Thermal Physics

Overview

This page is the current-syllabus map for Thermal Physics. The wiki uses two teaching routes so that the ideas can be learned in manageable stages:

• Thermal Physics A
• Thermal Physics B

These A/B labels are wiki organisation labels, not the official 2026 syllabus boundary. In syllabus 9749, official Topic 8 combines temperature with ideal gases and kinetic theory, while official Topic 9 combines specific heat capacity, specific latent heat, internal energy and the first law. You will therefore move between A and B when following the official sequence.

Core Ideas

The two routes emphasise different viewpoints:

• Thermal Physics A focuses on macroscopic thermal ideas such as temperature scales, thermal equilibrium, heat capacity, latent heat, calorimetry, heating curves, and thermal practicals.
• Thermal Physics B focuses on microscopic and energy-accounting ideas such as kinetic theory, ideal gases, internal energy and the first law. Its detailed thermodynamic-process and p–V-cycle material is retained as clearly labelled enrichment.

Both topics still use a few shared words such as temperature, heat, and internal energy, but they use them at different levels:

• Thermal Physics A focuses on measurement, energy accounting, and heating/change-of-state calculations
• Thermal Physics B focuses on particle explanations, gas models, and thermodynamic process reasoning

The 2026 syllabus core

Topic 8: Temperature and ideal gases

You should be able to:

• recognise that regions at the same temperature are in thermal equilibrium;
• explain how gas-law evidence leads to an absolute thermodynamic temperature scale;
• use $T/\mathrm{K}=\theta /{}^{\circ }\mathrm{C}+273.15$;
• use $pV=nRT$, $N=n{N}_A$ and $N_A=6.02\times 10^{23}\mathrm{mo}{\mathrm{l}}^{-1}$;
• state the ideal-gas kinetic-theory assumptions;
• derive $pV=\frac{1}{3}Nm⟨c^2⟩$ from molecular collisions and isotropy; and
• use $\frac{1}{2}m⟨c^2⟩=\frac{3}{2}kT$.

Topic 9: First law of thermodynamics

You should be able to:

• define and use specific heat capacity, $Q=mc\Delta T$;
• define and use specific latent heat, $|Q|=ml$;
• explain internal energy as the sum of random molecular kinetic and intermolecular potential energies;
• connect a temperature rise to an increase in internal energy; and
• use $\Delta U=Q+W_{\mathrm{on}}$ with a consistent sign convention.

Heat, temperature and internal energy are different

Temperature describes thermal state and is linked to mean random molecular kinetic energy. Heat is energy transferred because of a temperature difference; it is not stored in a body. Internal energy is energy stored microscopically in the system and is a state property.

How to read scope labels

• Core means explicitly required by the 2026 syllabus.
• Supporting means useful for understanding or practical transfer, but not a separately named syllabus outcome.
• Enrichment means beyond the explicit syllabus. It is retained for insight and must not displace core revision.

Exam Relevance

Use this hub to choose the correct revision route before answering a thermal physics question. If the question is about temperature scales, heat capacity, latent heat or calorimetry, begin with Thermal Physics A. If it is about ideal gases, kinetic theory, internal energy or the first law, begin with Thermal Physics B.

For 9749 exam preparation, prioritise the sections marked core in the A/B notes. Use supporting practical material for experimental reasoning and treat p–V cycles or extended thermodynamic-process formulae as enrichment unless the question explicitly supplies the needed model.

Where To Go

Thermal Physics A

Use Thermal Physics A for:

• temperature and thermal equilibrium
• thermometric properties
• Celsius and Kelvin scales
• heat capacity and specific heat capacity
• calorimetry and mixing
• latent heat
• heating curves
• electrical determination of thermal quantities

Main support notes:

• Thermal Measurement and Scales
• Heat Capacity and Latent Heat
• Thermal Practicals
• Thermal Physics A Common Exam Traps

Thermal Physics B

Use Thermal Physics B for:

• kinetic theory of matter
• internal energy
• changes of state from particle viewpoint
• ideal gases and gas laws
• ideal gas equation
• the pressure derivation and mean molecular kinetic energy
• first law of thermodynamics
• clearly labelled enrichment on thermodynamic processes and p–V diagrams

Main support notes:

• Kinetic Theory and Ideal Gases
• First Law and Thermodynamic Processes
• p-V Diagrams and Cycles
• Thermal Physics B Common Exam Traps

Legacy Notes

Some older broad notes still remain in the repo as bridge material during migration, for example:

• Temperature and Heat
• Thermal Properties of Matter
• Kinetic Theory of Matter
• Ideal Gases
• First Law of Thermodynamics
• Thermodynamic Processes
• Thermal Physics Common Exam Traps

These are no longer the main canonical structure.

Links

• Thermal Physics A
• Thermal Physics B
• Thermal Measurement and Scales
• Heat Capacity and Latent Heat
• Kinetic Theory and Ideal Gases
• First Law and Thermodynamic Processes
• p-V Diagrams and Cycles
• Thermal Practicals

Summary

Thermal physics is easiest to master by keeping three questions separate:

1. What describes the thermal state? Temperature and state variables.
2. How is energy stored microscopically? Internal energy.
3. How is energy transferred? Heating and work.

The wiki then uses two linked teaching routes:

• Thermal Physics A
• Thermal Physics B

Use the syllabus-core map above to decide what is examinable, and follow enrichment links only after the core reasoning is secure.