IGCSE · 25 September 2026 · 6 min read
IGCSE Physics thermal explanations: particles, convection and cooling
Thermal physics often feels familiar because everyone has touched a cold spoon or watched steam above a cup. Familiarity can hide imprecise explanations. A complete answer must connect what is observed with the mechanism causing it, while keeping temperature, internal energy and energy transfer distinct.
The original questions in this guide practise those connections. Some ask for qualitative comparisons; one uses a calculation to clarify why equal energy transfers do not always produce equal temperature rises. The aim is to build an explanation that still works when the material or setting changes.
Keep temperature separate from energy transferred
Temperature is related to the average kinetic energy of particles, while internal energy includes the microscopic kinetic and potential energies of the particles in a system. A large quantity of warm water can have a greater internal energy than a tiny quantity of hotter water. Temperature alone does not tell you the total energy stored.
Heating describes an energy transfer caused by a temperature difference. In a simple situation, net thermal energy transfer is from the hotter object to the cooler one until they reach the same temperature. Avoid saying that an object contains heat as though heat were a material substance. State which object loses energy, which gains it and how the transfer occurs.
Worked explanation: why a metal spoon feels colder than wood
A metal spoon and a wooden spoon have been in the same room long enough to reach room temperature. Why can the metal feel colder? Your hand is warmer than both. Metal conducts thermal energy away from the contact region more rapidly, so energy leaves your skin faster. The sensation reflects that faster transfer; it does not establish that the metal initially had a lower temperature.
In a metal, mobile electrons transfer energy through the material as well as energy being transferred through interactions between particles. In a solid, particles vibrate about positions; they do not travel from the hot end to the cold end as a bulk flow. Naming conduction alone is incomplete when the question asks why the materials feel different: compare the rate of transfer and its effect on the skin.
Worked explanation: a convection current above a heater
Air near a heater gains energy and expands under approximately constant atmospheric pressure. The same mass occupies a greater volume, so its density decreases. The surrounding cooler, denser air exerts a resultant upward buoyant force on the warmer region. Warm air rises and cooler air moves in to replace it, establishing a circulation that transfers energy through bulk fluid movement.
The statement heat rises misses the mechanism: the warmed fluid rises in this arrangement. Thermal radiation can travel in any direction, and conduction can transfer energy downwards through a solid. Convection needs a fluid that can move, which is why the bulk process does not occur through a solid block even though conduction can occur there.
Explain cooling by evaporation without invoking cold particles
Particles in a liquid have a range of energies. Some near the surface have enough energy to escape the attractive interactions and leave as vapour. The escaping particles tend to have higher energies, so the average kinetic energy of the particles remaining in the liquid decreases if that energy is not replaced quickly enough. The liquid's temperature can therefore fall.
A larger exposed surface can increase the rate of evaporation because more particles are at the surface. Moving air can remove vapour from above that surface, allowing continued evaporation. Evaporation occurs below the boiling point and at the surface; boiling takes place throughout a liquid when vapour bubbles form under the appropriate pressure conditions. Do not describe the two as the same process at different speeds.
Worked calculation: equal energy does not mean equal temperature rise
Two samples each have mass 0.20 kg and receive 1800 J, with losses neglected. Sample A has specific heat capacity 900 J/(kg °C); sample B has 450 J/(kg °C). Using ΔT = E / mc, A rises by 1800 / (0.20 × 900) = 10°C, while B rises by 1800 / (0.20 × 450) = 20°C. The lower specific heat capacity gives the larger temperature rise for the same mass and energy.
This model applies when no change of state occurs over the temperature interval. During a pure substance's change of state at constant pressure, energy can change particle potential energy while temperature remains approximately constant. A flat part of a heating graph therefore does not mean the heater has stopped transferring energy; it means the energy is affecting the material differently.
Independent practice: compare surfaces in a cooling experiment
Two otherwise identical containers hold equal masses of water at the same starting temperature. One has a dull black outer surface and the other a shiny silver surface. In cooler surroundings, which is the better thermal radiator, and what must the experiment control? The dull black surface is the better emitter of infrared radiation. Keep material, shape, exposed area, water mass, starting temperature and surrounding conditions comparable.
Convection and evaporation may also transfer energy, so an experiment should control those conditions rather than attributing every difference to radiation automatically. A good answer names the mechanism, compares the surfaces and identifies relevant controls. Mark your explanation against those three jobs, then write it again for a different context such as insulation or a vacuum flask to check that the reasoning transfers.
Questions, explained
Choose a question for a direct answer, then explore the explanation and supporting resources. Each answer has its own link to save or share.
Why is saying heat rises not a complete physics explanation?
In convection, warmed fluid can expand, become less dense and rise through surrounding denser fluid. The fluid's bulk movement carries energy. Heat itself is not a substance that must move upwards: radiation travels in different directions and conduction can transfer energy down a solid. Name the fluid and link expansion, density and motion to explain the observation.
How does evaporation cool a liquid?
Particles have a range of energies, and higher-energy particles are more likely to escape from the surface. Their departure reduces the average kinetic energy of the particles remaining, so the liquid can cool unless energy is supplied quickly enough to replace the loss. The escaping vapour does not remove cold particles; it carries energy away from the liquid.
Why can temperature stay constant while a substance is heated?
During a change of state of a pure substance at constant pressure, transferred energy changes the microscopic potential energy associated with particle arrangements rather than increasing average kinetic energy. The temperature therefore remains approximately constant through the phase change. Energy is still being transferred, so a flat section on a heating curve should not be described as no heating.