Radiation Flashcards
What is the electromagnetic spectrum of an object
Characteristics distribution of electromagnetic radiation emitted or absorbed by that particular object
What are the electromagnetic spectrums
-Radiowaves
-Microwaves
-Infrared
-Visible
-Ultraviolet
-X-ray
-Gamma Ray
What is the trends of electromagnetic spectrum?
Wavelength gets shorter as you go down the spectrum
The frequency increases as you go down
What 3 things happen when thermal radiation hits a surface
-Can be absorbed by the surface
-Can be reflected off the surface
-Can be transmitted through the surface
How does temperature effect thermal radiation
Its intensity and spectrum
What happens when temperature increases in relation to wavelengths and energy
Wavelengths decrease and energy increases
What are the exchanges of radiation
-All bodies emit radiation to their surroundings
-All bodies receive radiation from their surrondings
What is prevosts theory
Body emits radiation at a rate which depends only on its temperature and nature of surface
A body absorbs radiation at a rate which depends only on the temperature of its surroundings and on the nature of its surface
“Consider a perfectly insulated body at a
temperature TB: it cannot lose or gain heat by
conduction or convection, only by radiation.
* If it is surrounded by some form of enclosure
which is maintained at a constant temperature TS”
Explain what happens in all situations of this?
If TB>TS the body emits radiation at a greater rate than it absorbs it so TB falls until it is equal to TS
If TS>TB the body absorbs radiation at a greater rate than it absorbs so TB rises until it is equal to TS
If TB=TS the rate of absorption is equal to the rate of emission and TB doesnt change
What colours are the best absorbers and emitters?
-Black (Best)
-White
-Gray (worst)
Difference between transmission and emission?
Transmission: Part of the radiation passes through the body
Emission: The amount of thermal radiation released or given off by a body
What is the Black Body?
An object that absorbs all the radiation which strikes it
What does emissivity describe?
Describes the surface radiation properties of a body
What is emissivity?
The ratio of energy radiated/absorbed by a particular material to energy radiated/absorbed by a black body at the same temperature
What do the emissivity range mean?
0-1
0:A body that does not emit or absorb at all
1:A body that emit at the maximum possible rate an absorbs all the energy striking it
A mirror has an emissivity of = 0.1. A laser beam with a
power of 5 mW is incident on the mirror. What is the
power of the reflected beam
Since the emissivity is 0.1, so is the absorption factor.
10% of the power is absorbed, and 90% is reflected.
So the reflected power is 50.9 = 4.5 mW.
The rest of the power has been absorbed into the material of the mirror
and converted to heat
Explain how punching a small hole in the lid of an empty meal tin is a close approximation to a black body?
Light enters the hole and is reflected many times by the wall which will get a % absorbed each time before it reaches the hole again.
Is the distribution of energy even?
No, the distributions of energy are not even and it depends on the source temperature
(for ex: heating metal red will appear first as it the longest wavelength then yellow then orange then blue and the combination appears white)
What is Weins Law
Product of the wavelength of the peak of the spectrum and the temperature of the black body is a constant
Describe the radiated power over wavelength graph
At lower wavelengths there is little photons so less radiated power and as wavelength increases there is a peak radiated power around the middle as there is a high number of photons at high energy
What is weins formula?
Wavelengthp=0.0029/T (In kelvin)
If the temperature of a blackbody is 500K what will the peak be?
p = 0.0029/500 m
= 5.8x10^-6 = 5.8 micrometer
What is Stefans Law?
Gives the total intensity of the radiation over all the over all wavelengths
Formula for Stefans Law
Q=oAT^4 (o=5.7x10^-8 Stefans constant, A is the area radiating and T is absolute temperature)
What is Weins law?
the product of the wavelength of the peak of the
spectrum and the temperature of the blackbody is a constant
What is the formula for weins law?
Wavelength= Weins constant/Temperature in Kelvin (0.0029/T)
Describe the Radiated Power vs Wavelength graph
Due to weins law curves with a higher radiated power peaks are at lowers wavelengths because as temperature increases wavelength max decreases
Lower temperature objects emit mainly in the infrared region
What is Stefans Law
gives the total intensity (power) of
the radiation over all wavelengths
How will Stefans Law be modified if the object isnt a perfect black body
Q=e (emissivity) x o (stefans consstant) x Area x Temperature
What is the value of Stefans constant
5.7x10^-8Wm^-2K^-4
A 100 W light bulb has a filament which is 0.60 m long with a
diameter of 8.010−5 m. The filament may be regarded as a black
body. Estimate its working temperature. If the filament is not a
perfect blackbody, would the working temperature be higher or
lower? Why?
Surface area of filament (excluding ends) = pixdxL
= pi x 8.0x10^−5x0.60 = 1.51x10−4 m2
For a blackbody, Stefan’s law:
The power of the light bulb is 100 W,
So 100 = 5.710−81.5110−4T4
Hence T4 = 1.161013 or T = 1846 K
The working temperature of the filament is 1846 K
If the filament is not a perfect black body, i.e. <1, since the electrical power
supplied stays the same, the radiated power stays the same, the working
temperature will be higher. For example, if
= 0.9, T will rise to 1895 K.
However, the filament will be a less efficient radiator.
What will happen if a black body at T1 is placed in an enclosure T2
-Heat is radiated at: Q=oxSxT1^4
Heat is absorbed at: oAT2^4
Net energy flow from the enclosure to the blackbody:oxA(T2^4-T1^4) (Heat absorbed-Heat radiated)
A potato at a temperature of 150ºC is placed in a refrigerator whose
walls are at 2ºC. Assuming the potato is a perfect black body, and is
spherical with a diameter of 7 cm, and all heat transfer is by
radiation, calculate the rate at which it loses energy.
Surface area of potato =pi x d^2 = 1.54x10^−2 m2.
Power emitted is 5.7x10^−8x1.54x10^−2 x (273+150)^4 = 28.1 W
Power absorbed is
5.7x10^−8x1.54x10^−2x(273+2)^4 = 5.0 W
Thus, the potato therefore initially loses heat at 23.1 W.
This will fall with time, as it cools.
– If instead the potato was at 2ºC, and was in an oven at 150ºC, the calculation
would be the other way round: the potato would gain energy at 23.1 W.
– Microwave ovens supply much more power by restricting the wavelengths
radiated to a very narrow band which excites water molecules – they are not
black bodies
Formula for energy of a photon
E=hf
E= Energy of photon
h= Plancks Constant (6.626x10^34)
f= Frequency
Speed equation
c=f x wavelength
c= speed
f=frequency
wavelength
Relationship between Energy, Frequency, and plancks constant and wavelength
E=hc/wavelength
E=energy
h= Wavelength (6.626x10^34)
c=speed of light (3x10^8)
Formula for intensity
I=P/A
I=Intensity
P=Power
A=Area
Surface area for a sphere?
A=4xpixr^2
Surface area of a filament
A = 2 π r L
What is the area of a cylinder? and what is it without the ends?
2nr^2+2nrh
n=pi
r=radius
h=height
Without ends:
n x d x L
