1. Radiation

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