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Thermodynamics

Black body radiation and Wien's law

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  • Papers 2 and 3

Overview

Recall questions on what a black body radiator is, the shape of the radiation curve and how it changes with temperature, and Wien's law applied to a star.

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Answer each one before you reveal it.

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Process

How does the radiation curve change as the temperature of the black body is increased?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What is meant by a black body radiator?

    Model answer

    It is a body that absorbs all the electromagnetic radiation falling on it and is also the best possible emitter of radiation at every wavelength.

  2. 2

    Why is the radiation from a black body described as a continuous spectrum?

    Model answer

    It emits at every wavelength rather than at separate discrete wavelengths, so the intensity varies smoothly across the spectrum.

  3. 3

    What shape does the radiation curve for a black body have?

    Model answer

    The intensity rises from almost zero at very short wavelengths to a single peak, then falls away more gradually at longer wavelengths without reaching zero.

  4. 4

    How does the radiation curve change as the temperature of the black body is increased?

    Model answer

    The peak shifts to a shorter wavelength and the intensity rises at every wavelength, so the area under the curve becomes much larger.

  5. 5

    Can the radiation curves for two different temperatures ever cross?

    Model answer

    No, because the curve for the higher temperature lies above the other at every single wavelength.

  6. 6

    State Wien's law:

    Model answer

    Wien's law is λₘₐₓT = 2.898 × 10⁻³ m K.

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