| Topic: |
Science > Physics |
| User: |
"" |
| Date: |
21 Aug 2006 12:15:36 PM |
| Object: |
Which radiates more FIR: metal or water? |
Hi,
If we place a sheet of metal next to a puddle of water in complete
darkness, and both objects are the same temperature, which radiates
more FIR (Far Infrared Radiation)?
Today I picked up a piece of metal that was in the Sun. Of course we
all know a piece of metal in the Sun *feels* hot. Yet, is this just an
illusion because metal has high thermal conductivity?
I read somewhere about a physics law that basically states a good
absorber of radiation is a good radiator; i.e., it generates more FIR
due to temperature. Does anyone recall the name of this physics law?
Also, is there a similar law that states a good reflector is a bad
absorber, and hence a bad radiator?
Metals have 96% to 99% reflectance in FIR and water is one of the best
FIR radiators, close to 100%. Therefore, it seems to me that in
complete darkness a puddle of water should radiate at least 10 times
more FIR than a sheet of metal if both objects are at the same
temperature.
Thanks for any input,
Paul
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| User: "Tom" |
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| Title: Re: Which radiates more FIR: metal or water? |
21 Aug 2006 12:58:20 PM |
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<softwarelabus@yahoo.com> wrote in message
news:1156180536.903481.120210@74g2000cwt.googlegroups.com...
Hi,
If we place a sheet of metal next to a puddle of water in complete
darkness, and both objects are the same temperature, which radiates
more FIR (Far Infrared Radiation)?
checkout emissivity
Today I picked up a piece of metal that was in the Sun. Of course we
all know a piece of metal in the Sun *feels* hot. Yet, is this just an
illusion because metal has high thermal conductivity?
no, the object is converting energy in the band of sunlight into the heat
(IR band)
the emissivitys are different in the two bands.
I read somewhere about a physics law that basically states a good
absorber of radiation is a good radiator;
In that optical band
i.e., it generates more FIR
due to temperature. Does anyone recall the name of this physics law?
Also, is there a similar law that states a good reflector is a bad
absorber, and hence a bad radiator?
Metals have 96% to 99% reflectance in FIR and water is one of the best
FIR radiators, close to 100%.
reflectance or absorbance?
Which band of FIR?
Therefore, it seems to me that in
complete darkness a puddle of water should radiate at least 10 times
more FIR than a sheet of metal if both objects are at the same
temperature.
Thanks for any input,
Paul
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| User: "" |
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| Title: Re: Which radiates more FIR: metal or water? |
21 Aug 2006 02:16:03 PM |
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Thanks for reply.
Tom wrote:
<softwarelabus@yahoo.com> wrote in message
news:1156180536.903481.120210@74g2000cwt.googlegroups.com...
Hi,
If we place a sheet of metal next to a puddle of water in complete
darkness, and both objects are the same temperature, which radiates
more FIR (Far Infrared Radiation)?
checkout emissivity
Thanks. My example of water was poorly chosen. Perhaps a better choice
is lampblack carbon. Do you happen to know the emissivity of metals in
the FIR band? The below web page shows emissivity of numerous metals,
but does not state the band:
http://www.electro-optical.com/bb_rad/emissivity/matlemisivty.htm
I know typical engineers like to go by the grey body assumption, but
realistically material emissivity changes with wavelength. I found the
name of the law I was seeking, called Kirchhoff's law, which states
emissivity equals absorptivity for an object in thermal equilibrium.
Therefore, if an object has low emissivity in the band of interest,
then the object is obviously a low absorber since it is reflecting, not
absorbing. Therefore, could I state the following law: Good reflectors
are poor radiators.
According to the table in the above website, polished aluminum has low
emissivity. Therefore could I conclude that metal is colder than
lampblack carbon in direct Sunlight? I am referring to true
temperature, not temperature according to touch.
i.e., it generates more FIR
due to temperature. Does anyone recall the name of this physics law?
Also, is there a similar law that states a good reflector is a bad
absorber, and hence a bad radiator?
Metals have 96% to 99% reflectance in FIR and water is one of the best
FIR radiators, close to 100%.
reflectance or absorbance?
I was simply distinguishing two separate references. 1) Metals have 96%
to 99% reflectance in the entire FIR band. 2) Water is one of the best
FIR radiators, close to 100%.
Which band of FIR?
I'm interested in 5 THz to 50 THz, but the graphs I've seen on metal
(bare, no coatings) reflectivity show no drastic changes in the entire
FIR band. Sure, there are bumps in the graphs, but nothing I'd call
drastic; e.g., dropping from 96% to 10%
Paul
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| User: "Don Klipstein" |
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| Title: Re: Which radiates more FIR: metal or water? |
21 Aug 2006 04:15:40 PM |
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In article <1156187763.631450.29720@m73g2000cwd.googlegroups.com>,
softwarelabus@yahoo.com wrote:
Thanks for reply.
Tom wrote:
<softwarelabus@yahoo.com> wrote in message
news:1156180536.903481.120210@74g2000cwt.googlegroups.com...
Hi,
If we place a sheet of metal next to a puddle of water in complete
darkness, and both objects are the same temperature, which radiates
more FIR (Far Infrared Radiation)?
checkout emissivity
Thanks. My example of water was poorly chosen. Perhaps a better choice
is lampblack carbon. Do you happen to know the emissivity of metals in
the FIR band? The below web page shows emissivity of numerous metals,
but does not state the band:
http://www.electro-optical.com/bb_rad/emissivity/matlemisivty.htm
That appears to me to be emissivity for "room temperature" blackbody
radiation.
I know typical engineers like to go by the grey body assumption, but
realistically material emissivity changes with wavelength. I found the
name of the law I was seeking, called Kirchhoff's law, which states
emissivity equals absorptivity for an object in thermal equilibrium.
Therefore, if an object has low emissivity in the band of interest,
then the object is obviously a low absorber since it is reflecting, not
absorbing. Therefore, could I state the following law: Good reflectors
are poor radiators.
According to the table in the above website, polished aluminum has low
emissivity. Therefore could I conclude that metal is colder than
lampblack carbon in direct Sunlight? I am referring to true
temperature, not temperature according to touch.
Barring convection and conduction, temperature rise will vary directly
with ratio of solar absorption to room temperature emissivity. If the
lampblack has a solar absorption of .95 and an emissivity of .9, and the
metal has a solar absorption of .12 and a room temperature emissivity of
..05, then barring cooling by air the metal will get hotter.
- Don Klipstein (don@misty.com)
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| User: "Alex Terrell" |
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| Title: Re: Which radiates more FIR: metal or water? |
24 Aug 2006 03:18:25 AM |
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Don Klipstein wrote:
In article <1156187763.631450.29720@m73g2000cwd.googlegroups.com>,
softwarelabus@yahoo.com wrote:
Thanks for reply.
Tom wrote:
<softwarelabus@yahoo.com> wrote in message
news:1156180536.903481.120210@74g2000cwt.googlegroups.com...
Hi,
If we place a sheet of metal next to a puddle of water in complete
darkness, and both objects are the same temperature, which radiates
more FIR (Far Infrared Radiation)?
checkout emissivity
Thanks. My example of water was poorly chosen. Perhaps a better choice
is lampblack carbon. Do you happen to know the emissivity of metals in
the FIR band? The below web page shows emissivity of numerous metals,
but does not state the band:
http://www.electro-optical.com/bb_rad/emissivity/matlemisivty.htm
That appears to me to be emissivity for "room temperature" blackbody
radiation.
I know typical engineers like to go by the grey body assumption, but
realistically material emissivity changes with wavelength. I found the
name of the law I was seeking, called Kirchhoff's law, which states
emissivity equals absorptivity for an object in thermal equilibrium.
Therefore, if an object has low emissivity in the band of interest,
then the object is obviously a low absorber since it is reflecting, not
absorbing. Therefore, could I state the following law: Good reflectors
are poor radiators.
According to the table in the above website, polished aluminum has low
emissivity. Therefore could I conclude that metal is colder than
lampblack carbon in direct Sunlight? I am referring to true
temperature, not temperature according to touch.
Barring convection and conduction, temperature rise will vary directly
with ratio of solar absorption to room temperature emissivity.
Convection can be barred by placing object in vacuum. Conduction is
trickier. Coming to something close to the original example, place a
block of shiny aluminium and a block of black carbon at say 500 Kelvin,
in vacuum chamber. Which cools down faster?
Initially, the carbon will radiate more. However, the outer surface
will rapidly cool, thereby redcuing the level of radiation. If the
block is large, then heat transfer is limited by internal conductivity,
rather than radiation. The metal block on the other hand will maintain
roughly the same temperature throughout, and after some time will
radiate more than the carbon block.
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| User: "" |
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| Title: Re: Which radiates more FIR: metal or water? |
21 Aug 2006 05:36:11 PM |
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Don Klipstein wrote:
In article <1156187763.631450.29720@m73g2000cwd.googlegroups.com>,
softwarelabus@yahoo.com wrote:
Thanks for reply.
Tom wrote:
<softwarelabus@yahoo.com> wrote in message
news:1156180536.903481.120210@74g2000cwt.googlegroups.com...
Hi,
If we place a sheet of metal next to a puddle of water in complete
darkness, and both objects are the same temperature, which radiates
more FIR (Far Infrared Radiation)?
checkout emissivity
Thanks. My example of water was poorly chosen. Perhaps a better choice
is lampblack carbon. Do you happen to know the emissivity of metals in
the FIR band? The below web page shows emissivity of numerous metals,
but does not state the band:
http://www.electro-optical.com/bb_rad/emissivity/matlemisivty.htm
That appears to me to be emissivity for "room temperature" blackbody
radiation.
I know typical engineers like to go by the grey body assumption, but
realistically material emissivity changes with wavelength. I found the
name of the law I was seeking, called Kirchhoff's law, which states
emissivity equals absorptivity for an object in thermal equilibrium.
Therefore, if an object has low emissivity in the band of interest,
then the object is obviously a low absorber since it is reflecting, not
absorbing. Therefore, could I state the following law: Good reflectors
are poor radiators.
According to the table in the above website, polished aluminum has low
emissivity. Therefore could I conclude that metal is colder than
lampblack carbon in direct Sunlight? I am referring to true
temperature, not temperature according to touch.
Barring convection and conduction, temperature rise will vary directly
with ratio of solar absorption to room temperature emissivity. If the
lampblack has a solar absorption of .95 and an emissivity of .9, and the
metal has a solar absorption of .12 and a room temperature emissivity of
.05, then barring cooling by air the metal will get hotter.
I see. I did not consider the fact that an object could indeed radiate
a different amount than it absorbs. Ugg! Why didn't I think of that? I
think the following example clarifies this. Project a very intense red
laser beam on an object. The laser beam heats the object, but due to
heat the object does not radiate just red light. The object emits a
wide band of radiation. So as you pointed out, the objects temperature
depends on the difference between absorption and emissivity.
Thanks,
Paul
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| User: "urs boegli" |
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| Title: Re: Which radiates more FIR: metal or water? |
22 Aug 2006 01:59:32 AM |
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I see. I did not consider the fact that an object could indeed radiate
a different amount than it absorbs. Ugg! Why didn't I think of that? I
think the following example clarifies this. Project a very intense red
laser beam on an object. The laser beam heats the object, but due to
heat the object does not radiate just red light. The object emits a
wide band of radiation. So as you pointed out, the objects temperature
depends on the difference between absorption and emissivity.
Thanks,
Paul
Paul,
basically it is just the conservation of energy law. The red laser
energy is absorbed to a certain percentage, called absorption
coefficient of this material at the laser's wavelength In a first
approach this would heat up the material linearely for ever.
On the other hand as soon as the material's temperature is higher than
than the environment's one more energy is radiated than absorbed. This
energy is the integral of the emissivity times Plank curve over all the
wavelengths, but in this case it's approx. the IR emissivity around 10um
where is the peak of the emission at room temperature.
The temperature of the material rises until the emitted power is equal
to the absorbed one.
I would be interested in THz coefficients, too!
Urs
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| User: "" |
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| Title: Re: Which radiates more FIR: metal or water? |
22 Aug 2006 01:27:21 PM |
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urs boegli wrote:
I see. I did not consider the fact that an object could indeed radiate
a different amount than it absorbs. Ugg! Why didn't I think of that? I
think the following example clarifies this. Project a very intense red
laser beam on an object. The laser beam heats the object, but due to
heat the object does not radiate just red light. The object emits a
wide band of radiation. So as you pointed out, the objects temperature
depends on the difference between absorption and emissivity.
Thanks,
Paul
Paul,
basically it is just the conservation of energy law. The red laser
energy is absorbed to a certain percentage, called absorption
coefficient of this material at the laser's wavelength In a first
approach this would heat up the material linearely for ever.
On the other hand as soon as the material's temperature is higher than
than the environment's one more energy is radiated than absorbed. This
energy is the integral of the emissivity times Plank curve over all the
wavelengths, but in this case it's approx. the IR emissivity around 10um
where is the peak of the emission at room temperature.
The temperature of the material rises until the emitted power is equal
to the absorbed one.
I would be interested in THz coefficients, too!
Thanks. I agree, there should be more THz coefficients. Some of the
data I've seen on emissivity is questionable. Consider the following
data:
Emissivity:
Plexiglass, Perpex: 0.86 (2 - 5.6 um?)
Quartz: 0.93 (8 - 14 um?)
Teflon overcoating: 0.38 Note, no mention of coating thickness.
The following graph shows Quartz, Plexiglas, and Teflon as being
transparent to THz with low losses:
http://www.mtinstruments.com/thzresources/THz%20properties%20of%20materials.pdf
The following pdf states and shows THz imaging traverses through an
"18mm-thick block of Teflon." See page 6 of 10.
http://www.mtinstruments.com/thzresources/BWO%20THz%20Imaging.pdf
As far as I know, THz imaging and Spectroscopy shows all electrically
insulating materials are reasonably transparent up to at least 50 THz
(6 um), which makes sense to me. Albeit these wavelengths are shorter
than microwaves, but we know microwaves have no effect on electrically
insulated materials. The following site defines microwaves as being
from 300 MHz to 300 GHz:
http://amsglossary.allenpress.com/glossary/browse?s=m&p=38
I'd appreciate any clarity on this topic.
Paul
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| User: "Sam Wormley" |
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| Title: Re: Which radiates more FIR: metal or water? |
21 Aug 2006 01:37:03 PM |
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wrote:
Hi,
If we place a sheet of metal next to a puddle of water in complete
darkness, and both objects are the same temperature, which radiates
more FIR (Far Infrared Radiation)?
Today I picked up a piece of metal that was in the Sun. Of course we
all know a piece of metal in the Sun *feels* hot. Yet, is this just an
illusion because metal has high thermal conductivity?
I read somewhere about a physics law that basically states a good
absorber of radiation is a good radiator; i.e., it generates more FIR
due to temperature. Does anyone recall the name of this physics law?
Also, is there a similar law that states a good reflector is a bad
absorber, and hence a bad radiator?
Metals have 96% to 99% reflectance in FIR and water is one of the best
FIR radiators, close to 100%. Therefore, it seems to me that in
complete darkness a puddle of water should radiate at least 10 times
more FIR than a sheet of metal if both objects are at the same
temperature.
Thanks for any input,
Paul
Emissivity Coefficient
http://scienceworld.wolfram.com/physics/EmissivityCoefficient.html
Emissivity Values for Common Materials (including water)
http://www.infrared-thermography.com/material-1.htm
Material Emissivity Properties
http://www.omega.com/literature/transactions/volume1/emissivitya.html
http://www.electro-optical.com/bb_rad/emissivity/matlemisivty.htm
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| User: "" |
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| Title: Re: Which radiates more FIR: metal or water? |
21 Aug 2006 03:29:10 PM |
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Sam Wormley wrote:
softwarelabus@yahoo.com wrote:
Hi,
If we place a sheet of metal next to a puddle of water in complete
darkness, and both objects are the same temperature, which radiates
more FIR (Far Infrared Radiation)?
Today I picked up a piece of metal that was in the Sun. Of course we
all know a piece of metal in the Sun *feels* hot. Yet, is this just an
illusion because metal has high thermal conductivity?
I read somewhere about a physics law that basically states a good
absorber of radiation is a good radiator; i.e., it generates more FIR
due to temperature. Does anyone recall the name of this physics law?
Also, is there a similar law that states a good reflector is a bad
absorber, and hence a bad radiator?
Metals have 96% to 99% reflectance in FIR and water is one of the best
FIR radiators, close to 100%. Therefore, it seems to me that in
complete darkness a puddle of water should radiate at least 10 times
more FIR than a sheet of metal if both objects are at the same
temperature.
Thanks for any input,
Paul
Emissivity Coefficient
http://scienceworld.wolfram.com/physics/EmissivityCoefficient.html
Emissivity Values for Common Materials (including water)
http://www.infrared-thermography.com/material-1.htm
Material Emissivity Properties
http://www.omega.com/literature/transactions/volume1/emissivitya.html
http://www.electro-optical.com/bb_rad/emissivity/matlemisivty.htm
Thanks for the links. I particularly like
http://www.infrared-thermography.com/material-1.htm except not sure
what units of wavelength he's specifying. He uses "m", which seems to
be microns, but if you click on "Measured Normal Emissivities" he uses
"mm."
Paul
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| User: "Tom" |
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| Title: Re: Which radiates more FIR: metal or water? |
21 Aug 2006 04:31:24 PM |
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<softwarelabus@yahoo.com> wrote in message
news:1156192149.929134.311320@74g2000cwt.googlegroups.com...
Sam Wormley wrote:
softwarelabus@yahoo.com wrote:
Hi,
do the quiz over here, fun!
http://www.snellinfrared.com/iriq/index.asp
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| User: "" |
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| Title: Re: Which radiates more FIR: metal or water? |
21 Aug 2006 05:16:33 PM |
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Tom wrote:
<softwarelabus@yahoo.com> wrote in message
news:1156192149.929134.311320@74g2000cwt.googlegroups.com...
Sam Wormley wrote:
softwarelabus@yahoo.com wrote:
Hi,
do the quiz over here, fun!
http://www.snellinfrared.com/iriq/index.asp
That's cool. I particularly like Q 8 on first quiz. Here's the answer:
"The horizontal surfaces will reach the dew point first because they
cool faster than the other surfaces. Why? This is due to increased
cooling by radiation to the night sky. On cloudy nights, radiational
cooling is minimized."
Q 9 is good-- similar to 8.
But most all Q 10 was most interesting, "10. A material has a
reflectivity of 0.10, and its transmissivity is 0.10. What is its
emissivity?" Answer, "0.8 emissivity. This is the Law of Conservation
of Energy (also known as the "RAT Law"). R+A+T=1.0."
BTW, I got 100% on Level 1, hehe. It says I'm "Smokin' hot, which I
doubt!
I think the quiz answered my existing forum question. RAT's law! So
it's true that a good reflector (reflectivity) makes a poor absorber
(emissivity). If reflectivity is high, 0.9+, then emissivity must be <=
0.1
Thanks!
Paul
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| User: "Richard J Kinch" |
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| Title: Re: Which radiates more FIR: metal or water? |
21 Aug 2006 07:11:05 PM |
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If we place a sheet of metal next to a puddle of water in complete
darkness, and both objects are the same temperature, which radiates
more FIR (Far Infrared Radiation)?
What metal, and what color or coating? (Consider why auto radiators are
painted black).
.
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| User: "tj Frazir" |
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| Title: Re: Which radiates more FIR: metal or water? |
21 Aug 2006 09:04:52 PM |
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The same temp is the same fir.
metal just wount be the same temp as the water on the ground.
If it was the same temp exsact you could not tell the letal from the
water .
( thermal ,,)
but reflective you can tell because metal would reflect better. Gold the
best.
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