Kinetic theory of gases
Cambridge A-Level Physics (9702) · Unit 15: Ideal gases · 7 flashcards
Kinetic theory of gases is topic 15.3 in the Cambridge A-Level Physics (9702) syllabus , positioned in Unit 15 — Ideal gases , alongside The mole and Equation of state. In one line: 1. Gas consists of identical molecules in random, continuous motion. 2. Volume of molecules is negligible compared to gas volume. 3. No intermolecular forces except during collisions. 4. Collisions are perfectly elastic. 5. Duration of collision is negligible compared to time between collisions.
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 7 flashcards — 3 definitions, 2 key concepts and 2 calculations — covering the precise wording mark schemes reward. Use the 3 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
Basic assumptions of the kinetic theory of gases
1. Gas consists of identical molecules in random, continuous motion. 2. Volume of molecules is negligible compared to gas volume. 3. No intermolecular forces except during collisions. 4. Collisions are perfectly elastic. 5. Duration of collision is negligible compared to time between collisions.
What the Cambridge 9702 syllabus says
Official 2025-2027 spec · A2 LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- state the basic assumptions of the kinetic theory of gases
- explain how molecular movement causes the pressure exerted by a gas and derive and use the relationship pV = 3 1 Nm<c2>, where <c2> is the mean-square speed (a simple model considering one-dimensional collisions and then extending to three dimensions using 3 1 <c2> = <cx 2> is sufficient)
- understand that the root-mean-square speed cr.m.s. is given by c < >
- compare pV = 3 1 Nm<c2> with pV = NkT to deduce that the average translational kinetic energy of a
- kT, and recall and use this expression
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9702 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Kinetic theory of gases
- › For ideal gases, state clearly that internal energy is the sum of only the kinetic energies of the atoms/molecules.
- › Use specific syllabus terminology; refer to the motion, collisions, and volume of molecules when discussing kinetic theory.
State the basic assumptions of the kinetic theory of gases.
1. Gas consists of identical molecules in random, continuous motion. 2. Volume of molecules is negligible compared to gas volume. 3. No intermolecular forces except during collisions. 4. Collisions are perfectly elastic. 5. Duration of collision is negligible compared to time between collisions.
Explain how molecular movement causes gas pressure.
Gas pressure arises from the multitude of collisions of gas molecules with the walls of the container. Each collision exerts a force, and the sum of these forces over the area of the wall results in pressure. Higher molecular speeds or more frequent collisions lead to higher pressure.
State the relationship between pressure (p), volume (V), number of molecules (N), mass of molecule (m), and mean-square speed (<c²>).
The relationship is given by pV = (1/3)Nm<c²>, where <c²> represents the average of the squares of the speeds of the gas molecules.
What is the root-mean-square speed (cᵣₘₛ)?
The root-mean-square speed (c<sub>rms</sub>) is the square root of the mean (average) of the squares of the speeds of the molecules in a gas. It is calculated as c<sub>rms</sub> = √<c²>.
How is the average translational kinetic energy of a molecule related to absolute temperature (T)?
The average translational kinetic energy of a molecule is directly proportional to the absolute temperature. It is given by (1/2)m<c²> = (3/2)kT, where k is the Boltzmann constant.
How can you derive the relationship between root-mean-square speed and temperature?
Starting with pV = (1/3)Nm<c²> and pV = NkT, equate to obtain (1/3)Nm<c²> = NkT. Simplify to <c²> = 3kT/m. Therefore, c<sub>rms</sub> = √(3kT/m).
How does increasing the temperature of a gas affect the root-mean-square speed of its molecules?
Increasing the temperature of a gas increases the root-mean-square speed of its molecules. Since c<sub>rms</sub> = √(3kT/m), a higher temperature (T) results in a higher c<sub>rms</sub>, indicating faster-moving molecules.
Review the material
Read full revision notes on Kinetic theory of gases — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 15 — Ideal gases
Kinetic theory of gases sits alongside these A-Level Physics decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Kinetic theory of gases deck
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
How to study this Kinetic theory of gases deck
Start in Study Mode, attempt each card before flipping, then rate Hard, Okay or Easy. Cards you rate Hard come back within a day; cards you rate Easy push out to weeks. Your progress is saved in your browser, so come back daily for 5–10 minute reviews until every card reads Mastered.
Study Mode
Rate each card Hard, Okay, or Easy after flipping.