| Topic: |
Science > Physics |
| User: |
"Hlafordlaes" |
| Date: |
03 Oct 2005 06:21:03 AM |
| Object: |
Dumb Question |
Boy, does this show my ignorance, but...
Could someone tell me why lower energy EM frequencies (infrared and
microwave) transmit heat better than higher energy frequencies? My
utterly simple guess is that the wavelengths in question interact with
atoms more than other wavelengths (eg x-ray "misses" and therefore does
not excite many particles???).
(In my defense, I missed high school physics altogether because I was
sent overseas and the topic wasn't covered those years in the place I
went.)
If you've got the time, you might write a bit more about heat in
general...
A very embarrassed thanks.
.
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| User: "Steven Gray" |
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| Title: Re: Dumb Question |
03 Oct 2005 07:05:08 PM |
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"Hlafordlaes" <hlafordlaes@gmail.com> wrote in
news:1128338463.358918.153180@g49g2000cwa.googlegroups.com:
Boy, does this show my ignorance, but...
Could someone tell me why lower energy EM frequencies (infrared and
microwave) transmit heat better than higher energy frequencies? My
utterly simple guess is that the wavelengths in question interact with
atoms more than other wavelengths (eg x-ray "misses" and therefore does
not excite many particles???).
(In my defense, I missed high school physics altogether because I was
sent overseas and the topic wasn't covered those years in the place I
went.)
If you've got the time, you might write a bit more about heat in
general...
A very embarrassed thanks.
I can't add much to Andy's reply either, but consider that your thought
"lower energy EM frequencies ...transmit heat better..." isn't exactly
true. It's a matter of temperature. The filament of an incandescent
lightbulb is quite hot, and quite a bit of its energy is carried away as
visible light. It's glowing in the visible spectrum. An object at 100
degrees F is also "glowing" but it's glowing in the infrared.
If you draw a curve of the energy output of an object as a function of
frequency, it has a peak in it. The peak moves to higher frequencies as
the temperature of the object goes up.
--
Steve Gray
sgray2@cfl.rr.com
.
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| User: "James Copeland" |
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| Title: Re: Dumb Question |
03 Oct 2005 09:21:07 PM |
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"Steven Gray" <sgray2@NOcfl.rr.comSPAM> wrote in message
news:Xns96E4CC875D2E3sgray2cflrrcom@65.32.5.122...
"Hlafordlaes" <hlafordlaes@gmail.com> wrote in
news:1128338463.358918.153180@g49g2000cwa.googlegroups.com:
If you draw a curve of the energy output of an object as a function of
frequency, it has a peak in it. The peak moves to higher frequencies as
the temperature of the object goes up.
--
Steve Gray
sgray2@cfl.rr.com
If you draw the same type of curve of energy output as a function of
wavelength (instead of frequency) it too has a peak (maximum) in it, and the
wavelength corresponding to this peak is inversely proportional to the
absolute temperature, T. I.e, the peak moves to shorter wavelengths as the
temperature of the object rises. This relationship is known as "Wien's
displacement law":
Lambda(max) = A/T where A is a constant.
Jim C.
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| User: "Steven Gray" |
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| Title: Re: Dumb Question |
04 Oct 2005 07:14:13 PM |
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"James Copeland" <chemcope@ksu.edu> wrote in
news:dhsovt$o66$1@cnn.cns.ksu.edu:
If you draw a curve of the energy output of an object as a function of
frequency, it has a peak in it. The peak moves to higher frequencies
as the temperature of the object goes up.
If you draw the same type of curve of energy output as a function of
wavelength (instead of frequency) it too has a peak (maximum) in it,
and the wavelength corresponding to this peak is inversely proportional
to the absolute temperature, T. I.e, the peak moves to shorter
wavelengths as the temperature of the object rises. This relationship
is known as "Wien's displacement law":
Lambda(max) = A/T where A is a constant.
I would think so. It's basically the same thing, isn't it?
--
Steve Gray
sgray2@cfl.rr.com
.
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| User: "James Copeland" |
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| Title: Re: Dumb Question |
05 Oct 2005 10:11:26 AM |
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"Steven Gray" <sgray2@NOcfl.rr.comSPAM> wrote in message
news:Xns96E5CE068AF7Bsgray2cflrrcom@65.32.5.121...
"James Copeland" <chemcope@ksu.edu> wrote in
news:dhsovt$o66$1@cnn.cns.ksu.edu:
If you draw a curve of the energy output of an object as a function of
frequency, it has a peak in it. The peak moves to higher frequencies
as the temperature of the object goes up.
If you draw the same type of curve of energy output as a function of
wavelength (instead of frequency) it too has a peak (maximum) in it,
and the wavelength corresponding to this peak is inversely proportional
to the absolute temperature, T. I.e, the peak moves to shorter
wavelengths as the temperature of the object rises. This relationship
is known as "Wien's displacement law":
Lambda(max) = A/T where A is a constant.
I would think so. It's basically the same thing, isn't it?
--
Steve Gray
Of course it is. I was merely pointing out that the wavelength plot has been
historically called Wien's displacement law. Just trying to add a bit to the
overall education data base. ;-)
Jim C.
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| User: "Androcles Androcles@ MyPlace.org" |
|
| Title: Re: Dumb Question |
03 Oct 2005 06:42:51 AM |
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"Hlafordlaes" <hlafordlaes@gmail.com> wrote in message
news:1128338463.358918.153180@g49g2000cwa.googlegroups.com...
| Boy, does this show my ignorance, but...
|
| Could someone tell me why lower energy EM frequencies (infrared and
| microwave) transmit heat better than higher energy frequencies? My
| utterly simple guess is that the wavelengths in question interact with
| atoms more than other wavelengths (eg x-ray "misses" and therefore
does
| not excite many particles???).
|
| (In my defense, I missed high school physics altogether because I was
| sent overseas and the topic wasn't covered those years in the place I
| went.)
|
| If you've got the time, you might write a bit more about heat in
| general...
|
| A very embarrassed thanks.
Boats at the bottom of Niagara Falls get bounced around (heat analogy),
but shooting at them with a rifle makes holes in them (x-ray analogy).
Androcles
.
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| User: "Dirk Van de moortel" |
|
| Title: Re: Dumb Question |
03 Oct 2005 11:29:11 AM |
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"Hlafordlaes" <hlafordlaes@gmail.com> wrote in message news:1128338463.358918.153180@g49g2000cwa.googlegroups.com...
Boy, does this show my ignorance, but...
Could someone tell me why lower energy EM frequencies (infrared and
microwave) transmit heat better than higher energy frequencies? My
utterly simple guess is that the wavelengths in question interact with
atoms more than other wavelengths (eg x-ray "misses" and therefore does
not excite many particles???).
Can't add much to Andy's detailed reply.
Perhaps this...
Waves with longer wavelengths (lower frequency) manage to go
"around" the obstacles easier than waves with shorter wavelengths
(higher frequencies). The latter are dispersed more than the
former. That is why the sky is blue and the sun yellow or even
red.
So your guess was not so good: the lower energy frequencies
interact *less* with the atoms and pass less of their energy to the
obstacles they meet.
Dirk Vdm
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| User: "Andy Resnick" |
|
| Title: Re: Dumb Question |
03 Oct 2005 08:07:14 AM |
|
|
Hlafordlaes wrote:
Boy, does this show my ignorance, but...
Could someone tell me why lower energy EM frequencies (infrared and
microwave) transmit heat better than higher energy frequencies? My
utterly simple guess is that the wavelengths in question interact with
atoms more than other wavelengths (eg x-ray "misses" and therefore does
not excite many particles???).
(In my defense, I missed high school physics altogether because I was
sent overseas and the topic wasn't covered those years in the place I
went.)
If you've got the time, you might write a bit more about heat in
general...
It can seem a little confusing: hot objects emit radiation into the
visible and beyond. Part of the explanation involves the terms 'elastic
scattering' and 'inelastic scattering'.
Typically, when we speak of 'heat', what we really mean is a description
of molecules moving around: 'Hot atoms move fast, cold atoms move
slowly'. This isn't an exact analogy.
Let's apply EM energy to a material: the material absorbs all the EM
radiation we shine on it, but the photons scatter only elastically. At
very high frequencies, like X-ray, the energy is large enough to
actually knock individual electrons off atoms; the EM energy is then
converted into motion of electrons. UV light damages your cells by
stripping electrons off large molecules like DNA, and as your body tries
to repair the damage, mutations result.
As we tune our EM frequencies lower to the visible, we now don't have
enough energy to strip off electrons, but we do still smash individual
electrons: fluorescence and lasing.
Keep tuning lower, into the IR band (wavelengths around 1 to 100
microns), instead of interacting with individual electrons, we are now
interacting with whole atoms and molecules. This larger-scale motion is
what corresponds to thermal energy in a material: atoms vibrating,
molecules shaking, that kind of thing.
Still lower, into the microwave and millimeter wave, we are interacting
with things like the rotational modes of motion of water molecules (2.54
GHz).
Hopefully,I am explaining that atoms and molecules respond differently
to applied energy, depending on the magnitude of the energy. Large
doses of energy affect individual electrons, while smaller doses affect
coordinated modes of motion of atoms and molecules. And what we call
'heat' corresponds to these large-scale motions (sometimes also called
phonons), rather than the motion of individual electrons.
Now, if we sit out in the sun we get warm. We are absorbing visible
radiation (the spectrum of the sun peaks around 555 nm), but our skin
doesn't fluoresce. Instead, the visible EM radiation is converted into
longer-wavelength radiation via inelastic scattering: the high-energy EM
photons are downconverted into lower energy photons and phonons.
Hope this helps...
--
Andrew Resnick, Ph.D.
Department of Physiology and Biophysics
Case Western Reserve University
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| User: "Androcles Androcles@ MyPlace.org" |
|
| Title: Re: Dumb Question |
03 Oct 2005 08:43:43 AM |
|
|
"Andy Resnick" <andy.resnick@op.case.edu> wrote in message
news:dhraa9$g3f$1@eeyore.INS.cwru.edu...
| Hlafordlaes wrote:
|
| > Boy, does this show my ignorance, but...
| >
| > Could someone tell me why lower energy EM frequencies (infrared and
| > microwave) transmit heat better than higher energy frequencies? My
| > utterly simple guess is that the wavelengths in question interact
with
| > atoms more than other wavelengths (eg x-ray "misses" and therefore
does
| > not excite many particles???).
| >
| > (In my defense, I missed high school physics altogether because I
was
| > sent overseas and the topic wasn't covered those years in the place
I
| > went.)
| >
| > If you've got the time, you might write a bit more about heat in
| > general...
|
| It can seem a little confusing: hot objects emit radiation into the
| visible and beyond. Part of the explanation involves the terms
'elastic
| scattering' and 'inelastic scattering'.
|
| Typically, when we speak of 'heat', what we really mean is a
description
| of molecules moving around: 'Hot atoms move fast, cold atoms move
| slowly'. This isn't an exact analogy.
|
| Let's apply EM energy to a material: the material absorbs all the EM
| radiation we shine on it, but the photons scatter only elastically.
At
| very high frequencies, like X-ray, the energy is large enough to
| actually knock individual electrons off atoms; the EM energy is then
| converted into motion of electrons. UV light damages your cells by
| stripping electrons off large molecules like DNA, and as your body
tries
| to repair the damage, mutations result.
|
| As we tune our EM frequencies lower to the visible, we now don't have
| enough energy to strip off electrons, but we do still smash individual
| electrons: fluorescence and lasing.
|
| Keep tuning lower, into the IR band (wavelengths around 1 to 100
| microns), instead of interacting with individual electrons, we are now
| interacting with whole atoms and molecules. This larger-scale motion
is
| what corresponds to thermal energy in a material: atoms vibrating,
| molecules shaking, that kind of thing.
|
| Still lower, into the microwave and millimeter wave, we are
interacting
| with things like the rotational modes of motion of water molecules
(2.54
| GHz).
|
| Hopefully,I am explaining that atoms and molecules respond differently
| to applied energy, depending on the magnitude of the energy. Large
| doses of energy affect individual electrons, while smaller doses
affect
| coordinated modes of motion of atoms and molecules. And what we call
| 'heat' corresponds to these large-scale motions (sometimes also called
| phonons), rather than the motion of individual electrons.
|
| Now, if we sit out in the sun we get warm. We are absorbing visible
| radiation (the spectrum of the sun peaks around 555 nm), but our skin
| doesn't fluoresce. Instead, the visible EM radiation is converted
into
| longer-wavelength radiation via inelastic scattering: the high-energy
EM
| photons are downconverted into lower energy photons and phonons.
|
| Hope this helps...
| --
| Andrew Resnick, Ph.D.
| Department of Physiology and Biophysics
| Case Western Reserve University
It does seem a little confusing.
I told him
Boats at the bottom of Niagara Falls get bounced around (heat analogy),
but shooting at them with a rifle makes holes in them (x-ray analogy).
(Which I hope is less confusing.)
Androcles.
.
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