RadiationHot MetalStefan-Boltzmann LawBlackbodySurface TemperatureAbsorbing HeatRadiationRadiationHot MetalHot MetalWhen metal is heated When metal is heated enough it glows.enough it glows.The color changes with The color changes with temperature.temperature.•Red – orange – yellow – Red – orange – yellow – whitewhiteLight can cross a vacuum.Light can cross a vacuum.•Carries energy with it.Carries energy with it.Stefan-Boltzmann LawStefan-Boltzmann LawRadiation increases with Radiation increases with temperature.temperature.Energy rate increases as 4Energy rate increases as 4thth power of temperaturepower of temperatureStefan-Boltzmann Law.Stefan-Boltzmann Law.•PP = power (W) = power (W)•AA = surface area (m = surface area (m22))•TT = temperature (K) = temperature (K)•ee = emissivity (0 to 1) = emissivity (0 to 1) = constant (5.67 x 10= constant (5.67 x 10-8-8 W/mW/m22KK44))4ATePBlackbodyBlackbodyEmissivity measures the ability of an object to radiate Emissivity measures the ability of an object to radiate heat.heat.It is the same to absorb heat.It is the same to absorb heat.A perfect emissivity of 1 is a blackbody.A perfect emissivity of 1 is a blackbody.Low e near 0 High e near 1Surface TemperatureSurface TemperatureAssume the sun is a Assume the sun is a blackbody radiating 3.9 x blackbody radiating 3.9 x 10102626 W, and its radius is 7.0 x W, and its radius is 7.0 x 101088 m. m.What is the surface What is the surface temperature?temperature?First find the surface area.First find the surface area.AA = 4 = 4rr22Now apply the Stefan-Now apply the Stefan-Boltzmann LawBoltzmann LawTT = 5.8 x 10 = 5.8 x 1033 K K4/12)4(rPT Absorbing HeatAbsorbing HeatObjects that radiate also Objects that radiate also absorb.absorb.The radiation heat can be The radiation heat can be converted into thermal converted into thermal
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