IB Physics Thermal Physics Questions: Exam Method

IB Physics thermal physics questions become much more manageable when you separate the situation into energy transfers, state changes, and fixed conditions. The most reliable method is to identify the system, determine what remains constant, select the relevant equation, convert every quantity to SI units, and then explain the result using particle behavior or energy conservation.

The current course does not use “Thermal Physics” as one official topic title. Most of the material appears in Theme B, The particulate nature of matter, particularly B.1 Thermal energy transfers, B.2 Greenhouse effect, B.3 Gas laws, and, at Higher Level, B.4 Thermodynamics. These areas are tested in recurring formats, so attempting questions and then watching complete worked solutions usually develops exam technique faster than repeatedly reading notes.

How thermal physics is examined in IB Physics

Under the course first assessed in 2025, Paper 1 contains Paper 1A multiple-choice questions and Paper 1B data-based questions. Paper 2 contains short-answer and extended-response questions, including questions that connect thermal physics with mechanics, electricity, climate models, or experimental analysis. The official IB specimen papers show the structure and expected style of these assessments.

Common thermal question formats include:

• Calculating energy using specific heat capacity or specific latent heat
• Interpreting heating, cooling, pressure-volume, or radiation graphs
• Applying the ideal gas equations under stated constraints
• Explaining conduction, convection, or thermal radiation using particles
• Calculating conduction rates or radiated power
• Analyzing planetary energy balance and greenhouse models
• Applying the first and second laws of thermodynamics at HL
• Evaluating assumptions, uncertainties, and sources of energy loss

The equations are not the main difficulty. Most lost marks come from selecting an equation before identifying the process, using Celsius in an absolute-temperature equation, overlooking a phase change, or failing to state why a variable remains constant.

A reliable method for answering thermal physics questions

1. Identify the system and the process

First decide what object or gas is being studied. A metal block, liquid, atmosphere, and ideal gas can exchange energy differently, so an answer that does not define the system can quickly develop sign or energy-accounting errors.

Then identify the process:

• Temperature change without phase change: use $Q = mc\Delta T$
• Phase change at constant temperature: use $Q = mL$
• Steady conduction: use the conduction-rate relationship
• Ideal-gas state change: use $PV = nRT$ or

For a multi-stage process, treat each stage separately. Heating ice, melting it, and then heating the resulting water requires three energy calculations rather than one.

2. State what remains constant

Gas-law questions are controlled by their constraints. If temperature is constant, $PV$ is constant for a fixed amount of ideal gas; if pressure is constant, $V/T$ is constant; and if volume is constant, $P/T$ is constant.

Do not infer a constant quantity merely because its value is not mentioned. Write the condition explicitly, such as fixed number of molecules, constant pressure, or constant volume. This both justifies the equation and reduces the chance of mixing incompatible gas laws.

3. Convert quantities before substituting

Use SI units unless the question clearly establishes another consistent system:

Quantity	Standard unit	Frequent trap
Mass	kg	Substituting grams into $Q = mc\Delta T$
Energy	J	Confusing energy with power
Power	W or J s⁻¹	Forgetting to multiply by time
Temperature	K	Using Celsius in gas or radiation laws
Temperature difference	K or °C	Either gives the same numerical change
Pressure	Pa	Leaving pressure in kPa
Volume	m³	Leaving volume in litres
Specific latent heat	J kg⁻¹	Treating it as total latent heat

A temperature difference of 20 °C equals 20 K, but an absolute temperature of 20 °C is 293 K. Gas laws, Stefan-Boltzmann calculations, and Wien’s displacement law require absolute temperature.

4. Show the physics before the arithmetic

Write the general relationship, substitute values with units, and then calculate. This allows method marks to remain available even if the final arithmetic is incorrect.

For an explanation question, use a physical chain rather than a vague statement. For example: particles in the hotter region have greater average kinetic energy; interactions with neighboring particles transfer energy; the resultant thermal energy transfer is toward the colder region.

5. Check the answer physically

Ask whether the sign, unit, and magnitude make sense. Heating a kilogram of water by several degrees should not require only a few joules, while a microscopic gas sample should not contain thousands of moles. If a gas is heated at constant volume, its pressure should rise rather than fall.

The RevisionDojo IB Physics data booklet resource is useful for practising equation selection under timed conditions. The aim is not simply to locate a formula, but to recognize the assumptions behind it.

Core question types and how to solve them

Specific heat capacity and latent heat

Use $Q = mc\Delta T$ when energy transfer changes temperature without changing phase. Use $Q = mL$ during melting, freezing, boiling, or condensation when the phase changes at constant temperature.

Suppose 0.20 kg of ice at 0 °C melts and the resulting water warms to 15 °C. Taking the specific latent heat of fusion as $3.34 \times 10^5$ J kg⁻¹ and the specific heat capacity of water as $4.18 \times 10^3$ J kg⁻¹ K⁻¹:

1. Melting: $Q_1 = mL = 0.20(3.34 \times 10^5) = 6.68 \times 10^4$ J

The recurring trap is applying $Q = mc\Delta T$ while the ice is melting. During the phase change, energy changes the particles’ arrangement and potential-energy contribution to internal energy rather than raising the temperature.

Heating power and efficiency

If a heater operates for time $t$, its supplied energy is $E = Pt$. If only a fraction $\eta$ reaches the substance, the useful energy is $\eta Pt$.

Questions may instead ask for efficiency:

$\eta = \text{useful energy output} / \text{total energy input}$

Energy losses to the container and surroundings mean that an experimental value of specific heat capacity may differ from a data-booklet value. A strong response names the direction of the effect rather than merely saying “heat was lost.” For example, if the calculation assumes all electrical energy heats the water, unaccounted energy loss can cause the calculated specific heat capacity to be too large.

Conduction, convection, and radiation

Be precise about the mechanism:

• Conduction transfers energy through interactions between neighboring particles; mobile electrons also contribute strongly in metals.
• Convection involves bulk motion in a fluid caused by density differences.
• Thermal radiation is electromagnetic radiation and does not require a medium.

For steady conduction through a uniform slab, the transfer rate depends on thermal conductivity $k$, cross-sectional area $A$, temperature difference, and thickness $\Delta x$. A thicker wall reduces the rate, while a larger area or temperature difference increases it.

Radiation questions often use $P = e\sigma AT^4$. Check whether the question asks for emitted power or net radiated power. For an object exchanging radiation with surroundings at temperature $T_s$, the net expression commonly involves $T^4$, not simply .

Ideal-gas questions

Choose between $PV = nRT$ and $PV = Nk_BT$ by checking whether the amount is given in moles or molecules. Keep pressure in pascals, volume in cubic metres, and temperature in kelvin.

Conceptual answers should connect macroscopic variables to molecular motion. Heating a fixed amount of gas at constant volume increases the particles’ average translational kinetic energy. Collisions with the container walls then occur with greater momentum change and usually greater frequency, increasing the pressure.

Targeted practice in the B.3 gas laws questionbank helps build the habit of identifying fixed variables before choosing a relationship.

Graph and data-based questions

Read both axes, units, scales, and any uncertainty bars before interpreting a graph. A gradient may represent power, cooling rate, or another derived quantity, while an area may represent energy or thermodynamic work depending on the axes.

For a cooling graph, a steep negative gradient means a high rate of temperature decrease. As the object approaches the surroundings’ temperature, the temperature difference becomes smaller, so the magnitude of the cooling rate generally decreases. Do not describe a changing gradient as a constant cooling rate.

Greenhouse effect and radiation models

B.2 questions combine thermal radiation with model evaluation. Students may calculate absorbed solar power, emitted infrared power, equilibrium temperature, emissivity, or albedo.

A planet intercepts sunlight over projected area $\pi R^2$ but emits across surface area $4\pi R^2$ in a simple uniform model. Confusing these areas creates a factor-of-four error. Explanations should state that greenhouse gases absorb and re-emit particular wavelengths of outgoing infrared radiation, rather than claiming that they simply “trap heat.” The RevisionDojo greenhouse effect notes provide further practice with planetary energy balance.

HL thermodynamics

B.4 Thermodynamics is HL-only in the current course. Questions can involve internal energy, work, heat transfer, $PV$ diagrams, entropy, and heat engines.

Sign conventions must be handled consistently. Define whether $W$ represents work done by the gas or on the gas before applying the first law. On a pressure-volume diagram, the area under a process curve represents work done by the gas during expansion under the usual convention; the enclosed area of a cycle gives net work.

Common thermal physics mistakes

Mistake	Better exam habit
Treating heat and temperature as synonyms	Describe heat as energy transferred because of a temperature difference
Using Celsius in an absolute law	Convert to kelvin before using gas or radiation equations
Using one equation across a phase change	Divide the process into heating and latent-heat stages
Confusing power and energy	Use $E = Pt$ and check for seconds
Omitting the fixed gas variable	State the constraint before selecting the gas law
Giving a one-word transfer mechanism	Explain using particle interactions, fluid motion, or radiation
Ignoring the surroundings in radiation	Decide whether emitted or net power is required
Copying excessive calculator digits	Round sensibly using the precision of the supplied data

The fastest way to improve your method

Start with a short review using the IB Physics revision notes, then attempt questions without looking at the solution. Mark exactly where your reasoning failed: process identification, equation choice, units, arithmetic, graph interpretation, or written explanation.

Next, watch the question worked through from the beginning. Thermal energy transfer videos and IB Physics video resources make the hidden decisions visible, including why one equation applies and another does not. Worked video solutions are particularly efficient because they demonstrate the sequence of reasoning that a markscheme alone may not reveal.

Then close the solution and complete the question again from a blank page. Use the Thermal Physics Questionbank or the broader IB Physics Questionbank to repeat the same question type with changed values and contexts. This attempt, review, and reattempt cycle is more effective for learning method than passively rereading a completed solution.

Conclusion

IB Physics thermal physics questions follow recognizable patterns involving energy accounting, phase changes, transfer mechanisms, gas constraints, radiation, graphs, and, at HL, thermodynamic processes. Secure answers come from defining the system, identifying the process and constants, using SI units, showing the governing relationship, and checking the physical meaning of the result.

RevisionDojo can support this method through topic notes, the Questionbank, data-booklet practice, and worked videos. For the most direct exam preparation, attempt thermal questions first and then use the per-question explanations and relevant video solutions to compare your reasoning with a complete method.

Sources and referenced URLs

• Official IB Physics specimen papers for first examinations in 2025
• Current IB Physics guide copy, first assessment 2025
• RevisionDojo Thermal Physics Questionbank
• RevisionDojo IB Physics Questionbank
• RevisionDojo B.1 thermal energy transfer videos
• RevisionDojo IB Physics video resources
• RevisionDojo IB Physics revision notes
• RevisionDojo IB Physics data booklet
• RevisionDojo B.3 gas laws questionbank
• RevisionDojo B.2 greenhouse effect notes