Black body radiation (Physics only)

    AQA
    GCSE

    A perfect black body is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence, reflecting and transmitting none. The spectral intensity of black body radiation depends solely on the object's temperature; as temperature increases, the intensity of emitted radiation increases at every wavelength, and the peak wavelength shifts toward the shorter, visible end of the spectrum. This concept is fundamental to understanding thermal equilibrium, where the rate of absorption equals the rate of emission, and is applied directly to the Earth's energy balance and the greenhouse effect.

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    Objectives
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    Exam Tips
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    Pitfalls
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    Key Terms
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    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for defining a perfect black body as an object that absorbs all electromagnetic radiation incident on it
    • Credit responses that explicitly state no radiation is reflected or transmitted by the object
    • Award 1 mark for linking constant temperature to the rate of absorption being equal to the rate of emission
    • For Higher Tier, award marks for stating that as temperature increases, the intensity of radiation increases at every wavelength
    • For Higher Tier, credit the observation that the peak intensity shifts to a shorter wavelength as temperature increases

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for defining a perfect black body as an object that absorbs all electromagnetic radiation incident on it
    • Credit responses that explicitly state no radiation is reflected or transmitted by the object
    • Award 1 mark for linking constant temperature to the rate of absorption being equal to the rate of emission
    • For Higher Tier, award marks for stating that as temperature increases, the intensity of radiation increases at every wavelength
    • For Higher Tier, credit the observation that the peak intensity shifts to a shorter wavelength as temperature increases

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When defining a black body, you must explicitly state that *no* radiation is reflected or transmitted; 'absorbs all radiation' alone is often insufficient for full marks
    • 💡For questions on the Earth's temperature, focus on the balance between incoming radiation from the Sun and outgoing radiation from the Earth; use the term 'rate' to gain credit
    • 💡When sketching black body curves for higher temperatures, ensure the new curve is above the old one at *all* wavelengths and the peak is shifted to the left

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Defining a black body merely as a black-coloured object rather than an idealized absorber of all incident radiation
    • Failing to use the word 'rate' when explaining temperature changes (e.g., saying 'it absorbs more than it emits' instead of 'rate of absorption exceeds rate of emission')
    • On spectral graphs (Higher Tier), drawing a higher temperature curve that crosses the lower temperature curve (it should be higher at all wavelengths)

    Key Terminology

    Essential terms to know

    Likely Command Words

    How questions on this topic are typically asked

    State
    Explain
    Describe
    Sketch
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    Practical Links

    Related required practicals

    • •{"code":"Leslie's Cube Investigation","title":"Investigation of emission of infrared radiation","relevance":"Demonstrates that matte black surfaces are the best emitters and absorbers of IR"}

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