Unit Overview

The particulate nature of matter

IB Physics SL· 5 min read 📊 19-21% of overall IB Physics SL exam

1. Unit at a Glance

This unit follows a logical progression from core definitions up to full thermodynamic systems, with the unifying theme that all thermal phenomena arise from the motion and interaction of tiny particles.

You will start by learning how to define and measure temperature and thermal energy, then move to modeling ideal gas behavior, explore how matter responds to thermal input, and finish with the first law of thermodynamics for closed systems.

This unit is split into 4 connected sub-topics:

Temperature and thermal energy

Introduces absolute temperature, internal energy, and the distinction between thermal energy and temperature.

★★⏱ 10 min

Gases

Covers ideal gas assumptions, the ideal gas law, and kinetic molecular theory of ideal gases.

★★★⏱ 12 min

Thermal properties of matter

Explores specific heat capacity, latent heat, and thermal energy transfers during temperature and phase changes.

★★⏱ 11 min

Thermodynamics

Introduces the first law of thermodynamics and energy analysis of common ideal gas processes.

★★★⏱ 13 min

2. Common Pitfalls

Wrong move:

Confusing temperature and internal energy, assuming higher temperature always means higher internal energy

Why:

Internal energy is the total energy of all particles in a system, so it depends on both temperature and the amount of substance

Correct move:

Always define the system boundary and relate internal energy to total particle energy, not just temperature alone

Wrong move:

Mixing up specific heat and latent heat in problems that include both temperature change and phase change

Why:

Latent heat applies only to phase transitions at constant temperature, while specific heat applies to temperature changes

Correct move:

Split calculation problems into separate steps for temperature change ( $Q = m c Δ T$ ) and phase change ( $Q = m L$ )

3. Quick Reference Cheatsheet

Concept	Key Formula / Relationship
Ideal Gas Law	$P V = n R T = N k_{B} T$
Thermal Energy (Temperature Change)	$Q = m c Δ T$
Thermal Energy (Phase Change)	$Q = m L$
First Law of Thermodynamics	$Δ U = Q + W$ (W = work done on the system)
Average Particle Kinetic Energy (Ideal Gas)	$⟨ E_{k} ⟩ = \frac{3}{2} k_{B} T$

What's Next

Begin this unit by learning the foundational definitions of temperature and thermal energy to build your understanding of thermal physics. Once you complete all sub-topics in this unit, you will move on to the next unit on wave phenomena, which builds on your existing knowledge of energy and particle behavior.