IGCSE Physics Thermal physics notes

The changes of state you must name: melting (solid → liquid), freezing or solidification (liquid → solid), boiling (liquid → gas throughout), condensation (gas → liquid), and evaporation (liquid → gas from the surface only). Transfers between gas and solid are not required.

Brownian motion is the evidence behind the model: microscopic particles (like smoke specks) suspended in air are seen to jitter randomly under a microscope. They are being knocked about by collisions with the fast-moving, invisible air molecules — light particles hitting something you can see. Supplement: the words atoms and molecules name the tiny particles of the gas; the smoke specks are microscopic particles, a different, much larger thing — keep the two ideas distinct.

Temperature measures the average kinetic energy of the particles. There is a lowest possible temperature, absolute zero (−273 °C), where particles have the least kinetic energy they can have. The kelvin scale starts there: T (in K) = θ (in °C) + 273, so 0 °C = 273 K and 100 °C = 373 K.

For a fixed mass of gas there are two everyday cases. Heating at constant volume: particles gain kinetic energy, so collisions with the walls are harder and more frequent — pressure rises. Compressing at constant temperature: the same speed of particles now hits the walls more often because the walls are closer — pressure rises again.

Supplement: at constant temperature the two quantities are locked together by pV = constant. Plotting p against V for a fixed mass of gas at constant temperature gives a smooth curve; plotting p against 1/V gives a straight line through the origin.

Thermal expansion: as temperature rises, solids, liquids and gases expand (at constant pressure). In a solid the particles vibrate harder about fixed positions and jostle slightly further apart on average — the object gets bigger without its particles growing. Liquids expand too (thermometers use mercury or alcohol); gases expand dramatically if free to do so. Applications and consequences: expansion gaps in bridges and railway lines, bimetallic strips in thermostats bending as they heat, overhead cables strung slack in summer so winter contraction does not snap them, and rivets fitted hot so they clamp tight as they cool.

Supplement: the relative sizes matter: for the same temperature rise, gases expand most, liquids less, solids least. In a gas the particles are free to rush apart, and the pressure pushes the volume out; in a solid the strong bonds between fixed particles resist the extra jostling.

Heating an object raises its internal energy — the total energy stored by its particles. Supplement: a temperature rise means the average kinetic energy of all the particles has increased. How much energy a given rise takes depends on the material’s specific heat capacity:

Supplement: to measure c, deliver a known energy to a known mass and record the temperature rise: for a metal block, an electric heater in a drilled hole, with V, I and t giving ΔE = IVt and a thermometer in a second hole; for a liquid, a heater in an insulated calorimeter cup, with insulation cutting energy lost to the surroundings. Both experiments should use an insulated container and stir liquids for an even temperature.

Melting and boiling take in energy without a change in temperature: the energy goes into breaking the arrangement, not into moving the particles faster. For water at standard atmospheric pressure the melting point is 0 °C and the boiling point is 100 °C. Condensation and solidification reverse the process: as particles lose energy they slow, the attractions pull them closer, and heat is released to the surroundings.

Evaporation is different: the most energetic particles escape from the liquid’s surface, leaving the slower, cooler ones behind — which is why evaporation cools what is left. Supplement: evaporation differs from boiling in three ways — it happens only at the surface, at any temperature below the boiling point, and without bubbles; boiling happens throughout the liquid at one fixed temperature. Evaporation speeds up when you raise the temperature (more fast particles), increase the surface area (more escape routes) or blow air across the surface (escaping particles are swept away instead of landing again).

Supplement: why does an evaporating liquid cool whatever it touches? The fastest particles leave, so the average kinetic energy — and the temperature — of the remaining liquid drops; the liquid then draws thermal energy from the warm object in contact with it. Sweating, a damp cloth on a fevered brow and the earthenware water cooler all work this way.

Conduction is thermal energy passing through a material without the material moving along with it. Classic demonstration: rods of different metals with wax-fixed balls at the ends, heated at one end — the balls drop in order of conductivity. Metals conduct best; wood, plastic and air barely conduct at all (insulators). Supplement: in a solid, faster-vibrating particles pass energy along the lattice by jostling their neighbours; in metals, free (delocalised) electrons zoom through carrying energy quickly — that is why metals beat non-metals. Gases and most liquids conduct badly: their particles are far apart, so the jostling handover is slow. (Many everyday solids conduct better than insulators but worse than copper — think stone or brick.)

Convection happens only in liquids and gases. Heat a fluid from below: the warmed patch expands, its density falls, and it floats upwards like a bubble while cooler, denser fluid sinks to replace it — a rolling convection current. Demonstrate with a potassium manganate(VII) crystal dropping in warmed water and trace the purple loop. Sea breezes, boiling water and home heating all run on convection.

Thermal radiation is infrared radiation, and every object emits it — no medium needed, which is how the Sun’s energy reaches us through space. Matt black surfaces are the best emitters and absorbers; shiny white or silvery surfaces are the worst (they reflect infrared). Experiments (Supplement): a Leslie-cube style cube with different faces and an infrared detector compares emitters at the same temperature; a dull black vs shiny plate facing a heater with corks stuck on by wax compares absorbers — the black plate’s cork drops first.

Supplement: an object stays at constant temperature when it emits energy at exactly the rate it absorbs it. Absorb more than you emit and you warm up; emit more than you absorb and you cool. The Earth’s temperature is set by the same balance between incoming solar radiation and infrared radiated from the surface — factors such as the atmosphere’s composition shift the balance and change the average temperature. Emission also grows with surface temperature and with surface area: a large hot radiator sheds energy faster than a small cool one.

Everyday applications: a kitchen pan conducts heat through its metal base to the food; a radiator warms a room by convection — hot air rises from it, cools, sinks and circulates; a wood or coal fire (Supplement) uses all three: radiation reaches you directly, convection carries heated air up the chimney and around the room, and conduction heats the poker left in the coals. A car radiator (Supplement) uses water’s high heat capacity to carry engine heat away by convection, then the fan and airflow remove it by conduction and radiation.