Lesson 9.3.1

9.3.1 Black body radiators, Stefan-Boltzmann and Wien's laws Quiz: Pearson Edexcel Physics, Unit 9

20 questions

Written by Revision Ninja · Last reviewed 11 October 2026

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Lesson 9.3.1, Black body radiators, Stefan-Boltzmann and Wien's laws: 20 multiple choice questions for the Pearson Edexcel Physics (9PH0), Unit 9: Thermodynamics, written with Revision Ninja.

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The 20 questions

  1. What is a black body radiator?

    • A perfect reflector that emits no radiation at any temperature.
    • An object that absorbs all incident radiation and emits a spectrum that depends only on its temperature.
    • A dark object that emits only infrared radiation whatever its temperature.
    • An object that reflects all visible light and so looks black at any temperature.

  2. Which expression is the Stefan-Boltzmann law for the total power radiated by a black body?

    • L = σ T^4 / A
    • L = σ A T^4
    • L = σ A T
    • L = σ A T^2

  3. What are the SI units of the Stefan-Boltzmann constant σ?

    • J m^-3 K^-4
    • W m^-2 K^4
    • W m^-2 K^-4
    • W m^2 K^-4

  4. Wien's law gives λmax T = 2.898 x 10^-3 m K. What happens to the peak wavelength as the temperature of a black body rises?

    • It doubles each time the temperature doubles.
    • It stays the same, since the peak depends only on the surface area.
    • It increases, since the peak wavelength is proportional to T.
    • It decreases, since the peak wavelength is inversely proportional to T.

  5. A star's spectrum peaks at a wavelength of 500 nm. Using Wien's law, what is its approximate surface temperature?

    • about 2900 K
    • about 5800 K
    • about 11600 K
    • about 580 K

  6. A black body has surface area 0.50 m^2 and temperature 1000 K. Taking σ = 5.67 x 10^-8 W m^-2 K^-4, what is its total power output?

    • about 28 kW
    • about 283 kW
    • about 2.8 kW
    • about 5.7 kW

  7. If the absolute temperature of a black body doubles, by what factor does the power emitted per unit area increase?

    • 2
    • 8
    • 16
    • 4

  8. What is the peak wavelength of a black body at 3000 K?

    • about 0.97 micrometres
    • about 0.97 nanometres
    • about 0.097 micrometres
    • about 9.7 micrometres

  9. A black body has its peak wavelength at 1.2 micrometres. What is its temperature?

    • about 2400 K
    • about 1200 K
    • about 3500 K
    • about 4800 K

  10. A Sun-like star has radius 7.0 x 10^8 m and surface temperature 5800 K, and is treated as a black body with σ = 5.67 x 10^-8 W m^-2 K^-4. Approximately what is its luminosity?

    • about 7.9 x 10^26 W
    • about 1.2 x 10^26 W
    • about 3.9 x 10^26 W
    • about 3.9 x 10^22 W

  11. Star B has the same surface temperature as star A but twice the radius. What is L_B / L_A?

    • 4
    • 16
    • 8
    • 2

  12. A star's peak wavelength falls from 900 nm to 600 nm. What is the ratio of its final to initial absolute temperature?

    • 1.5
    • 0.44
    • 0.67
    • 2.25

  13. A black body's absolute temperature doubles while its surface area halves. Its power output was P before. What is it now?

    • 8P
    • 4P
    • 16P
    • 32P

  14. Why does a real object emit less power than a black body at the same temperature?

    • Real surfaces emit only visible light, not infrared radiation at the same temperature.
    • Real surfaces have emissivity below 1, so they radiate less power than a black body.
    • Real surfaces are always cooler than their measured temperature, so they radiate less.
    • Real surfaces have zero surface area in the emission calculation, which lowers their power.

  15. Which statement about black body radiation curves is correct?

    • As temperature rises, the peak shifts to shorter wavelengths and the total area under the curve increases.
    • The area under the curve falls as temperature rises.
    • As temperature rises, the peak shifts to longer wavelengths and the area is unchanged.
    • The curve keeps the same shape at all temperatures and only moves up.

  16. Star X is hotter than star Y, and both have the same radius. Which is more luminous, and why?

    • Star X, because L is proportional to T^4 for the same surface area.
    • They are equally luminous, since luminosity depends only on radius.
    • Star Y, because its peak wavelength is shorter.
    • Star Y, because cooler stars emit more energy per second.

  17. What is the main reason the peak wavelength of a hotter object shifts to shorter values?

    • Heat makes emitted photons travel more slowly through space.
    • Hotter objects absorb more visible light, so they emit less red light.
    • Hotter objects emit their energy at higher frequencies, so the peak moves to shorter wavelengths.
    • Higher temperature increases the speed of light inside the object.

  18. Two stars have equal luminosity but different surface temperatures. Which has the larger radius?

    • The cooler star, since a lower T needs a larger surface area to give the same L.
    • They must have equal radii.
    • The hotter star, since its peak wavelength is shorter.
    • The hotter star, since it has the greater energy per unit area.

  19. Why is Wien's law only an approximation for real stars?

    • Stars are not perfect black bodies, so their spectra differ from the ideal black body curve.
    • Wien's law only applies to objects that are cooler than 1000 K.
    • Stars emit no light at the peak wavelength, so the law cannot be used.
    • Wien's law requires the star's radius to be measured in metres.

  20. A sphere of radius 0.10 m is at 400 K and is treated as a black body. What power does it emit? Use σ = 5.67 x 10^-8 W m^-2 K^-4.

    • about 180 W
    • about 45 W
    • about 1800 W
    • about 18 W

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