6. Radiation

Question 1

What is the equation for the amount of heat emitted from a black body?

Answer 1

q = sigma T^4
sigma = 5.67x10^-8

Question 2

What is a black body?

Answer 2

A body which absorbs all incident light and emits the most given its temperature

Question 3

What is irradiation (G)

Answer 3

G = total rate at which radiation arrives per unit area
G = ( α + tau +ρ) G
α = absorbed, tau = transmitted ρ= reflected

Question 4

What is radiosity (J)

Answer 4

Total rate at which radiation leaves per unit area
J = εσT^4 + ρG
J = emitted + reflected

Question 5

What is the effective resistance to emission/absorption?

Answer 5

1-ε/(εA)

Question 6

What is the effective resistance to heat travelling from the surface of 1 to the surface of 2?

Answer 6

1/(A1 * F_12)

Question 7

How do you calculate the proportion of heat leaving one body which reaches another? (F12)

Answer 7

F12 = A2/A1

Question 8

What is a view factor (F_12)

Answer 8

Fraction of energy leaving surface 1 that reaches surface 2

Question 9

What is the view factor for a flat surface

Answer 9

0

Question 10

What is the view factor for a concave inner shell

Answer 10

1

Question 11

What is the reciprosity relationship

Answer 11

If T1 = T2, then F_12A1 = F_21A2

Question 12

What is Kirchoff’s Law

Answer 12

If T1 = T2, then epsilon = alpha (emitted = absorbed)

Question 13

What are the two types of scattering

Answer 13

Rayleigh and Mei scattering

Question 14

What molecules absorb infrared radiation from the earth best (greenhouse effect)

Answer 14

CO2, H2O, CH4 (methane), Chlorofluorocarbons (CFCs)

Question 15

How does the Greenhouse effect work

Answer 15

Short wavelength (high energy) radiation from the sun, penetrates the atmosphere and is absorbed on the surface

Earth warms and transmits long wavelength infrared radiation back

This infrared radiation is absorbed by certain molecules instead of leaving the atmosphere

Question 16

Why is the sky blue?

Answer 16

Rayleigh scattering intensity is proportional to 1 over wavelength^4. Hence shorter wavelengths are more intense (blue)

Question 17

What are the turbidity factors for clear air and smoggy air respectively

Answer 17

Clear air, n = 2

Smoggy air, n = 4

Question 18

What is Wien’s displacement law?

Answer 18

gamma max (wavelength at which E peaks) is proportional to 1 / T

Question 19

What is the range of wavelengths of light which is visible to the human eye

Answer 19

400 nm - 700 nm

Question 20

What is a radiation shield

Answer 20

A surface which intercepts the flow of radiation between a source and a sink. The shield itself becomes both the sink and the source. Its purpose is to reduce the net radiative heat transfer

Question 21

What is the extinction coefficient?

Answer 21

The ability of a material to absorb radiative energy

I_x=I_0 exp(-ax)