Post on 03-Jan-2020
Thermal Energy Transmission - Radiation
• Wavelength and frequency
• Wien’s Law
• Power/Intensity
Unit 05 Thermal Radiation - Slide 1
Electro-magnetic waves
*fv Velocity =How long it took you
How far you go=
Period
Wavelength
T
Unit 05 Thermal Radiation - Slide 2
Example 5.1 Wave Length
The frequency of blue visible light is around 620 X 1012
Hz. What is the wavelength of its radio wave?
The frequency of the country music station “US 99”
is 99.5 MHz (99.5 X 106 cycles/sec). What is the
wavelength of its radio wave?
Remember! All electromagnetic waves have the same speed!
v=c=3X108 m/secUnit 07 Electricity - Slide 3
Longer wavesHigher frequency
Enough
energy to
damage
cells
Thermal Radiation – Unit 05, Slide 4
60Hz =5X106m
How much energy via radiation?
4*** TAreaPower Heat per time
(Watts)
Emissivity
Stefan’s constant
= 5.67 X 10-8 W/(m2-K4)
-> amount absorbed (1 for black)
Stefan-Boltzman’s Law
Unit 07 Electricity - Slide 5
Everything Emits Radiation!
Thermal Radiation – Unit 05, Slide 6
As long as T = 0!
Example 5.2: Power from the sun
4*** TAreaP
What is the rate of energy released from
the sun via radiation?
• Tsun = 5887K
• sun~1
• rsun=7X108m
24 rSurface area of Sphere =
(on formula sheet)
Unit 05 Thermal Radiation - Slide 7
Example 5.3: Temperature of the Earth
Unit 05 Thermal Radiation - Slide 8
4*** TAreaP
We just calculated that the sun puts out
4.14X1026 Watt of power via radiation.
How much of that power hits the earth?
The radius of the earth is rearth=6.4X106m.
The earth is rorbit=1.5X1011m from the sun.
A) 1.9X1017Watts
B) 0.9X1017Watts
C) 2.4X1012Watts
D) 3.4X1011Watts
Example 5.3: Temperature of the Earth
Unit 05 Thermal Radiation - Slide 9
4*** TAreaP
We just calculated that 1.9X1017Watts of power
hits the earth via radiation. 30% is reflected by
the atmosphere, so the earth absorbs
1.3X1017 Watts
If the same amount of power must leave the
earth, what should the temperature of the earth
be?
A) 410K
B) 350K
C) 260K
D) 150K
earth~1, rearth=6.4X106m
24 rSurface area of Sphere =
(on formula sheet)
Wien’s Law
T
mmKpeak
9.2
Why is earth not freezing all the time?
Wien’s Law – peak depends on T
• out is longer than in
• earth is smaller for longer
• “Green house effect”
Unit 05 Thermal Radiation - Slide 10
Example 5.4: Peak
The temperature of the sun is 5887K.
What is the peak wave length, peak, of the
radiation beginning transmitted?
What is its frequency of the wave, f ?
What is the period of the wave, T ?
What is the velocity of the wave?
Unit 05 Thermal Radiation - Slide 11
Easy! All electromagnetic waves have the same speed v=c=3X108 m/sec
Longer wavesHigher frequency
Enough
energy to
damage
cells
Thermal Radiation – Unit 05, Slide 12
Wien’s Law
Thermal Radiation – Unit 05, Slide 13
Example 5.5: Peak
The temperature of the earth is around 287K.
What is the peak wave length of the radiation
beginning transmitted?
Unit 05 Thermal Radiation - Slide 14
Longer waves
Higher frequency
Enough
energy to
damage
cells
Thermal Radiation – Unit 05, Slide 15
Question
Thermal Radiation – Unit 05, Slide 16
We just determined that the peak wave
length of the electromagnetic waves coming
from the earth peaks at around 4.9mm.
What is the velocity of this wave?
A) 4.9 m/sec
B) 4.9 mm/sec
C) 3X108m/sec
D) 4.9 X 108m/sec
E) 1/4.9 mm/sec
Example 5.6: Do you glow?
Do you emit light?
The temperature of your skin is
around 98.6oF. What is the
peak wave length of the
radiation you transmit?
Unit 05 Thermal Radiation - Slide 17
Question
Unit 05 Thermal Radiation - Slide 18
The temperature of your skin is
around 98.6oF. What is the
temperature of your skin in Kelvins?
A) 0K
B) 98.6K
C) 37K
D) 310K
E) 522K
Question
Unit 05 Thermal Radiation - Slide 19
The temperature of your skin is
around 98.6oF. What is the peak
wavelength of the light you emit?
A) 2.9X10-3m
B) 2.9X10-6m
C) 3.1X10-6m
D) 6.2X10-6m
E) 9.4X10-6m
Longer wavesHigher frequency
Enough
energy to
damage
cells
Thermal Radiation – Unit 05, Slide 20
Look for heat losses?
Thermal Radiation – Unit 05, Slide 21
Thermal Radiation – Unit 05, Slide 22
Heat Loss at my house
Thermal Radiation – Unit 05, Slide 23
Thermal Radiation – Unit 05, Slide 24
Older Window
(double pane)
Colder at center
Modern Window
(triple pane)
Colder at frame
Embarrassing item #1
8” of concrete (e.g. uninsulated basement wall) has R ~ 1.35
Thermal Radiation – Unit 05, Slide 25
Brand new zillion
dollar window
R~5.1
Exposed concrete
basement wall
R~1.35
Embarrassing item #2
2” of wood (e.g. uninsulated band board + siding) has R ~ 2.2
Thermal Radiation – Unit 05, Slide 26
Really BIG brand new
zillion dollar window
R~5.1
Exposed concrete
basement wall
R~1.35
Insulated Wall
R~13
2”X10” Bandboard +
siding
R~2.2
(Note: Exterior Shot)
Embarrassing item #3
A wood-on-wood connection makes a very poor air seal
Thermal Radiation – Unit 05, Slide 27
Cold air pouring into basement at joint
between band board and foundation
wall
Cold air pouring into kitchen at joint
between wall frame and floor
(Note: Photos taken during 5Pa vacuum air
tightness test)
Embarrassing item #4
Insulation doesn’t always stay where you put it and in the condition you put it there
Thermal Radiation – Unit 05, Slide 28
New insulation fell during sheetrock
installation?Insulation damaged by
past water leak