Radiation
Hot Metal   When metal is heated    enough it glows.   The color changes with    temperature.    • Red – orange – yellow...
Stefan-Boltzmann Law   Radiation increases with    temperature.   Energy rate increases as 4th    power of temperature ...
Blackbody   Emissivity measures the ability of an object to radiate    heat.   It is the same to absorb heat.   A perfe...
Surface Temperature   Assume the sun is a                 First find the surface area.    blackbody radiating 3.9 x 1026...
Absorbing Heat   Objects that radiate also    absorb.   The radiation heat can be    converted into thermal    energy.
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P210 13f

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P210 13f

  1. 1. Radiation
  2. 2. Hot Metal When metal is heated enough it glows. The color changes with temperature. • Red – orange – yellow – white Light can cross a vacuum. • Carries energy with it.
  3. 3. Stefan-Boltzmann Law Radiation increases with temperature. Energy rate increases as 4th power of temperature Stefan-Boltzmann Law. P = eσAT 4 • P = power (W) • A = surface area (m2) • T = temperature (K) • e = emissivity (0 to 1) ∀ σ = constant (5.67 x 10-8 W/m2K4)
  4. 4. Blackbody Emissivity measures the ability of an object to radiate heat. It is the same to absorb heat. A perfect emissivity of 1 is a blackbody. Low e near 0 High e near 1
  5. 5. Surface Temperature Assume the sun is a  First find the surface area. blackbody radiating 3.9 x 1026  A = 4πr2 W, and its radius is 7.0 x 108 m.  Now apply the Stefan- Boltzmann Law What is the surface P 1/ 4 T =( ) temperature? 4πr σ 2  T = 5.8 x 103 K
  6. 6. Absorbing Heat Objects that radiate also absorb. The radiation heat can be converted into thermal energy.
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