Lecture 12a Flashcards
Radiation does not require
a medium to take place
radiation occurs in
solids, liquid and gases
Radiation can occur between
two bodies even when they are separated by a medium colder than both
radiation is
electromagnetic waves
type of electromagnetic relevant to heat transfer
thermal radiation, wavelength 10^2 mirco metres to 10^-1 micro metres
what emits thermal radiation
all matter about absolute zero
thermal radiation increases with
temperature
What is a light source
a body that emits radiation in visible range ie the sun
at room temperature thermal radiation is
infrared region of the spectrum
radiation is a
volumetric phenomenon, but we treat it as a surface
a blackbody emits
the maximum amount of radiation by surface at a given temperature
and absorbs all incident radiation
a blackbody is
a perfect emitted and absorber of radiation, idealized body to serve as standard
Radiation energy emitted by a blackbody =
Eb = sigma *T^4
what is important to remember when dealing with radiation equation
always do in kelvin
What is spectral blackbody emissive power
amount of radiation energy emitted by a blackbody at a thermodynamic temperature T per unit time per unit surface area and per unit wavelength, about the wavelength lander
wavelength at which peak occurs for a specified tempearture is given by
wiens displacement law
lander *T = 2897
observation from variation of the blackbody emissive power with wavelength figure
emitted radiation is a continuous function of wavelength
amount of emitted radiation increases with temperature
as temperature increases curve shifts left to shorter wavelength, larger fraction of radiation is short wavelengths at high temps
Area under a Eblander lander chart represents
total radiation energy emitted by blackbody at that temperature
How to use Eblander lander chart to find energy emitted in wavelength band
find area under graph between wavelengths
what does blackbody radiation functions equal
fraction of radiation emitted from a blackbody at temperature T in the wavelength band from lander = 0 to lander
What does intensity describe
magnitude of radiation emitted or incident in a specified direction in space denoted by I
if opaque what can we consider radiation
a surface phenomenon
what is emissivity
ratio of radiation emitted by the surface at a given temperature to radiation emitted by a blackbody at the same temperature
how closely a surface approximates a blackbody
what will we always consider emissivity
constant grey and diffuse
emissivity equal to the value in the normal direction
if a surface is diffuse
properties are independent of direction
if surface is grey
properties are independent of wavelength
absorptivity =
alhpa = Absorbed radiation / Incident radiation = Gabs /G
What is G
irradiation G radiation flux incident on a surface
reflectivity
pho = Reflected radiation/G = Gref/G
Tranmissivity
tau = Transmitted radiation/G = Gtr/G
Draw a diagram of how incident radiation is split
see book
for opaque surfaces
tau = 0 therefore alpha + rho = 1
G =
Gabs + Gref + Gtr
alpha + rho + tau = 1
Specular reflection
angle of reflection equals angle of incidence (mirrorlike)
Diffuse reflection
radiation is reflected equally in all direction
What is reflection generally assumed
diffuse or specular
Kirchoffs law
Gabs = alhpa G = alpha *sigma*T^4 Eemit = em*sigma*T^4 As*em*sigma*T^4 = As*alpha*sigma*T^4
relationship between emissivity and absorptivity
epsilon (T) = alpha (T) for all cases
spectral form of kirchoffs law
