Thermal & Modern Physics: A Level Physics questions and answers
10 free A Level Physics Thermal & Modern Physics questions written in the style of Cambridge Physics papers, each with its answer and mark scheme. 4 of them are A* challenge questions, built to catch the mistakes strong students still make.
This topic covers Temperature, Ideal gases, Thermodynamics, Quantum physics, Nuclear physics, Medical physics, Astronomy & cosmology. Work through the questions below, then generate a full Thermal & Modern Physics paper with a fresh set every time.
The spectrum of a star has peak intensity at a wavelength of 483 nm. The luminosity of the star is 3.8 × 1026 W.
(a)
Use Wien’s displacement law to calculate the surface temperature.
[2]
(b)
Calculate the radius of the star.
[3]
(c)
A spectral line of wavelength 656 nm measured in the laboratory is observed in light from a distant galaxy at 668 nm. Calculate the recession speed of the galaxy and its distance, using H0 = 2.2 × 10−18 s−1.
[3]
(d)
State what is meant by a standard candle.
[1]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
6000 K
λmaxT = 2.9 × 10−3 m K C1 6000 A1
2
(b)
6.41 × 108 m
L = 4πσr2T4C1 rearranged C1 6.41 × 108A1
3
(c)
5.49 × 106 m s−1, 2.49 × 1024 m
Δλ/λ = v/c C1 5.49 × 106A1 2.49 × 1024A1
3
(d)
an object of known luminosity
known luminosity B1
1
Question 2Structured · 4 marksA* challenge
Internal energy is the sum of the kinetic and potential energies of the molecules.
(a)
Explain why the internal energy of an ideal gas depends only on its temperature.
[2]
(b)
Explain why the internal energy of water increases when it boils at constant temperature.
[2]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
no intermolecular forces so no PE
there are no intermolecular forces so no potential energy B1 kinetic energy depends on temperature B1
2
(b)
potential energy increases
energy breaks bonds/separates molecules B1 potential energy increases while kinetic energy is constant B1
2
Question 3Structured · 7 marksA* challenge
An aluminium block of mass 1.0 kg is heated by a 50 W heater for 300 s. Its temperature rises by 13.3 K.
(a)
Define specific heat capacity.
[1]
(b)
Calculate the specific heat capacity from these data.
[2]
(c)
The accepted value is 900 J kg−1 K−1. Explain the difference and suggest an improvement.
[2]
(d)
Explain why the temperature of a substance remains constant while it melts.
[2]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
energy per unit mass per unit temperature rise
thermal energy per unit mass per unit temperature change B1
1
(b)
1130 J kg−1 K−1
E = Pt = 15000 J C1 c = 1130 J kg−1 K−1A1
2
(c)
heat losses; insulate
thermal energy is lost to the surroundings so the temperature rise is smaller B1 insulate the block / use lagging B1
2
(d)
energy increases PE, not KE
energy is used to break bonds / increase potential energy of particles B1 mean kinetic energy, and so temperature, stays constant B1
2
Question 4Structured · 5 marksA* challenge
Electron diffraction provides evidence for the wave nature of particles.
(a)
Calculate the de Broglie wavelength of an electron accelerated through 2000 V.
[3]
(b)
Explain how the diffraction pattern changes if the accelerating voltage increases.
[2]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
2.7 × 10−11 m
eV = p2/2m C1 p = 2.41 × 10−23 N s C1 λ = h/p = 2.7 × 10−11 m A1
3
(b)
rings smaller
momentum increases so wavelength decreases B1 rings become smaller (less diffraction) B1
2
Question 5Structured · 9 marks
Fig. 5.1 shows some of the energy levels of a hydrogen atom. The arrow shows one electron transition.
Fig. 5.1
(a)
Explain why the energy levels are given negative values.
[2]
(b)
Calculate the wavelength of the photon emitted in the transition shown in Fig. 5.1.
[3]
(c)
State the region of the electromagnetic spectrum of this photon.
[1]
(d)
Explain how the line spectrum of hydrogen provides evidence for discrete energy levels.
[2]
(e)
Calculate the ionisation energy of hydrogen in joules.
[1]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
zero energy = electron free (at infinity); bound electron has less energy
the energy of a free electron (at infinity) is defined as zero B1 bound electrons have less energy so their energies are negative B1
2
(b)
4.88 × 10−7 m
ΔE = 2.55 eV = 4.08 × 10−19 J C1 λ = hc/ΔE C1 4.88 × 10−7 m A1
3
(c)
visible
visible B1
1
(d)
only specific wavelengths emitted; each corresponds to a transition between fixed levels
only certain wavelengths (lines) are emitted B1 each photon energy equals the difference between two fixed energy levels, so energy levels are discrete B1
2
(e)
2.18 × 10−18 J
13.6 × 1.60 × 10−19 = 2.18 × 10−18 J B1
1
Question 6Structured · 11 marks
This question is about medical imaging.
(a)
Explain how ultrasound is produced using the piezo-electric effect.
[2]
(b)
Define specific acoustic impedance.
[1]
(c)
Ultrasound passes from soft tissue (Z = 1.6 × 106 kg m−2 s−1) to bone (Z = 6.5 × 106 kg m−2 s−1). Calculate the fraction of the intensity reflected.
[2]
(d)
Explain why a gel is applied between the transducer and the skin.
[2]
(e)
An X-ray beam passes through 3.0 cm of tissue with linear attenuation coefficient 0.50 cm−1. Calculate the percentage of the intensity transmitted.
[2]
(f)
Explain why a CT scan gives more information than a single X-ray image, and why it involves a higher dose.
[2]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
alternating p.d. across crystal makes it vibrate at its resonant frequency
an alternating p.d. applied across a piezo-electric crystal makes it change shape / vibrate B1 at the resonant frequency of the crystal this produces ultrasound B1
2
(b)
Z = ρc
product of the density of the medium and the speed of sound in it B1
1
(c)
0.366
IR/I = (Z2 − Z1)2/(Z2 + Z1)2C1 0.366 A1
2
(d)
air has very different Z to skin; nearly all reflected; gel matches Z
air and skin have very different acoustic impedances so almost all ultrasound would be reflected B1 the gel has an impedance similar to skin, allowing transmission B1
2
(e)
22.3%
I = I0e−μxC1 22.3% A1
2
(f)
many images from different angles combined into 3D / slices; more exposures
images are taken from many angles round the patient and combined into cross-sections / a 3D image B1 the patient is exposed to many X-ray images, so a larger dose B1
2
Question 7Structured · 6 marks
A container of volume 2.0 × 10−3 m3 holds an ideal gas at 2.0 × 105 Pa and 300 K.
(a)
Calculate the number of molecules.
[2]
(b)
Calculate the mean translational kinetic energy of a molecule.
[2]
(c)
State two assumptions of the kinetic theory of gases.
[2]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
9.66 × 1022
n = pV/RT C1 × NA = 9.66 × 1022A1
2
(b)
6.21 × 10−21 J
(3/2)kT C1 6.21 × 10−21 J A1
2
(c)
elastic collisions; negligible volume; no intermolecular forces
Any 2 of: any two: collisions elastic; molecules of negligible volume; no intermolecular forces except during collisions; random motion; time of collision negligible B2
2
Question 8Structured · 8 marks
The count rate from a radioactive sample is measured every hour. The background count rate is 80 counts per minute. Table 8.1 shows the results.
time / hours
0
1
2
3
4
5
6
count rate / counts per minute
1280
1089
929
794
680
585
504
Table 8.1
(a)
Define the decay constant.
[1]
(b)
Use Table 8.1 to determine the half-life of the sample. Show your working.
[3]
(c)
Use your answer to (b) to calculate the decay constant in s⁻¹.
[2]
(d)
Explain why the background count rate must be subtracted.
[1]
(e)
Repeated measurements of the count rate at the same time give slightly different values. Explain why.
[1]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
probability per unit time that a nucleus decays
the probability per unit time that a nucleus will decay B1
1
(b)
4.00 hours
subtract background: corrected rates 1200 … C1 time for corrected rate to halve / gradient of ln(corrected rate) against t C1 4.00 hours (± 0.1) A1
3
(c)
4.81 × 10−5 s−1
λ = ln 2 ÷ t½, with t in seconds C1 4.81 × 10−5 s−1A1
2
(d)
background not from source; would make decay appear slower
Any 1 of: background radiation is not from the sample; including it gives too large a count rate / too long a half-life B1
1
(e)
decay is random
radioactive decay is random (spontaneous), so the number of decays in each interval varies B1
1
Question 9Structured · 9 marks
This question is about temperature scales and thermometers.
(a)
Explain what is meant by thermal equilibrium.
[2]
(b)
Explain why the thermodynamic temperature scale does not depend on the property of any particular substance.
[2]
(c)
Convert −196 °C and 373.15 K to the other scale.
[2]
(d)
Compare a thermocouple with a liquid-in-glass thermometer for measuring the rapidly changing temperature of a small object.
[3]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
no net transfer of thermal energy; same temperature
two objects are in thermal equilibrium when there is no net transfer of thermal energy between them B1 they are at the same temperature B1
2
(b)
based on absolute zero and triple point, theoretical
it is defined theoretically, with absolute zero (0 K) and the triple point of water (273.16 K) as fixed points B1 it does not rely on a measurable property of a substance B1
2
(c)
77 K; 100 °C
77 K B1 100 °C B1
2
(d)
thermocouple: small heat capacity, fast response, small size, remote reading; liquid-in-glass: slow, large
thermocouple has a small thermal capacity so responds quickly B1 it is small so affects the object little and can be read remotely / recorded B1 a liquid-in-glass thermometer responds slowly and has larger thermal capacity B1
3
Question 10Structured · 5 marks
Line spectra are evidence for discrete energy levels.
(a)
Explain how an emission line spectrum is produced.
[3]
(b)
A transition between levels −3.40 eV and −13.6 eV occurs. Calculate the wavelength emitted.
[2]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
electrons drop between levels emitting photons of specific energy
electrons in atoms are excited to higher levels B1 they drop to lower levels B1 emitting photons with energy equal to the difference, so specific wavelengths B1
What is in A Level Physics Thermal & Modern Physics?
In the 9702 syllabus, Thermal & Modern Physics covers Temperature, Ideal gases, Thermodynamics, Quantum physics, Nuclear physics, Medical physics, Astronomy & cosmology.
Where can I get Thermal & Modern Physics questions with mark schemes?
This page has 10 Thermal & Modern Physics questions with answers. For more, the free generator on mentorriyaz.in builds a new Thermal & Modern Physics paper with a full mark scheme each time you click.
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Original Cambridge-style practice questions written for mentorriyaz.in. These are not Cambridge past paper questions, and this site is not affiliated with or endorsed by Cambridge University Press & Assessment. Updated 2026-11-04. Paper generator · mentorriyaz.in