# Wien's fifth power Law and Stephan Boltzmann's fourth power laws of emissive power Wien's fifth po

Wien's fifth power Law and Stephan Boltzmann's fourth power laws of emissive power
Wien's fifth power law says that emissive power is proportional to the temperature raised to the fifth power. On the other hand, the Stefan–Boltzmann law says emissive power is proportional to the temperature raised to the fourth power. How can both of these be true?
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Braxton Gallagher
Wien's fifth power law applies to the maximum height of the emissive power density. The Stefan–Boltzmann law applies to the total emissive power (the integration of the emissive power density).
From Wien's displacement law , ${\lambda }_{max}T=b$ is constant. So, from Planck's law (e.g. see here):
${I}^{\prime }\left({\lambda }_{max},T\right)\propto \frac{{\lambda }_{max}^{-5}}{{e}^{hc/\left({\lambda }_{max}kT\right)}-1}=\frac{{T}^{5}/{b}^{5}}{{e}^{hc/\left(kb\right)}-1}\propto {T}^{5}$
and integrating Plank's law we get, by changing variables to $x=hc/\left(\lambda kT\right)$ so that ${\lambda }^{-5}d\lambda \propto -{T}^{4}{x}^{3}dx$ (the negative is absorbed by changing the integration interval limits):
${\int }_{0}^{\mathrm{\infty }}{I}^{\prime }\left(\lambda ,T\right)d\lambda \propto {\int }_{0}^{\mathrm{\infty }}\frac{{T}^{4}{x}^{3}dx}{{e}^{x}-1}\propto {T}^{4}$