Celsius value directly into a formula that uses absolute temperature. Convert the temperature first.

B.1.3 Exam Analysis

Assessment in practice

1 marks
How it is assessed

The evidence includes a multiple-choice conversion and a one-mark structured conversion from Celsius to kelvin.

Command terms

Calculate / State / Determine

What earns marks

Use T(K)=θ(°C)+273.15 for an absolute temperature. Show the conversion and select the answer with the correct sign and scale; in a calculation, report kelvin when the question asks for absolute temperature.

Watch for

Using the same numerical value for Celsius and kelvin, or choosing a negative kelvin temperature.

Representative question

Question 1

[Maximum number: 1]

Calculate the temperature at C .

Synthesize B.1 Thermal Energy Transfers

Microscopic story

Matter contains moving particles. Temperature tracks average random kinetic energy, while internal energy also includes intermolecular potential energy. Phase changes alter particle behaviour at constant temperature.

Transfer story

A temperature difference gives the net direction of thermal energy transfer. Conduction transfers energy through local interactions, convection through moving fluids, and radiation through electromagnetic waves.

Equation map

ρ=mV\rho=\frac{m}{V}

Q=mcΔT,Q=mLQ=mc\Delta T,\quad Q=mL

ΔQΔt=kAΔTΔx\frac{\Delta Q}{\Delta t}=\frac{kA\Delta T}{\Delta x}

L=σAT4L=\sigma AT^4

b=L4πd2b=\frac{L}{4\pi d^2}

λmax⁡T=2.9×10−3 m K\lambda_{\max}T=2.9\times10^{-3}\,\mathrm{m\,K}

Question strategy

  1. Identify whether the question concerns a state property, a transfer mechanism or a rate.
  2. Convert to SI units and use kelvin whenever an absolute temperature appears.
  3. Check whether temperature changes, remains constant during a phase change, or enters a fourth-power/inverse-square relation.
  4. State the physical reason, not only the numerical substitution.