CIE A Level Physics
CIE A Level Physics Thermodynamics Exam Questions
Free CIE A Level Physics exam questions for Thermodynamics, with a mark scheme for every pack. Covers internal energy, temperature and molecular kinetic energy, work done by a gas and the first law of thermodynamics.
Thermodynamics exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About CIE A Level Physics Thermodynamics
Thermodynamics is Topic 16 of the Cambridge International AS & A Level Physics (9702) syllabus. It defines internal energy as the sum of the random kinetic and potential energies of the molecules and applies the first law of thermodynamics to changes in a system. You calculate work done when a gas changes volume at constant pressure, and apply ΔU = q + W with the correct sign convention. It builds on Temperature and Ideal gases. Examiners test the sign convention heavily, often through tables showing compression, heating and expansion in a gas cycle.
What you need to know
- Internal energy Internal energy is the sum of a random distribution of kinetic and potential energies of the molecules in a system. A rise in temperature increases the kinetic energy and so the internal energy.
- Ideal gas internal energy In an ideal gas there are no intermolecular forces, so there is no potential energy and internal energy depends only on temperature.
- Work done by a gas When a gas changes volume at constant pressure, W = pΔV. Expansion means work is done by the gas; compression means work is done on the gas.
- First law ΔU = q + W, where ΔU is the increase in internal energy, q is the energy transferred to the system by heating and W is the work done on the system.
- Gas cycles Over a complete cycle the gas returns to its original state, so the total ΔU is zero. The net work done is the area enclosed on a p-V diagram.
Common mistakes in Thermodynamics
- Using W as work done by the gas in ΔU = q + W. In Cambridge's convention W is work done on the system, so expansion gives negative W.
- Saying internal energy is just kinetic energy for all substances. Only for an ideal gas is potential energy zero.
- Using pΔV when pressure is changing. It only applies at constant pressure; otherwise use the area under the p-V graph.
- Saying temperature rises whenever heat is supplied. If the gas does enough work, internal energy may stay the same or fall.
- Forgetting that total ΔU over a full cycle is zero.
Try three Thermodynamics exam-style questions
1. A gas expands at a constant pressure of 1.0 × 10^5 Pa and its volume increases by 2.0 × 10^-3 m^3. Calculate the work done by the gas. (1 mark)
Show the mark scheme answer
W = pΔV = 1.0 × 10^5 × 2.0 × 10^-3 = 200 J.
2. During the expansion in the previous question the gas is supplied with 500 J of thermal energy. Calculate the change in internal energy of the gas. (2 marks)
Show the mark scheme answer
Work done on the gas W = -200 J; ΔU = q + W = 500 - 200 = +300 J (increase).
3. Explain why the internal energy of an ideal gas depends only on its temperature. (2 marks)
Show the mark scheme answer
There are no intermolecular forces in an ideal gas so the molecules have no potential energy; internal energy is only random kinetic energy, which is proportional to thermodynamic temperature.
Thermodynamics FAQs
What does Cambridge AS & A Level Physics Thermodynamics cover?
It covers internal energy, the link between temperature and molecular kinetic energy, work done by a gas at constant pressure and the first law of thermodynamics.
Which paper is Thermodynamics examined in?
It is an A Level topic assessed in Paper 4 (A Level Structured Questions).
How do I remember the sign convention for the first law?
Use ΔU = q + W where both q and W are energy going into the gas: heating the gas gives positive q, and compressing it gives positive W.
