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NIU PHYS 210 - RADIATION

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RadiationHot MetalStefan-Boltzmann LawBlackbodySurface TemperatureAbsorbing HeatRadiationRadiationHot MetalHot MetalWhen 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 – whitewhiteLight can cross a vacuum.Light can cross a vacuum.•Carries energy with it.Carries energy with it.Stefan-Boltzmann LawStefan-Boltzmann LawRadiation increases with Radiation increases with temperature.temperature.Energy rate increases as 4Energy rate increases as 4thth power of temperaturepower of temperatureStefan-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))4ATePBlackbodyBlackbodyEmissivity 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 TemperatureAssume 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 = 4rr22Now apply the Stefan-Now apply the Stefan-Boltzmann LawBoltzmann LawTT = 5.8 x 10 = 5.8 x 1033 K K4/12)4(rPT Absorbing HeatAbsorbing HeatObjects 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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NIU PHYS 210 - RADIATION

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