The Particulate Nature of Matter
IB Physics Guide 2023: Theme B - B.1 Thermal energy transfers, B.2 Greenhouse effect, B.3 Gas laws, B.5 Current and circuits
This topic is where IB Physics stops being abstract and starts being kitchen-and-weather-forecast real. Examiners love it because it rewards students who can move fluidly between a microscopic picture (jiggling particles) and a macroscopic one (pressure gauges, ammeters). Most marks lost here aren't calculation errors — they're mixing up temperature scales, confusing heat with temperature, or writing with the wrong symbol for charge. Get the definitions razor-sharp first; the maths is genuinely the easy part.
Overview: The Particulate Model
Everything in this topic rests on one idea: matter is made of particles in constant random motion, and the average kinetic energy of those particles is what we call temperature. Everything else — heat transfer, gas pressure, even electric current — is just this idea applied to a different situation. Solids have particles vibrating in fixed positions; liquids have particles sliding past each other; gases have particles flying freely between collisions. The particulate model explains phase change, thermal expansion, pressure, and (via drifting charge carriers) current in a single unified story.
- Thermal physics (B.1-B.3) is about energy transfer between particles and the resulting macroscopic quantities: temperature, internal energy, pressure, volume.
- Current and circuits (B.5) reuses the particle picture — charge carriers (usually electrons) drifting through a conductor — to build up , , and circuit analysis.
- The syllabus deliberately separates 'heat' (energy transferred due to a temperature difference) from 'temperature' (a measure of average particle KE) — conflating them costs marks throughout Paper 1 and 2.
The shape of the chapter
Command terms you'll meet in this topic
| Command term | What it demands | AO | Mark-earning move |
|---|---|---|---|
| Determine | Obtain a numerical answer showing the working/method used | AO2 | Full method + substitution + correct unit; bare answer without working loses method marks |
| Explain | Give a reason or justification, referring to physics principles (e.g. particle motion) | AO2 | Must link cause to effect explicitly — 'particles move faster' alone earns nothing without linking to KE/temperature |
| Sketch | Draw a graph/diagram with correct general shape and labelled axes, not necessarily to scale | AO3 | Missing axis labels or wrong curvature (e.g. straight line instead of curve) loses the mark even if shape is 'close' |
| State | Give a short factual answer with no explanation required | AO1 | Extra unrequested explanation wastes time but isn't penalised |
| Deduce | Reach a conclusion using the data/information given in the question | AO3 | Must reference the specific given values, not just general theory |
Key point
Overview