AQA A Level Physics
AQA A Level Physics 3.6.2 Thermal Physics Exam Questions
Free AQA A Level Physics exam questions for 3.6.2 Thermal Physics, with a mark scheme for every pack. Covers internal energy, specific heat and latent heat, the gas laws, the ideal gas equation and molecular kinetic theory.
3.6.2 Thermal Physics exam questions
2 question packs 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 AQA A Level Physics 3.6.2 Thermal Physics
Thermal Physics is AQA section 3.6.2 and is examined on Paper 2. It covers thermal energy transfer, internal energy and the first law idea that heating or doing work changes internal energy, specific heat capacity and specific latent heat, the three gas laws, absolute zero, the ideal gas equation and work done by a gas. Section 3.6.2.3 develops the molecular kinetic theory model, including Brownian motion, the assumptions of an ideal gas and the derivation of pV = 1/3 Nm(crms)^2. Examiners test multi-stage heating calculations, gas law graphs, the Boyle's law and Charles's law required practical, and explanations of pressure in terms of molecular collisions.
What you need to know
- Internal energy Internal energy is the sum of the randomly distributed kinetic and potential energies of the molecules. During a change of state the potential energy changes but the kinetic energy, and so the temperature, stays constant.
- Specific heat and latent heat Q = mc delta theta for a temperature change and Q = ml for a change of state. Know methods involving electrical heating and continuous flow.
- Gas laws Boyle's law pV = constant (constant T), Charles's law V/T = constant (constant p), pressure law p/T = constant (constant V). Temperatures must be in kelvin; absolute zero is 0 K, about -273 degrees C. Investigating Boyle's and Charles's law is a required practical.
- Ideal gas equation pV = nRT = NkT, where R = 8.31 J mol-1 K-1, k = R/NA = 1.38 x 10^-23 J K-1 and NA = 6.02 x 10^23 mol-1. Work done by a gas at constant pressure = p delta V.
- 3.6.2.3 Kinetic theory pV = 1/3 Nm(crms)^2 is derived from momentum change in collisions with the walls. The assumptions include many molecules in random motion, negligible molecular volume, elastic collisions, negligible collision time and no intermolecular forces except during collisions.
- 3.6.2.3 Molecular energy Mean kinetic energy per molecule = 1/2 m(crms)^2 = 3/2 kT = 3RT/2NA, so it is proportional to absolute temperature. Know how Boyle, Charles, Brown and others built the model as an example of how scientific knowledge develops.
Common mistakes in 3.6.2 Thermal Physics
- Using degrees Celsius in the gas equations. Always convert to kelvin by adding 273.
- Saying temperature rises while a substance melts. Energy goes into increasing potential energy, so temperature stays constant until the change of state is complete.
- Confusing n (moles) with N (number of molecules). Use pV = nRT with moles and pV = NkT with molecules.
- Explaining gas pressure as molecules colliding with each other. Pressure comes from the force due to momentum change when molecules hit the container walls.
- Squaring the mean speed instead of finding the root mean square speed. Square each speed, find the mean, then take the square root.
Try three 3.6.2 Thermal Physics exam-style questions
1. A gas occupies 0.020 m^3 at a pressure of 1.0 x 10^5 Pa and a temperature of 300 K. Calculate the number of moles and the mean kinetic energy of a molecule. (4 marks)
Show the mark scheme answer
n = pV/RT = (1.0 x 10^5 x 0.020) / (8.31 x 300); n = 0.80 mol; mean Ek = 3/2 kT = 1.5 x 1.38 x 10^-23 x 300; = 6.2 x 10^-21 J.
2. Calculate the energy needed to heat 0.50 kg of water from 20 degrees C to 100 degrees C and then boil off 0.10 kg of it. (c = 4200 J kg-1 K-1, l = 2.26 x 10^6 J kg-1) (3 marks)
Show the mark scheme answer
Heating: Q = 0.50 x 4200 x 80 = 1.68 x 10^5 J; boiling: Q = 0.10 x 2.26 x 10^6 = 2.26 x 10^5 J; total = 3.9 x 10^5 J.
3. Use kinetic theory to explain why the pressure of a gas in a sealed rigid container increases when it is heated. (3 marks)
Show the mark scheme answer
The mean kinetic energy and so the mean speed of the molecules increases; molecules hit the walls more often; each collision gives a larger change in momentum so a larger force; volume is constant so pressure increases.
3.6.2 Thermal Physics FAQs
What does AQA A Level Physics 3.6.2 Thermal Physics cover?
It covers internal energy, specific heat capacity, specific latent heat, the gas laws, absolute zero, the ideal gas equation, and the molecular kinetic theory model of an ideal gas.
Which paper is 3.6.2 Thermal Physics examined in?
Thermal physics is assessed on Paper 2, and the gas laws required practical can also be tested on Paper 3.
What equations do I need for thermal physics?
Learn Q = mc delta theta, Q = ml, pV = nRT, pV = NkT, pV = 1/3 Nm(crms)^2 and mean Ek = 3/2 kT. Practise the derivation of the kinetic theory equation as it is a common long-answer question.
