IGCSE Physics · Unit 2: Thermal Physics · Core + Extended

Thermal Physics CoreExtended adds pV=constant, mechanisms & complex applications

This note covers the whole of Thermal Physics: how the kinetic particle model explains the states of matter and gas behaviour, how substances expand and store thermal energy, what really happens during melting/boiling/evaporation, and the three ways thermal energy can move from place to place — conduction, convection and radiation.

Why this matters Almost every idea in this unit comes back to one picture: tiny particles in constant, random motion, moving faster as temperature rises. Once that picture is solid, conduction, convection, radiation, expansion, and even gas pressure all become logical consequences of it rather than separate facts to memorise.

1. States of Matter

ArrangementSeparationMotion
Solidregular, fixed patternvery close togethervibrate about fixed positions
Liquidirregular, close togetherclose together, can slidemove around each other, sliding freely
Gasirregular, far apartfar apartmove rapidly in random directions

The changes between states have specific names: solid → liquid is melting; liquid → solid is solidification (freezing); liquid → gas is boiling/evaporation; gas → liquid is condensation. (Direct solid ↔ gas transitions are not required at this level.)

2. The Particle Model: Temperature, Pressure and Brownian Motion

As temperature increases, particles move faster (have more kinetic energy) and vibrate or move more vigorously. There is a lowest possible temperature, called absolute zero (−273°C), at which particles have the least possible kinetic energy.

Gas pressure arises from gas particles colliding with the walls of their container. Each collision exerts a tiny force; the combined effect of billions of collisions per second produces measurable pressure. Extended: more precisely, this is described as particles colliding with a surface, exerting a force, and hence a pressure (force per unit area).

Brownian motion The random, jittery motion of microscopic particles suspended in a fluid (e.g. smoke particles in air, viewed under a microscope) is evidence for the kinetic particle model of matter. This motion happens because the much smaller, faster-moving molecules of the surrounding gas or liquid are constantly and randomly colliding with the microscopic particles from all directions — since these collisions are unequal and random at any instant, the microscopic particle is knocked about unpredictably. Extended: it is important to distinguish between the microscopic particle we can see (e.g. a smoke particle) and the individual atoms or molecules of the gas causing the collisions, which are far too small to see directly.
Try it — Particle Model
  1. Describe, in terms of particles, what happens to the average speed of gas particles as the gas is heated.
  2. Explain why smoke particles observed under a microscope appear to move in a random, jittery way.
  3. State what is meant by absolute zero.
Show answers
  1. The average speed of the gas particles increases, since they gain kinetic energy as the gas is heated.
  2. The much smaller, invisible air molecules are moving rapidly and randomly, constantly colliding with the smoke particles from all directions. Because these collisions are unequal and unpredictable at any given moment, the visible smoke particles are knocked in random directions, producing the jittery motion observed — this is Brownian motion.
  3. Absolute zero (−273°C) is the lowest possible temperature, at which particles have the least possible kinetic energy.

3. Gases and the Absolute Scale of Temperature

For a fixed mass of gas: increasing the temperature at constant volume increases the pressure (particles move faster, hitting the walls harder and more often). Decreasing the volume at constant temperature increases the pressure (particles hit the walls more often because there is less space to travel between collisions).

T (in K) = θ (in °C) + 273
Worked example — converting between scales Convert 27°C to kelvin, and convert 373 K to °C.
27°C → T = 27 + 273 = 300 K
373 K → θ = 373 − 273 = 100°C
pV = constant Extended For a fixed mass of gas at constant temperature: pV = constant. This means pressure and volume are inversely proportional — compressing a gas to half its volume (at constant temperature) doubles its pressure. Graphically, a plot of p against V for constant temperature is a curve (specifically, p ∝ 1/V), while a plot of p against 1/V is a straight line through the origin.
Worked example — using pV = constant A gas has a volume of 0.8 m³ at a pressure of 150 kPa. Find its new pressure if it is compressed to a volume of 0.3 m³ at the same temperature.
p₁V₁ = p₂V₂
150 × 0.8 = p₂ × 0.3
120 = 0.3p₂ → p₂ = 400 kPa
Try it — Gas Behaviour
  1. Convert 15°C to kelvin.
  2. Explain, in terms of particles, why the pressure of a fixed mass of gas increases if it is heated at constant volume.
  3. A gas at a pressure of 200 kPa occupies 0.5 m³. Find its pressure if the volume is increased to 1.25 m³ at constant temperature.
Show answers
  1. T = 15 + 273 = 288 K
  2. Heating the gas increases the average kinetic energy (and speed) of its particles. Faster particles collide with the container walls more frequently and with greater force, increasing the pressure.
  3. p₁V₁ = p₂V₂ → 200 × 0.5 = p₂ × 1.25 → 100 = 1.25p₂ → p₂ = 80 kPa

4. Thermal Expansion

Solids, liquids and gases all expand when heated at constant pressure (their particles move further apart on average as they vibrate/move more vigorously). Gases expand the most, then liquids, then solids the least. Extended: this order arises because gas particles are already far apart and only weakly attracted to each other (so extra motion easily pushes them further apart), liquid particles are close together with moderate attractions, and solid particles are held tightly in place by strong attractive forces, so an increase in vibration produces only a small increase in average separation.

Everyday applications and consequences Bimetallic strips (used in thermostats) bend because the two metals expand at different rates; gaps are left in bridges and railway tracks to allow for expansion on hot days; a tight metal jar lid can be loosened by running it under hot water, since the metal expands more than the glass.

5. Specific Heat Capacity

A rise in the temperature of an object increases its internal energy. Extended: more precisely, this corresponds to an increase in the average kinetic energy of all the particles in the object.

Specific heat capacity is defined as the energy required per unit mass per unit temperature increase.

c = ΔE / (mΔθ)
Worked example — calculating energy needed Find the energy needed to raise the temperature of 2 kg of water by 15°C. (Specific heat capacity of water = 4200 J/(kg°C))
ΔE = mcΔθ = 2 × 4200 × 15 = 126000 J (126 kJ)
Worked example — finding specific heat capacity A 0.5 kg block of metal absorbs 9000 J of energy, and its temperature rises from 20°C to 60°C. Find its specific heat capacity.
Δθ = 60 − 20 = 40°C
c = ΔE / (mΔθ) = 9000 / (0.5 × 40) = 9000 / 20 = 450 J/(kg°C)
Experiment to measure specific heat capacity Extended For a solid: insert an electrical heater and a thermometer into a block of known mass. Record the starting temperature, then switch on the heater for a measured time, noting the current, voltage (to calculate electrical energy supplied, E = IVt) and the final temperature. Calculate c using c = ΔE/(mΔθ).
For a liquid: heat a known mass of liquid in an insulated container using an immersion heater, recording the same measurements, and calculate c the same way. Insulation and a lid help reduce heat loss to the surroundings, which would otherwise make the calculated c too high.
Try it — Specific Heat Capacity
  1. Find the energy needed to raise the temperature of 3 kg of a substance by 25°C, given its specific heat capacity is 900 J/(kg°C).
  2. A 2 kg block absorbs 16000 J of energy and its temperature rises by 20°C. Find its specific heat capacity.
  3. Explain why insulating the container is important in an experiment to measure specific heat capacity.
Show answers
  1. ΔE = mcΔθ = 3 × 900 × 25 = 67500 J
  2. c = ΔE/(mΔθ) = 16000/(2 × 20) = 16000/40 = 400 J/(kg°C)
  3. Without insulation, some of the energy supplied is lost to the surroundings rather than raising the temperature of the substance being tested. This would mean less of the measured energy actually went into the substance than assumed, leading to an inaccurate (too high) calculated value of specific heat capacity.

6. Melting, Boiling and Evaporation

Melting and boiling both involve energy being put into a substance without a change in temperature — the energy instead goes into breaking the forces holding particles in their fixed arrangement (melting) or separating them completely (boiling). Water melts at 0°C and boils at 100°C at standard atmospheric pressure.

Evaporation is the escape of more-energetic particles from the surface of a liquid, and it happens at any temperature (not just at the boiling point). Because it is the fastest, most energetic particles that escape, the average kinetic energy (and hence temperature) of the remaining liquid decreases — this is why evaporation causes cooling.

Boiling vs. evaporation Extended
BoilingEvaporation
Where it happensthroughout the whole liquidonly at the surface of the liquid
Temperatureonly at the boiling pointat any temperature
Speedrapid, with visible bubblesslow, no bubbles
Factors affecting evaporation, and the cooling effect explained Extended Evaporation happens faster when: temperature is higher (more particles have enough energy to escape); surface area is larger (more particles are at the surface, able to escape); and air movement over the surface is greater (escaped particles are carried away quickly, preventing them from returning to the liquid, which keeps the rate of escape high).
Explaining the cooling of an object in contact with an evaporating liquid: as the most energetic (fastest) particles escape from the liquid’s surface, the average kinetic energy of the particles remaining behind decreases. Since temperature is related to average particle kinetic energy, this drop means the liquid’s temperature falls — and since the object is in contact with the liquid, thermal energy is then transferred from the object into the now-cooler liquid, cooling the object too.
Try it — Melting, Boiling and Evaporation
  1. Explain why the temperature of water does not rise while it is boiling, even though energy is still being supplied.
  2. State two differences between boiling and evaporation.
  3. Explain why sweating helps cool the human body.
Show answers
  1. The energy supplied during boiling goes into overcoming the forces of attraction between particles, allowing them to separate and escape as a gas, rather than increasing their average kinetic energy — so the temperature stays constant at the boiling point until all the liquid has boiled away.
  2. Boiling happens throughout the liquid at a fixed temperature (the boiling point) and is rapid with visible bubbles; evaporation happens only at the surface, at any temperature, and is a slower process with no bubbles.
  3. Sweat is a liquid on the skin’s surface. As it evaporates, the most energetic water particles escape, leaving behind particles with lower average kinetic energy, which lowers the temperature of the remaining sweat. Since the sweat is in contact with the skin, thermal energy transfers from the skin into the cooler sweat, cooling the body.

7. Conduction

Good thermal conductors (e.g. metals) transfer thermal energy quickly; bad thermal conductors (insulators) (e.g. wood, plastic, air) transfer it slowly. This can be demonstrated by heating one end of several different rods (of the same size) simultaneously and observing how quickly heat reaches the far end (e.g. using wax to hold small objects in place, which fall off as the heat arrives).

Mechanism of conduction Extended In all solids, particles vibrate more vigorously when heated, and these vibrations pass on energy to neighbouring particles through the lattice — this is lattice vibration, and it happens in both metals and non-metals.
Metals are much better conductors because they also contain free (delocalised) electrons, which can move quickly through the metal, carrying kinetic energy with them and transferring it rapidly throughout the material — this is the dominant mechanism in metals, on top of lattice vibration.
Conduction is poor in gases and most liquids because their particles are far more spread out (or more loosely packed, in the case of liquids) than in a solid, so there is much less direct particle-to-particle contact to pass vibrations along.
Note that some solids conduct thermal energy better than typical insulators, but still less well than good conductors like metals — conduction ability exists on a spectrum, not just “good” or “bad.”

8. Convection

Convection is an important method of thermal energy transfer in liquids and gases (fluids) — it cannot happen in solids, since it relies on particles being free to move.

Explaining convection When a fluid is heated, the particles in that region gain kinetic energy and spread further apart, making that part of the fluid less dense than the surrounding, cooler fluid. This less dense, warmer fluid rises, while cooler, denser fluid sinks to take its place — setting up a circulating flow called a convection current. This can be demonstrated by placing a crystal of potassium permanganate in water and gently heating the water from below near the crystal: coloured streaks show the rising warm water and the circulating flow.

9. Radiation

Thermal radiation is infrared radiation, and all objects emit it, regardless of temperature (hotter objects simply emit more). Unlike conduction and convection, thermal radiation transfer does not require a medium — it can travel through a vacuum, which is how energy from the Sun reaches the Earth.

Surface colour and texture affect emission, absorption and reflection of infrared radiation: dull, black surfaces are good emitters and good absorbers of infrared radiation; shiny, white (or light-coloured) surfaces are poor emitters and poor absorbers, but good reflectors.

Experiments to compare surfaces Comparing emitters: two identical containers, one painted matt black and one shiny/silver, are filled with hot water at the same starting temperature. A thermometer in each records how quickly each cools — the black container cools faster, showing it is a better emitter.
Comparing absorbers: two identical thermometers (or metal plates) — one with a black surface, one with a shiny surface — are placed the same distance from a radiant heater. The black one heats up faster, showing it is a better absorber.
Radiation, emission rate, and Earth’s temperature Extended The rate at which an object emits radiation depends on its surface temperature (hotter objects emit more, and more strongly, per second) and its surface area (a larger area emits more overall).
For an object to stay at a constant temperature, it must transfer energy away at the same rate that it receives energy. If an object receives energy faster than it transfers it away, its temperature rises; if it transfers energy away faster than it receives it, its temperature falls.
The Earth’s temperature depends on this balance between incoming radiation from the Sun and radiation emitted back out from the Earth’s surface — factors that disturb this balance (such as an increase in greenhouse gases trapping outgoing radiation) can cause the Earth’s average temperature to change.
Try it — Conduction, Convection and Radiation
  1. Explain, in terms of particles, why metals are much better conductors of heat than most non-metal solids.
  2. Explain why convection cannot occur in a solid.
  3. Explain why a matt black teapot loses heat to its surroundings faster than an identical, highly polished silver teapot.
Show answers
  1. In metals, heat is transferred both by lattice vibrations passing energy between neighbouring particles (as in all solids) and by free, delocalised electrons, which move rapidly through the metal carrying kinetic energy with them. Most non-metals lack these free electrons, so they rely only on the slower lattice vibration mechanism, making them poorer conductors.
  2. Convection relies on particles being able to move freely from one place to another so that warmer, less dense regions of fluid can rise and cooler, denser regions can sink. In a solid, particles are fixed in position (only vibrating, not moving from place to place), so this circulating movement cannot happen.
  3. A matt black surface is a good emitter of infrared radiation, while a highly polished silver surface is a poor emitter. Since the teapot loses heat mainly by radiation from its surface, the matt black teapot emits thermal radiation much more effectively, so it cools down faster than the shiny one.

10. Consequences of Thermal Energy Transfer

Basic everyday applications Heating a kitchen pan: the metal base (a good conductor) transfers heat quickly from the stove to the food; a plastic or wooden handle (a poor conductor) stays cool enough to hold.
Heating a room by convection: a radiator heats the air next to it, which becomes less dense and rises; cooler air sinks to take its place, setting up a convection current that circulates warm air around the whole room.
Complex applications (more than one transfer method) Extended A fire burning wood or coal: conduction transfers heat through the solid fuel and any metal grate; convection currents carry hot air and smoke upward and draw in fresh air to feed the flames; radiation from the flames and glowing embers warms nearby objects and people directly, without needing a medium.
A radiator in a car (the engine cooling system): conduction transfers heat from the hot engine block into the coolant liquid; convection (assisted by a pump) circulates the heated coolant to the radiator; conduction then transfers heat from the coolant, through the radiator’s metal fins, to the air; and that heated air is carried away partly by convection and partly by air forced through by the car’s motion or a fan.
Common mistakes to watch for
  • Forgetting to convert Celsius to kelvin before using a gas-law equation like pV = constant in combination with temperature changes.
  • Thinking temperature and internal/thermal energy are exactly the same thing — temperature relates to the average kinetic energy of particles, not the total energy of an object.
  • Believing a substance’s temperature rises throughout melting or boiling — it stays constant at the melting/boiling point until the change of state is complete.
  • Confusing evaporation with boiling — evaporation happens only at the surface, at any temperature, while boiling happens throughout the liquid, only at the boiling point.
  • Forgetting that thermal radiation is the only one of the three transfer methods that can travel through a vacuum (no medium needed).
  • Assuming dull black surfaces only absorb well — they are also the best emitters, which is why they cool down fastest as well as heating up fastest.

11. Quick-Fire Challenge Round

True or False?
  1. Evaporation only happens at a liquid’s boiling point.
  2. Thermal radiation requires a medium to travel through, just like conduction and convection.
  3. A dull black surface is both a good absorber and a good emitter of infrared radiation.
  4. Metals conduct heat well partly because of free, delocalised electrons.
Show answers
  1. False — evaporation can happen at any temperature, not just at the boiling point; only boiling is restricted to the boiling point.
  2. False — thermal radiation is the one transfer method that does NOT need a medium; it can travel through a vacuum, unlike conduction and convection.
  3. True — dull black surfaces are both good absorbers and good emitters of infrared radiation.
  4. True — free electrons move through the metal carrying kinetic energy, in addition to lattice vibrations, making metals much better conductors than most non-metals.
Spot the Error
  1. “A pan of water stays at 100°C while it boils because no more energy is being supplied to it.”
  2. “Convection can transfer heat through a solid metal bar just as well as through water.”
  3. “A gas is compressed to a third of its original volume at constant temperature, so its pressure decreases to a third of its original value.”
Show answers
  1. Energy is still being supplied while the water boils — the temperature stays constant because that energy is going into separating the particles (overcoming forces of attraction) to turn the liquid into gas, not into increasing the particles’ kinetic energy/temperature.
  2. Convection requires particles to move freely from place to place, which can happen in liquids and gases but not in solids (where particles are fixed in position, only vibrating). Heat transfer through a solid bar happens by conduction, not convection.
  3. By pV = constant, decreasing the volume to a third INCREASES the pressure, it doesn’t decrease it — specifically, the pressure would increase to three times its original value (p₁V₁ = p₂V₂, so if V₂ = V₁/3, then p₂ = 3p₁).

12. Mixed Practice — Bringing It All Together

  1. Convert 45°C to kelvin.
  2. Find the energy needed to raise the temperature of 1.5 kg of a liquid by 30°C, given its specific heat capacity is 2500 J/(kg°C).
  3. Explain why a person feels cold when stepping out of a swimming pool, even on a warm day, before they dry off.
  4. A fixed mass of gas has a volume of 0.6 m³ at a pressure of 100 kPa. Find its new pressure if compressed to 0.2 m³ at constant temperature.
  5. Explain, referring to more than one method of thermal energy transfer, how heat moves from a lit candle flame to warm a person’s hands nearby.
Show answers
  1. T = 45 + 273 = 318 K
  2. ΔE = mcΔθ = 1.5 × 2500 × 30 = 112500 J
  3. Water on the person’s skin evaporates. The most energetic water particles escape first, lowering the average kinetic energy (and temperature) of the remaining water, which then draws thermal energy away from the skin, cooling the person.
  4. p₁V₁ = p₂V₂ → 100 × 0.6 = p₂ × 0.2 → 60 = 0.2p₂ → p₂ = 300 kPa
  5. Conduction and convection carry heat through and away from the candle’s wax and the surrounding air, but the direct warmth felt on the hands mainly comes from thermal radiation (infrared) emitted by the flame, which travels directly through the air (and would even travel through a vacuum) to be absorbed by the skin.

Practice at Home

A large unit, so a large set — take it steadily across a few sittings. Parts C, D and E include Extended-only content.

Part A — States of matter and the particle model
  1. Describe the arrangement, separation and motion of particles in a gas.
  2. Explain, in terms of particles, why gas pressure increases when a fixed mass of gas is heated at constant volume.
  3. Explain what Brownian motion is evidence for.
Part B — Thermal expansion and specific heat capacity
  1. State the order (largest to smallest) in which solids, liquids and gases expand for the same temperature rise.
  2. Find the energy needed to raise the temperature of 4 kg of water by 10°C (specific heat capacity of water = 4200 J/(kg°C)).
  3. A 1 kg block absorbs 5000 J and its temperature rises by 12.5°C. Find its specific heat capacity.
Part C — Gas behaviour, melting, boiling, evaporation (Extended included)
  1. Convert 100°C to kelvin.
  2. A gas at 250 kPa occupies 0.4 m³. Find its pressure if the volume changes to 1.0 m³ at constant temperature.
  3. State two differences between boiling and evaporation.
  4. Explain how increasing air movement over a liquid’s surface increases its rate of evaporation.
Part D — Conduction and convection (Extended included)
  1. Describe an experiment to compare the thermal conductivity of different solid rods.
  2. Explain why metals conduct heat better than non-metallic solids, referring to free electrons.
  3. Explain, in terms of density, how a convection current is set up in a room heated by a radiator.
Part E — Radiation and consequences (Extended included)
  1. Explain why a matt black surface is a better emitter of infrared radiation than a shiny silver surface.
  2. Explain what it means for an object to be at a constant temperature, in terms of the rate at which it emits and receives radiation.
  3. Describe, referring to conduction, convection and radiation, how heat is transferred from a burning fire outward into a room.

Key Vocabulary Recap

Absolute zero — the lowest possible temperature (−273°C), where particles have least kinetic energy
Brownian motion — the random motion of microscopic particles caused by collisions with faster-moving molecules
Specific heat capacity — the energy required per unit mass per unit temperature increase (c = ΔE/(mΔθ))
Evaporation — the escape of the most energetic particles from a liquid’s surface, at any temperature
Conduction — thermal energy transfer through a material via particle vibrations (and free electrons in metals)
Convection — thermal energy transfer in fluids via circulating currents caused by density differences

Syllabus Reference

This note covers Cambridge IGCSE Physics (0625) section 2, Thermal physics, in full — 2.1 Kinetic particle model of matter (states of matter, particle model of temperature/pressure/Brownian motion, gases and the absolute temperature scale including pV = constant), 2.2 Thermal properties and temperature (thermal expansion, specific heat capacity, melting/boiling/evaporation), and 2.3 Transfer of thermal energy (conduction, convection, radiation, and the consequences of thermal energy transfer). Core and Supplement content are both included, with Extended-only material clearly flagged throughout.

Book a free trial class

Tell us a little about your requirement.