# Thermal Physics

> CIE IGCSE Physics · CIE IGCSE Physics (0625)
> Source: https://www.owlsprep.com/study/cie-0625-u2-overview/
> Weight: 12-15% of total assessment across MCQ, theory and practical components

This unit explores the behaviour of matter when heated, from particle-level changes to macroscopic thermal transfer processes, core to understanding everyday phenomena and applied physics concepts tested in the 0625 exam.

**Prerequisites:** Basic particle model from pre-IGCSE science; [CIE IGCSE Physics Unit 1: General Physics](https://www.owlsprep.com/study/cie-0625-u1-overview/)

## Learning objectives

- Explain the kinetic particle model of solids, liquids and gases and predict gas behaviour under changes to pressure, volume and temperature
- Calculate thermal expansion effects and solve quantitative problems using the specific heat capacity formula
- Distinguish between melting, boiling and evaporation and interpret heating/cooling curve graphs for state changes
- Compare conduction, convection and radiation, and evaluate real-world applications and consequences of thermal transfer

## Unit at a Glance

We start with the foundational kinetic particle model to explain state changes and gas behaviour, before moving to quantifiable thermal effects including expansion and specific heat capacity. The second half of the unit covers the three modes of thermal transfer and their real-world uses and consequences, with frequent links to practical exam scenarios.

Work through the subtopics in the order below to build your knowledge sequentially:
- [Kinetic Particle Model, States of Matter and Gas Behaviour](https://www.owlsprep.com/study/cie-0625-u2-kinetic-particle-model-states-of/) — Covers the kinetic particle model for the three states of matter and relationships between gas pressure, volume and temperature.
- [Thermal Expansion and Specific Heat Capacity](https://www.owlsprep.com/study/cie-0625-u2-thermal-expansion-and-specific-heat/) — Explains linear and volume thermal expansion in solids, liquids and gases, plus calculations for specific heat capacity.
- [Melting, Boiling and Evaporation](https://www.owlsprep.com/study/cie-0625-u2-melting-boiling-and-evaporation/) — Distinguishes between melting, boiling and evaporation, and teaches interpretation of heating and cooling curve graphs.
- [Conduction and Convection](https://www.owlsprep.com/study/cie-0625-u2-conduction-and-convection/) — Explores the mechanisms of conduction and convection, including examples of these processes in everyday and industrial contexts.
- [Radiation and Consequences of Thermal Transfer](https://www.owlsprep.com/study/cie-0625-u2-radiation-and-consequences-of-thermal/) — Covers infrared radiation properties, factors affecting emission/absorption, and real-world consequences of thermal transfer for climate and engineering.

## Common pitfalls

- **Wrong:** Confusing evaporation and boiling as identical processes
  - Why it fails: Evaporation occurs only at the liquid surface at any temperature, while boiling is a bulk process that only happens at a substance's fixed boiling point.
  - Correct: Always reference the location of particle escape and temperature requirements when distinguishing the two processes in exam answers.
- **Wrong:** Trying to use a temperature-dependent 'gas law' to link pressure and volume
  - Why it fails: At IGCSE the only quantitative gas relationship is pV = constant for a fixed mass of gas at constant temperature (Extended); there is no volume-temperature or pressure-temperature equation to apply.
  - Correct: Only apply pV = constant when temperature is held constant; describe the effects of changing temperature qualitatively in terms of particle motion and collisions.
- **Wrong:** Assuming all metals conduct heat at the same rate
  - Why it fails: Conduction rate varies significantly by material, with metals like copper conducting far faster than iron in standard exam scenarios.
  - Correct: Check material thermal conductivity values if provided, or use standard exam conventions for common material properties.

## Cheatsheet

| Concept/Formula | Definition/Usage | Relevant Subtopic |
| --- | --- | --- |
| Kinetic Particle Model | Particles in solids vibrate in fixed positions; liquids slide freely; gases move randomly at high speed | Kinetic Particle Model, States of Matter and Gas Behaviour |
| $pV = \text{constant}$ (fixed mass, fixed temperature) | Boyle's Law for inverse gas pressure and volume relationship | Kinetic Particle Model, States of Matter and Gas Behaviour |
| $E = mc\theta$ | Formula for energy required to change the temperature of a mass $m$ with specific heat capacity $c$ | Thermal Expansion and Specific Heat Capacity |
| Energy for a change of state | During melting or boiling, energy input changes the state at constant temperature - it is used to overcome the forces between particles, not to raise the temperature | Melting, Boiling and Evaporation |
| Conduction | Thermal transfer via particle collision in solids, with no bulk material movement | Conduction and Convection |
| Matte black surfaces | The best emitters and best absorbers of infrared (thermal) radiation; shiny, light-coloured surfaces are poor emitters/absorbers and good reflectors | Radiation and Consequences of Thermal Transfer |

## What's next

Start your study of Thermal Physics with the foundational kinetic particle model, which underpins all other concepts in this unit. Once you have completed all subtopics in this unit, you will move on to the next core unit on Wave Physics, which covers light, sound and electromagnetic radiation tested across all 0625 exam papers.

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