# Physics

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A person is standing outdoors in the shade where the temperature is 22 °C. (a) What is the radiant energy absorbed per second by his head when it is covered with hair? The surface area of the hair (assumed to be flat) is 180 cm2 and its emissivity is 0.84. (b) What would be the radiant energy absorbed per second by the same person if he were bald and the emissivity of his head were 0.63?

• Physics -

T=273+22=295 K
radiant emittance of the black body R*
Stefan-Boltzmann law
R*=σT⁴
Stefan-Boltzmann constant σ =5.67•10⁻⁸ W•m⁻²•K⁻⁴
R= =kR*=kσT⁴
E=RAt =kσT⁴At
E₁=R₁At =k₁σT⁴At
=0.84•5.67•10⁻⁸•180•10⁻⁶•295⁴•1=0.065 J
E₂=R₂At =k₂σT⁴At

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