The first law of thermodynamics
Cambridge A-Level Physics (9702) · Unit 16: Thermodynamics · 8 flashcards
The first law of thermodynamics is topic 16.2 in the Cambridge A-Level Physics (9702) syllabus , positioned in Unit 16 — Thermodynamics , alongside Internal energy. In one line: The work done (W) during a constant pressure (p) volume change (ΔV) is given by: W = pΔV. Remember to use consistent units (Pascals for pressure and m³ for volume to get Joules for work).
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 8 flashcards — 2 definitions, 3 key concepts and 3 calculations — 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.
The formula for work done during a constant pressure volume change
The work done (W) during a constant pressure (p) volume change (ΔV) is given by: W = pΔV. Remember to use consistent units (Pascals for pressure and m³ for volume to get Joules for work).
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.
- recall and use W = p∆V for the work done when the volume of a gas changes at constant pressure and understand the difference between the work done by the gas and the work done on the gas
- recall and use the first law of thermodynamics ∆U = q + W expressed in terms of the increase in internal energy, the heating of the system (energy transferred to the system by heating) and the work done on the system
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 The first law of thermodynamics
- › 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.
What is the formula for work done during a constant pressure volume change?
The work done (W) during a constant pressure (p) volume change (ΔV) is given by: W = pΔV. Remember to use consistent units (Pascals for pressure and m³ for volume to get Joules for work).
What is the sign convention for work done 'by' the gas versus work done 'on' the gas?
Work done 'by' the gas during expansion is considered positive. Work done 'on' the gas during compression is considered negative. This affects the sign in the first law of thermodynamics.
State the first law of thermodynamics in terms of internal energy, heating, and work.
The first law of thermodynamics states: ΔU = q + W, where ΔU is the increase in internal energy of the system, q is the energy transferred to the system by heating, and W is the work done on the system.
If a gas expands and does 500J of work while absorbing 300J of heat, what is the change in internal energy?
Since the gas does work, W = -500J (work is done 'by' the gas). q = +300J (heat absorbed 'by' the system). Therefore, ΔU = 300J + (-500J) = -200J. The internal energy decreases.
Explain the physical meaning of internal energy (U) in the context of thermodynamics.
Internal energy (U) represents the total energy of the particles (atoms, molecules) within a system. It includes kinetic energy (due to motion) and potential energy (due to intermolecular forces). ΔU represents the change in this total energy.
A gas is compressed at a constant pressure of 2.0 x 10⁵ Pa from a volume of 0.50 m³ to 0.25 m³. Calculate the work done on the gas.
ΔV = 0.25 m³ - 0.50 m³ = -0.25 m³. W = pΔV = (2.0 x 10⁵ Pa)(-0.25 m³) = -5.0 x 10⁴ J. Since W is negative, this represents work done on the gas (compression).
What does a positive value of 'q' signify in the first law of thermodynamics?
A positive value of 'q' indicates that energy is being transferred *to* the system as heat. This leads to an increase in the internal energy of the system, assuming work remains constant.
A system's internal energy increases by 400 J, and 100 J of work is done on it. How much heat was transferred, and was it added to or removed from the system?
Using ΔU = q + W, we have 400 J = q + 100 J. Solving for q, we get q = 300 J. Since q is positive, 300 J of heat was added to the system.
Review the material
Read full revision notes on The first law of thermodynamics — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 16 — Thermodynamics
The first law of thermodynamics 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 The first law of thermodynamics deck
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