Describe how the temperature of a perfect black body affects the radiation it emits.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A metal sphere is heated in a furnace. At low temperatures it appears dull and dark. As its temperature rises, it begins to glow.
Model answer (4 marks)
A perfect black body absorbs all incident radiation and emits none.
The amount of infrared radiation it emits increases with temperature – a hotter body gives a greater power output in a given time.
The wavelength at which the emission is greatest (the peak) moves to shorter wavelengths as the temperature rises, meaning the radiation shifts to higher frequencies.
At a constant temperature the power emitted equals the power absorbed, so the body is in thermal equilibrium.
The amount of infrared radiation it emits increases with temperature – a hotter body gives a greater power output in a given time.
The wavelength at which the emission is greatest (the peak) moves to shorter wavelengths as the temperature rises, meaning the radiation shifts to higher frequencies.
At a constant temperature the power emitted equals the power absorbed, so the body is in thermal equilibrium.
Examiner tips
- Use the phrase "perfect black body" to show you know the definition. Show the relationship between temperature and power (more IR when hotter). Mention the shift of the peak wavelength (Wien’s law). State the equilibrium condition (emitted = absorbed).
Common mistakes
- Confusing a black body with a normal metal surface – forgetting that a black body absorbs all radiation. Failing to mention the shift of the peak wavelength. Mixing up temperature with colour without linking to wavelength shift.
Mark scheme (4 marks)
- A perfect black body absorbs all radiation incident on it (and does not reflect or transmit any radiation)
- The hotter the object, the greater the infrared radiation emitted in a given time
- As temperature increases, the peak of the emitted radiation shifts to shorter wavelengths (higher frequencies)
- A body at constant temperature emits radiation at the same rate as it absorbs radiation
Key terms in this question
Related
- All AQA GCSE Physics (8463) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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