Thermal Physics
Particles, the three ways heat moves, specific and latent heat, and the gas laws, the section where "explain in terms of particles" is worth a third of the marks.
The kinetic model does the explaining
Everything in this section is answered with one picture: matter is made of particles in constant motion, and temperature measures their average kinetic energy. Solids: particles close, regular, vibrating about fixed positions. Liquids: close, irregular, sliding past each other. Gases: far apart, random, fast, colliding.
The evidence the paper wants you to cite is Brownian motion, smoke particles jittering under a microscope because they are being struck unevenly by air molecules too small to see. The smoke particle is large and slow; the air molecules are small, fast and random.
Temperature scales: kelvin = celsius + 273. Absolute zero, 0 K or −273 °C, is where particles have minimum kinetic energy. Gas law calculations must be in kelvin; a temperature "doubling" from 20 °C to 40 °C has not doubled in any physical sense.
Conduction, convection, radiation, and the words that score
Conduction: vibrating particles pass energy to neighbours. In metals, free electrons carry it much faster, which is why metals are good conductors and why the answer "because it is a metal" is not enough. Say free electrons.
Convection: only in fluids. The fluid near the heat source expands, becomes less dense, rises; cooler denser fluid sinks to replace it, and a current circulates. The mark scheme wants the words expands, less dense, rises. "Heat rises" scores nothing. Heat is not a substance.
Radiation: infrared electromagnetic waves, needs no medium, so it is how the Sun reaches us. Dull black surfaces are the best absorbers and emitters; shiny silver the best reflectors and worst emitters. That is why a vacuum flask is silvered inside and out, and why the vacuum is there. It stops conduction and convection, which both need particles.
Specific heat capacity and latent heat
E = mcΔT. Specific heat capacity c is the energy to raise 1 kg by 1 °C. Water's is high, 4 200 J/(kg °C), which is why it is used in radiators and why coastal climates are mild, a lot of energy for a small temperature change.
E = mL. Specific latent heat is the energy to change the state of 1 kg with no temperature change. Fusion for melting, vaporisation for boiling. The flat sections on a heating curve are where this energy goes: it separates the particles (raises their potential energy) instead of speeding them up.
Two-stage problem: water at 20 °C to steam at 100 °C. Warm it: m × 4 200 × 80. Boil it: m × 2 260 000. Add. Students who skip the second stage lose the majority of the marks, because vaporisation is the bigger number by far.
Evaporation is different from boiling: it happens at any temperature, from the surface only, and the fastest particles leave, so the average energy of what remains falls. That is cooling by evaporation, and it is faster with a larger surface, a higher temperature, or a draught.
The gas laws, three variables, hold one still
Gas pressure is caused by particles colliding with the walls. More collisions, or harder collisions, mean more pressure. That sentence explains every gas law.
Constant temperature (Boyle): pV = constant, so p₁V₁ = p₂V₂. Halve the volume, the same particles hit the walls twice as often, pressure doubles.
Constant volume: p/T = constant, T in kelvin. Heat the gas, particles move faster, hit harder and more often. A sealed can in a fire is dangerous for exactly this reason.
Combined: p₁V₁/T₁ = p₂V₂/T₂. Convert every temperature to kelvin before substituting. Gas at 300 K, 1 m³, 100 kPa taken to 600 K and 200 kPa: V₂ = (100 × 1 × 600) ÷ (300 × 200) = 1 m³, the pressure doubling exactly cancels the temperature doubling.
Explaining everyday thermal physics
The 4- and 6-mark questions are all of one kind: explain, in terms of particles or transfer. Learn the shape of the answer, not the specific object.
- Sea breeze. Land heats faster than sea (lower specific heat capacity). Air over land expands, less dense, rises. Cooler air from the sea moves in to replace it. At night it reverses.
- Freezer at the top of a fridge. Cold air is denser and sinks, so a convection current cools the whole cabinet.
- Bimetallic strip. Two metals expand by different amounts; the strip bends towards the one that expanded less. Used in thermostats to break a circuit.
- Expansion gaps in bridges and rails: solids expand when heated because particles vibrate further, not because they grow.
- Why sweating cools: evaporation removes the highest-energy particles.
Where marks go
- "Heat rises." Hot fluid rises because it is less dense. Say expands, less dense, rises.
- "Cold gets in." Cold is the absence of thermal energy; energy flows from hot to cold.
- Gas laws in celsius. Always kelvin. 27 °C is 300 K.
- Latent heat raises temperature. It does not, that is the definition.
- Evaporation confused with boiling. Any temperature, surface only, versus fixed temperature, throughout the liquid.
- Particles "expand" when heated. They vibrate more and take up more room. The particles themselves do not change size.
- Vacuum flask answers that miss the vacuum. Silvering handles radiation; the vacuum handles conduction and convection. Say both.
What to remember
- Temperature = average kinetic energy of particles. K = °C + 273.
- Conduction (free electrons in metals), convection (expands, less dense, rises), radiation (infrared; black absorbs, silver reflects).
- E = mcΔT for temperature change, E = mL for state change. Add the stages.
- Evaporation: surface, any temperature, fastest particles leave, so it cools.
- Gas pressure is collisions. p₁V₁/T₁ = p₂V₂/T₂, T in kelvin.
- Explain-why answers: name the mechanism, then the chain of consequences.
Watch one worked through
Here is one done slowly, with the reason for every move. Then one for you.
Why does the water at the top of a pan heat up when the flame is only at the bottom? Name the process and explain it.
- Water at the bottom is heated first and expands.heated water → expands → less denseSame mass in a bigger volume means lower density. That one line is the engine of the whole explanation.
- Less dense water rises through the denser water above.less dense → risesIt rises because the colder, denser water around it sinks and pushes it up, not because heat itself rises.
- Cooler water sinks to take its place and is heated in turn.cool water sinks → gets heated → rises = convection currentThe current is a loop. Describing it as a cycle rather than a one-way trip is what earns the marks.
- Name why it cannot be the other processes.conduction: water is a poor conductor convection: ✓ the fluid itself movesConvection needs a fluid that can flow, so it cannot happen in a solid. Conduction in water is far too slow to explain this.
Answer: Convection. Heated water expands, becomes less dense and rises; cooler denser water sinks to replace it, forming a current.
Which process transfers heat through a solid metal bar? One word.
Show the answer
conduction
Practise Thermal Physics
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