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.
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)
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
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
Absorbing Heat Objects that radiate also absorb. The radiation heat can be converted into thermal energy.