Radiation

e = 1

Perfect blackbody (ideal emitter and absorber)

e = 0

Perfect reflector or transmitter

Black/dark surfaces

Surfaces that are good absorbers and good emitters. They have a high e.

White/shiny/silver surfaces

Surface that are poor absorbers and poor emitters. Have a low e.

Yes, it is true!

Is it true that the total radiated power is proportional to the surface area A; More Area → More radiation emitted or absorbed.

EM radiation intensity

(1)

UV

(2)

Visible range (violet → red)

(3)

6000 K (white hot)

(4)

4000 K

(5)

3000 K (red hot)

(6)

λ (nm)

(7)

P

Symbol that represents the Power Radiated

σ

Symbol that represents the Stefan-Boltzmann constant.

σ = 5.67 × 10^-8 J/s • m^2 • K^4

Value of σ

e

Symbol that represents the emissivity

T

Symbol that represents the absolute Temperature in Kelvin

Radiated power grows very rapidly with temperature

Explain the T^4 dependance

P = σeAT^4

Mathematical representation of the Stefan-Boltzmann Law

Pnet = σeA((T_2)^4 - (T_1)^4)

Provide the mathematical representation of the net radiative heat transfer if an object at temperature T1 is surrounded by an environment at Temperature T2

When the environment is hotter than the object

When is the net radiative heat transfer positive?

When the object is hotter than the environment.

When is the net radiative heat transfer negative?

~0.97, black

Human skin has a high emissivity in the infrared (_____), making human nearly “_____” in IR, which is why we feel radiation so well and why IR cameras can detect people easily.

shortwave radiation, infrared radiation

Earth absorbs mainly visible and ________________ from the Sun and re-emits _______________.

Greenhouse Gases

Gases like CO₂, H₂O, and CH₄ that absorb much of the IR and re-radiate part of it back to Earth, keeping the surface ~40% warmer than with no atmosphere.

These gases enhance greenhouse effect and contribute to global warming when their concentration increases.

0.65

Average Emissivity of the Earth

Clouds

______ both reflect sunlight and re-radiate IR, affecting Earth’s energy balance.

Role of Clouds

Higher temperatures → More evaporation → More Clouds → More Reflection of Sunlight → Cooling effect that partially opposes the original warming (negative feedback)