Internal energy
Cambridge A-Level Physics (9702) · Unit 16: Thermodynamics · 6 flashcards
Internal energy is topic 16.1 in the Cambridge A-Level Physics (9702) syllabus , positioned in Unit 16 — Thermodynamics , alongside The first law of thermodynamics. In one line: Internal energy (U) is the total kinetic and potential energy of all the molecules within a system. It is a state function, meaning it only depends on the current state of the system, not how it reached that state.
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 6 flashcards — 2 definitions and 4 key concepts — covering the precise wording mark schemes reward. Use the 2 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.
'internal energy' of a system
Internal energy (U) is the total kinetic and potential energy of all the molecules within a system. It is a state function, meaning it only depends on the current state of the system, not how it reached that state.
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.
- understand that internal energy is determined by the state of the system and that it can be expressed as the sum of a random distribution of kinetic and potential energies associated with the molecules of a system
- relate a rise in temperature of an object to an increase in its internal energy
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 Internal energy
- › Always square root the mean-square speed value if the question specifically asks for r.m.s. speed.
- › Internal energy must be defined as the sum of a random distribution of kinetic and potential energies associated with the molecules.
- › For kinetic theory explanations, discuss molecular collisions, changes in mean square speed, and the direct proportionality between temperature and average kinetic energy.
- › Use the convention ΔU = q + w, where q is energy supplied TO the system and w is work done ON the system.
- › Recall that internal energy is the sum of a random distribution of kinetic and potential energies associated with the molecules of a system.
Define 'internal energy' of a system.
Internal energy (U) is the total kinetic and potential energy of all the molecules within a system. It is a state function, meaning it only depends on the current state of the system, not how it reached that state.
What two forms of energy contribute to the internal energy of a system?
Internal energy is the sum of the random distribution of kinetic energies (due to molecular motion) and potential energies (due to intermolecular forces) of the molecules within the system.
How does the temperature of an object relate to its internal energy?
An increase in the temperature of an object corresponds to an increase in its internal energy, specifically the kinetic energy component. Higher temperature means faster average molecular motion.
Does internal energy depend on the path taken to reach a particular state?
No, internal energy is a state function. This means the internal energy of a system depends only on its current state (temperature, pressure, volume, etc.) and not on the process or path taken to reach that state.
Describe the relationship between heat transfer to a system and its internal energy, assuming no work is done.
When heat is transferred to a system and no work is done (
Why is the motion of molecules considered 'random' when discussing internal energy?
The motion is random because individual molecules move in all different directions and with a variety of speeds. This randomness means we consider average kinetic energy when assessing the internal energy.
Review the material
Read full revision notes on Internal energy — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 16 — Thermodynamics
Internal energy 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 Internal energy 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 Internal energy deck
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