# Thermodynamics

> CIE A-Level Physics · CIE 9702
> Source: https://www.owlsprep.com/study/cie-9702-u20-overview/
> Weight: n/a

This unit introduces core thermal physics concepts, from temperature definition to the first law of thermodynamics, and special processes for ideal gases, forming a foundation for energy-based physics problems.

**Prerequisites:** [Basic knowledge of ideal gas laws](https://www.owlsprep.com/study/cie-9702-u19-ideal-gases-overview/)

## Learning objectives

- Define core thermal concepts including temperature, thermal equilibrium, and specific thermal properties
- Calculate heat transfer for both temperature changes and phase transitions in common materials
- Apply the first law of thermodynamics to energy changes in thermal systems
- Distinguish between isothermal and adiabatic changes for ideal gases

## Unit at a Glance

This unit builds progressively from fundamental definitions of temperature to advanced applications of energy conservation in thermal systems. We start with the core concept of thermal equilibrium, then move to how substances store and transfer heat during temperature changes and phase transitions.

We conclude by applying energy conservation to gas processes with the first law of thermodynamics, and distinguish the two most common special processes you will encounter in CIE exam questions.

This unit is structured into the following sub-topics:
- [Temperature and thermal equilibrium](https://www.owlsprep.com/study/cie-9702-u20-temperature-and-thermal-equilibrium/) — Introduces the definition of temperature, thermal equilibrium, and how thermometers function.
- [Specific heat capacity](https://www.owlsprep.com/study/cie-9702-u20-specific-heat-capacity/) — Covers calculation of heat transfer for temperature changes and calorimetry experiments.
- [Specific latent heat](https://www.owlsprep.com/study/cie-9702-u20-specific-latent-heat/) — Explores heat transfer during phase changes, with calculation and experimental methods.
- [First law of thermodynamics](https://www.owlsprep.com/study/cie-9702-u20-first-law-of-thermodynamics/) — Introduces energy conservation for thermal systems linking heat, work, and internal energy.
- [Isothermal and adiabatic changes](https://www.owlsprep.com/study/cie-9702-u20-isothermal-and-adiabatic-changes/) — Distinguishes between two special ideal gas processes and their application to the first law.

## Common pitfalls

- **Wrong:** Confusing specific heat capacity with specific latent heat
  - Why it fails: Students often use the wrong formula when solving problems involving phase changes or temperature changes.
  - Correct: Always confirm if the process involves a temperature change (use $c$, specific heat capacity) or a phase change (use $L$, specific latent heat).
- **Wrong:** Mixing up sign conventions for the first law of thermodynamics
  - Why it fails: Different sources use conflicting conventions, leading to incorrect signs for work done.
  - Correct: Follow CIE convention: $\boxed{\Delta U = Q + W}$, where $W$ is work done *on* the system.
- **Wrong:** Confusing adiabatic and isothermal processes
  - Why it fails: Students often assume adiabatic processes have constant temperature by mistake.
  - Correct: Isothermal = constant temperature; adiabatic = no heat exchange (temperature always changes).

## Cheatsheet

| Concept | Key Formula/Rule |
| --- | --- |
| Heat transfer for temperature change | $Q = mc\Delta T$ |
| Heat transfer for phase change | $Q = mL$ |
| First law of thermodynamics (CIE) | $\Delta U = Q + W$ |
| Isothermal change | Constant $T \implies \Delta U = 0 \implies Q = -W$ |
| Adiabatic change | No heat exchange $\implies Q = 0 \implies \Delta U = W$ |

## What's next

Start with the first topic in this unit to build your understanding from core definitions, and work through each sub-topic in order: concepts build progressively from basic heat transfer to the first law and special gas processes. Once you complete all topics in this unit, you will move on to the next unit covering oscillations.

- [Temperature and thermal equilibrium](https://www.owlsprep.com/study/cie-9702-u20-temperature-and-thermal-equilibrium/)
- [Specific Heat Capacity](https://www.owlsprep.com/study/cie-9702-u20-specific-heat-capacity/)
- [Specific latent heat](https://www.owlsprep.com/study/cie-9702-u20-specific-latent-heat/)

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From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/cie-9702-u20-overview/