IGCSE Physics: thermal physics, particles, gases and heat transfer

Solids, liquids and gases in the particle model

Edexcel lists particle arrangement and motion as 5.10P, examined on Paper 2. Cambridge has it as Core 2.1.2.

Key ideas, board by board

Cambridge Core (2.1) covers the properties of the three states, particle diagrams, the link between particle motion and temperature, absolute zero at -273 °C, gas pressure from particle collisions, Brownian motion as evidence for the particle model, the effect on gas pressure of changing temperature or volume (qualitatively), and converting between kelvin and °C. Supplement adds pV = constant for a fixed mass of gas at constant temperature, with its graph. In 2.2, thermal expansion, melting and boiling without a change in temperature, the melting and boiling points of water, and evaporation causing cooling are Core; specific heat capacity (c = change in E / (m x change in temperature)) and the difference between boiling and evaporation are Supplement.

Heat transfer (Cambridge 2.3): Core covers experiments on good and bad conductors, convection from density changes, and thermal radiation as infrared that needs no medium, with the effect of black or white and dull or shiny surfaces. Supplement explains conduction by lattice vibrations and free electrons, why gases conduct badly, the balance between energy received and emitted (including the Earth's temperature) and how emission rate depends on surface temperature and area.

Edexcel 4PH1 places conduction, convection and radiation in the energy section (4.6 to 4.10) on both papers. Changes of state, particle arrangement, the temperature-time graph practical and specific heat capacity are 5.8P to 5.14P, so only on Paper 2. Gas content (5.15 to 5.22) is on both papers: random motion and pressure, absolute zero, the Kelvin scale, kinetic energy proportional to Kelvin temperature, and the relationships p1 / T1 = p2 / T2 and p1V1 = p2V2.

Worked example 1: specific heat capacity

Question: how much energy is needed to heat 2.0 kg of water from 20 °C to 70 °C? The specific heat capacity of water is 4200 J/(kg °C). How long would a 2.0 kW kettle take, assuming no energy is wasted?

Change in temperature = 70 - 20 = 50 °C. E = m x c x change in temperature = 2.0 x 4200 x 50 = 420 000 J (420 kJ).

Time = energy / power = 420 000 ÷ 2000 = 210 s, which is 3.5 minutes. In practice it takes longer, because some energy heats the kettle and the surrounding air.

Worked example 2: gas laws

Boyle's law (Cambridge Supplement, Edexcel 5.22): 120 cm^3 of gas at 100 kPa is compressed slowly at constant temperature to 40 cm^3. p1V1 = p2V2, so p2 = 100 x 120 ÷ 40 = 300 kPa. A third of the volume gives three times the pressure, because particles hit the walls more often.

Pressure and temperature (Edexcel 5.21): a sealed can contains gas at 150 kPa and 27 °C. It is heated to 127 °C. Convert to kelvin first: T1 = 27 + 273 = 300 K and T2 = 127 + 273 = 400 K. p2 = p1 x T2 / T1 = 150 x 400 ÷ 300 = 200 kPa. Using °C directly would give the wrong answer of about 706 kPa.

Common mistakes

• Using °C instead of kelvin in the pressure-temperature relationship.
• Saying particles expand when heated. The particles stay the same size; they move faster and further apart.
• Saying temperature rises while a substance melts or boils. It stays constant while energy breaks bonds between particles.
• Writing that cold "flows" into a room. Thermal energy moves from hotter to colder places.
• Saying convection happens in solids. Particles in a solid cannot flow.
• Saying shiny surfaces are good emitters. Dull black surfaces are the best emitters and absorbers; shiny white or silver surfaces are the worst emitters and best reflectors.
• Explaining gas pressure with "particles hitting each other" instead of particles colliding with the container walls.

Exam technique and mark-scheme language

Gas pressure explanations score well when they chain three ideas: higher temperature means particles move faster (more kinetic energy); they collide with the walls more often and with more force; so the force per unit area, the pressure, increases. For compression at constant temperature: same speed, smaller volume, more frequent collisions with the walls, so greater pressure.

For convection, the expected chain is: the fluid near the heater is heated, expands and becomes less dense, so it rises; cooler, denser fluid sinks to replace it, forming a convection current. For evaporation cooling (Cambridge), say that the more energetic particles escape from the surface, so the average kinetic energy of the particles left behind falls and the temperature falls. In practical descriptions of specific heat capacity, mention insulating the block or beaker, measuring energy with a joulemeter (or from power x time), and stirring liquids.

How one-to-one lessons help with this topic

Thermal physics questions are mostly explanations, and students often know the idea but leave out a link in the chain. A tutor asks the student to explain, for example, why a gas's pressure rises, marks the answer against mark points, and has them repeat until every step is there. The tutor also practises the specific heat capacity method and the gas law calculations, with the Kelvin conversion drilled until it is automatic.

Self-check

1. Draw particle diagrams for a solid, a liquid and a gas and describe the motion in each.
2. Explain gas pressure, and why it changes with temperature and volume.
3. Convert between °C and kelvin and state absolute zero.
4. Use E = m x c x change in temperature, and describe how to measure c.
5. Explain why temperature stays constant during melting and boiling.
6. Describe conduction, convection and radiation, with an example of each.
7. Explain which surfaces are good emitters and absorbers of infrared.
8. Use p1V1 = p2V2 (and, for Edexcel, p1 / T1 = p2 / T2).