Topic 9: Thermodynamics — Edexcel A-Level Physics
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Topic 9: Thermodynamics explained
Wien’s law links the wavelength at which a black body radiator emits most strongly to its absolute temperature: λmaxT = 2.898 × 10⁻³ m K.
Read the full explanation
The product of peak wavelength and thermodynamic temperature is constant, so as T rises λmax falls. To use it, convert temperature to kelvin (add 273 to a Celsius value) and keep λmax in metres, since the constant carries units m K. For example, a star at 5800 K gives λmax = 2.898 × 10⁻³ ÷ 5800 ≈ 5.0 × 10⁻⁷ m, in the visible range. Rearranging to T = 2.898 × 10⁻³ ÷ λmax lets you find temperature from a measured peak wavelength. Check that the product λmaxT returns the constant, and watch powers of ten when wavelengths are quoted in nanometres or micrometres.
Your focus
- Apply λmaxT = 2.898 × 10⁻³ m K to calculate peak wavelength from temperature.
- Rearrange the equation to determine temperature from a measured peak wavelength.
- Convert temperatures to kelvin and wavelengths to metres before substitution.
Topic 9: Thermodynamics exam tips
Marking Points
- States Wien’s law as λmaxT = 2.898 × 10⁻³ m K and identifies the constant with its units.
- Converts temperature to kelvin before substitution, adding 273 to a Celsius value.
- Converts peak wavelength to metres, e.g. 500 nm = 5.0 × 10⁻⁷ m, before dividing.
- Rearranges correctly to λmax = 2.898 × 10⁻³ ÷ T or T = 2.898 × 10⁻³ ÷ λmax.
- Recognises that higher temperature gives a shorter peak wavelength and checks the product returns the constant.
Examiner Tips
- 💡Always write the equation, substitute with units shown, then state the final value with its unit (m or K).
- 💡Convert every quantity to SI base units before calculating to avoid power-of-ten slips.
- 💡Sanity-check by multiplying your λmax and T back together to confirm you recover 2.898 × 10⁻³ m K.
Common Mistakes
- Substituting a Celsius temperature directly: the law needs absolute temperature, so add 273 to convert to kelvin first.
- Leaving wavelength in nanometres: convert to metres so the units match the constant’s m K, otherwise the answer is out by a factor of 10⁹.
- Inverting the relationship and expecting λmax to rise with temperature: the product is constant, so λmax falls as T rises.