IB Physics thermal physics is built around a small set of connected ideas: particle motion, internal energy, thermal energy transfer, phase changes, ideal gases, and, at HL, thermodynamics. Exam questions test whether you can move between a microscopic particle model, macroscopic quantities such as pressure and temperature, and the equations that connect them.
Under the current course, first assessed in 2025, the relevant syllabus sections are B.1 Thermal energy transfers and B.3 Gas laws at SL and HL, plus B.4 Thermodynamics at HL only. The related B.2 Greenhouse effect applies thermal radiation to planetary energy balance, but it is formally a separate topic.
Where thermal physics appears in the current IB course
The official IB subject brief places these topics within theme B, The particulate nature of matter. Students sit Paper 1, comprising multiple-choice and data-based questions, and Paper 2, comprising short-answer and extended-response questions. Paper 1 contributes 36%, Paper 2 contributes 44%, and the scientific investigation contributes the remaining 20%.
Thermal physics can therefore appear as a rapid conceptual question, a graph or uncertainty problem, or a longer multi-step calculation. The official IB specimen papers show the style of the present assessment model.
| Syllabus section | Level | Central ideas |
|---|---|---|
| B.1 Thermal energy transfers | SL and HL | Internal energy, specific heat capacity, latent heat, conduction, convection and radiation |
| B.3 Gas laws | SL and HL | Ideal-gas model, pressure, moles, gas laws, kinetic theory and monatomic-gas internal energy |
| B.4 Thermodynamics | HL only | First and second laws, work, entropy, thermal processes and heat engines |
Temperature, thermal energy and internal energy
These terms are related but not interchangeable.
- Temperature measures how hot or cold a system is and, for an ideal gas, is proportional to the molecules' mean random translational kinetic energy.
- Internal energy is the total random kinetic energy of the particles plus their intermolecular potential energy.
- Thermal energy transfer, often labelled , is energy transferred because of a temperature difference. Energy passes spontaneously from a hotter body to a colder body until thermal equilibrium is reached.
For an ideal gas molecule,
The temperature must be in kelvin. A common multiple-choice trap is to substitute degrees Celsius into a gas-law or kinetic-theory equation. Use , following the precision expected from the data given.
Heating and changes of phase
When a substance remains in one phase, the transferred energy is
where is the specific heat capacity in . A large value of means more energy is needed to produce the same temperature increase.
During a phase change,
where is the specific latent heat in . Temperature remains constant during an idealized phase change because the supplied energy changes intermolecular potential energy rather than increasing average particle kinetic energy.
In a heating-curve question, divide the process into separate stages. Apply to sloping sections and (mL) to plateaux, then add the energies. Do not use one equation across the entire process.
Conduction, convection and radiation
Examiners frequently ask students to describe or explain a transfer mechanism. A strong response gives the mechanism, not merely its name.
- Conduction: energy is transferred through particle interactions; mobile electrons make metals particularly effective conductors.
- Convection: warmer fluid expands, becomes less dense and rises while cooler, denser fluid sinks, producing bulk circulation.
- Radiation: electromagnetic waves transfer energy and require no material medium.
For steady conduction through a uniform layer,
Radiated power is modelled using the Stefan-Boltzmann law, . If the surroundings also radiate toward the body, the net rate is . The fourth-power dependence makes kelvin conversion essential.
Gas laws and the particle model
The equation of state for an ideal gas is
Use SI units unless the question supplies a consistent alternative: pressure in pascals, volume in cubic metres and temperature in kelvin. Amount of substance is , where is molar mass.
An ideal gas is a model in which particles have negligible volume, move randomly, exert no intermolecular forces except during collisions, and undergo elastic collisions. Real gases approximate this behaviour most closely at low density, typically at low pressure and temperatures sufficiently far above condensation.
The empirical gas laws are special cases of the ideal-gas equation:
| Constant quantity | Relationship | Graphical implication |
|---|---|---|
| Temperature and amount | against is a decreasing curve | |
| Pressure and amount |
For a monatomic ideal gas, . Its internal energy depends only on absolute temperature, so an isothermal change has , even if heat enters or work is done.
HL thermodynamics and sign conventions
In B.4 Thermodynamics, the first law expresses energy conservation. Using the convention that is work done by the gas,
Thus, heat supplied to a gas is positive, and expansion work done by the gas is positive. Always check the convention established by the question or data booklet before assigning signs.
| Process | Fixed quantity or condition | Key result |
|---|---|---|
| Isovolumetric | constant | |
| Isobaric | constant | |
| Isothermal ideal gas |
On a pressure-volume diagram, work done by a gas equals the area under the process curve. For a complete cycle, the enclosed area gives net work; the direction determines its sign.
The second law explains why thermal processes have a preferred direction. The entropy of an isolated system does not decrease, and no cyclic heat engine can convert all absorbed thermal energy into useful work. For a reversible transfer at constant temperature, .
How examiners phrase thermal physics questions
Respond to the command term rather than writing everything you remember.
- State: give a concise fact, equation or value.
- Calculate: show substitution, maintain units and give a sensible final precision.
- Determine: obtain the result using the supplied information, which may include a graph.
- Explain: connect cause and effect using particle behaviour or an energy argument.
- Show that: present enough intermediate reasoning to establish the given result.
- Sketch: label axes and show the physically important shape, intercepts or gradients.
For example, “explain why the temperature remains constant while ice melts” requires reference to increasing intermolecular potential energy. Saying only that “latent heat is absorbed” names the phenomenon but does not explain it.
Common mistakes and an exam-focused method
The most frequent errors are using Celsius in absolute-temperature equations, confusing heat with internal energy, omitting a phase-change stage, mixing grams with kilograms, and reversing the thermodynamic work sign. Students also lose marks by describing convection as “heat rising”; the warmer fluid rises because its density changes.
Use this routine for calculations:
- Identify the system and physical process.
- Convert all values to compatible units.
- Select an equation from the process, not merely from familiar symbols.
- Substitute symbolically before evaluating.
- Check the sign, unit and physical plausibility.
After learning the theory, use the B.1 thermal energy transfer Questionbank and the broader IB Physics Questionbank. The thermal energy transfer video collection lets you watch worked methods being applied, while RevisionDojo's question pages provide per-question solutions where available. This is particularly useful for seeing how equations, diagrams and command terms convert into marks.
Conclusion
IB thermal physics becomes manageable when you organize it around energy: how particles store energy, how temperature changes, how energy crosses a boundary, and how gases exchange heat and work. SL and HL students need B.1 and B.3, while HL students must also master B.4 thermodynamics, entropy and heat engines.
Use the IB Physics resource hub to review Study Notes, then test each idea under exam conditions. RevisionDojo Questionbank practice, worked video solutions and Jojo AI feedback are most useful when you record why each error occurred and repeat the same question type later.
Sources and referenced URLs
- Official IB Physics subject brief, first assessment 2025
- Official IB Physics curriculum and assessment updates
- Official IB Physics specimen papers
- RevisionDojo IB Physics resource hub
- RevisionDojo IB Physics Questionbank
- RevisionDojo B.1 thermal energy transfer Questionbank
- RevisionDojo B.1 thermal energy transfer videos
