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. Radiation from Heat Heated objects give off electromagnetic waves.
Higher temperature has more radiation
A hot object gives off a spectrum of frequencies.
Shifts based on temperature
low energy
high energy
frequency
intensity

4. Stefan-Boltzmann Law Radiation increases with temperature.
Energy rate increases as 4th power of temperature
Stefan-Boltzmann Law.
P = power (W)
A = surface area (m2)
T = temperature (K)
e = emissivity (0 to 1)
s = constant (5.67 x 10-8 W/m2K4)

5. 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

6. Surface Temperature
Assume the sun is a blackbody radiating 3.9 x 1026 W, and its radius is 7.0 x 108 m.
What is the surface temperature?
First find the surface area.
A = 4pr2
Now apply the Stefan-Boltzmann Law
T = 5.8 x 103 K

7. Absorbing Heat Objects that radiate also absorb.
Absorption coefficients for different materials
The radiation heat can be converted into thermal energy.
Angle of radiation reduces energy

8. Seasons Earth absorbs radiation from the Sun.
The tilt of Earth’s axis changes the radiation absorbed.
Summer vs winter

9. Greenhouse Effect
The atmosphere only permits visible light and radio waves.
Visible light warms the surface.
The surface absorbs visible light and radiates infrared.
The infrared is trapped and warms the air.
atmosphere
visible light
infrared light
earth