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dreyer
2004-Sep-16, 02:18 AM
Hello everyone, my name is Sindre and I just stumbled across this site. Very nice site!

I figured I would go ahead and ask a question that has been bothering me for a while. Basically after reading this Q&A (http://badastronomy.com/mad/1999/space_feel.html) there is something I could need explained to me.

Now imagine a human (or any object really) in space, lets say I am in a spacesuit and my body reads a normal 37 C temperature. Now lets say I remove my spacesuit, as explained in the Q&A my body would then radiate away my body heat. But why?

Heat as I understand it is atoms/molecules bumping into each other or “vibrating” and thus generating heat. But why would they stop bumping into each other or “vibrating” when surrounded by a vacuum? Vacuum is empty space with no temperature, so how does it affect my body? I do not understand this.

Thanks!
Kind Regards,
Sindre

tuffel999
2004-Sep-16, 02:28 AM
Because there is nothing to bump into next to the outermost molecule so it slows and stops.

dreyer
2004-Sep-16, 02:28 AM
I can't believe I did not think of that. Thanks.

tuffel999
2004-Sep-16, 02:31 AM
NP. 8)

chiaroscuro25
2004-Sep-16, 02:45 AM
Because there is nothing to bump into next to the outermost molecule so it slows and stops.Absolutely totally completely wrong. #-o

Dreyer: there are several ways a body can lose heat. Conduction and advection require contact with another body or substance and don't work in a vaccuum, but all objects continually emit black body radiation. The frequency and intensity of this radiation is a function of the object's temperature; at human body temperature, most of this is in the infrared. (You've probably seen infrared camera footage that shows people as glowing compared to the background.) A person on Earth is also absorbing black body radiation from their surroundings, so radiative cooling is slow (slower than conduction and advection). In a vacuum though, we emit black body radiation without absorbing any from the surroundings, so radiative cooling would happen much faster.

tuffel999
2004-Sep-16, 02:48 AM
Because there is nothing to bump into next to the outermost molecule so it slows and stops.Absolutely totally completely wrong. #-o

Dreyer: there are several ways a body can lose heat. Conduction and advection require contact with another body or substance and don't work in a vaccuum, but all objects continually emit black body radiation. The frequency and intensity of this radiation is a function of the object's temperature; at human body temperature, most of this is in the infrared. (You've probably seen infrared camera footage that shows people as glowing with respect to their surroundings.) A person on Earth is also absorbing black body radiation from their surroundings, so radiative cooling is slow (slower than conduction and advection). In a vacuum though, we emit black body radiation without absorbing any from the surroundings, so radiative cooling would happen much faster.

Perhaps too simplistic but take a look:

http://www.newton.dep.anl.gov/askasci/gen99/gen99829.htm

and

http://www.newton.dep.anl.gov/askasci/phy00/phy00688.htm

Grand Vizier
2004-Sep-16, 02:49 AM
Because there is nothing to bump into next to the outermost molecule so it slows and stops.

Nice. I'm sure I would needlessly have said something far more complicated. As a first-approx answer that rules.

dreyer
2004-Sep-16, 02:52 AM
Thanks for the reply!

Could you explain in newbie terms why "radiative cooling" happends? What is this black body radiation really? What happens on the molecular level in simple terms (if that's possible).

Wouldn’t it be nice to be able to just order your bodily atoms to stop doing that! "Stop loosing my energy please!"

chiaroscuro25
2004-Sep-16, 02:52 AM
Perhaps too simplistic but take a look: http://www.newton.dep.anl.gov/askasci/gen99/gen99829.htm
Yes, that page is accurate enough, but it doesn't support your erroneous statement at all.

tuffel999
2004-Sep-16, 03:00 AM
"Because heat is a manifestation of atomic and molecular motion, the kinetic
energy (the energy of motion of any particle of matter) is expressed as KE =
1/2 mv2, where m represents the mass of the moving "particle" and v2
represents the velocity of the particle squared. If atoms and molecules are
cooled and thereby caused to slow down until they cease their vibrational,
rotational, and/or translational motions, they lose their kinetic energy and
thus have no energy to surrender to a temperature sensor. "


"Because there is nothing to bump into next to the outermost molecule so it slows and stops."

If there are no molecules next to the outermost and then there has to be a vacum at absolute zero,so that molecule or atom then cools and slows down the molecule until it is stationary and can no longer loose any kinetic energy. So too simplistic, yes.......and should have included in a vacum.

chiaroscuro25
2004-Sep-16, 03:06 AM
[Nice. I'm sure I would needlessly have said something far more complicated. As a first-approx answer that rules.
No, it's not a good first-approximation; it's completely wrong.

Let me explain why. If something is a solid, then its molecules are bound together by attractive forces. They jiggle around with respect to each other because of the thermal energy in the system, but these vibrations aren't large enough to overcome the attractive forces. (If you raise the temperature, their vibrations can become large enough to overcome the attractive forces, and the object evaporates.) So what happens to the molecule at the end of the line? It reaches the farthest point of its vibration and stops momentarily, but feels the attraction of the other molecules and is drawn back into object. Energy is conserved in these interactions; no energy is lost.


Thanks for the reply!
Could you explain in newbie terms why "radiative cooling" happends? What is this black body radiation really? What happens on the molecular level in simple terms (if that's possible).
Hmm. This is an interesting challenge; it's certainly the least obvious of the heat transfer mechanisms and is usually taught along with some non-trivial thermodynamics. Let me think about this.

chiaroscuro25
2004-Sep-16, 03:15 AM
"Because heat is a manifestation of atomic and molecular motion, the kinetic energy (the energy of motion of any particle of matter) is expressed as KE = 1/2 mv2, where m represents the mass of the moving "particle" and v2 represents the velocity of the particle squared. If atoms and molecules are cooled and thereby caused to slow down until they cease their vibrational, rotational, and/or translational motions, they lose their kinetic energy and thus have no energy to surrender to a temperature sensor. "

"Because there is nothing to bump into next to the outermost molecule so it slows and stops."You're quoting here is very misleading, as you seem to imply that the latter statement is contained on that web page along with the first. It is not.


If there are no molecules next to the outermost and then there has to be a vacum at absolute zero, so that molecule or atom then cools and slows down the molecule until it is stationary and can no longer loose any kinetic energy.There are a bunch of errors in that statement. A perfect vacuum has no temperature; temperature is a property of matter. Energy must be conserved, so a vacuum cannot simply absorb energy into nothingness, the energy must be carried away by a particle or radiation. If the outermost molecule escapes from the object, it will continue in a straight line at a constant velocity (Newton's 2nd law); no energy can be lost.

(This did make me realize I'd left off another heat transfer mechanism: evaporative cooling. But that wouldn't be a significant factor here.)

AGN Fuel
2004-Sep-16, 03:56 AM
Gotta agree with Chiaroscuro25 here. The cooling occurs on the 'shadowed' side of a body in a vacuum, through radiative transfer.


"Because heat is a manifestation of atomic and molecular motion, the kinetic energy (the energy of motion of any particle of matter) is expressed as KE = 1/2 mv2, where m represents the mass of the moving "particle" and v2 represents the velocity of the particle squared. If atoms and molecules are cooled and thereby caused to slow down until they cease their vibrational, rotational, and/or translational motions, they lose their kinetic energy and thus have no energy to surrender to a temperature sensor. "

My emphasis. The atoms/molecules at the surface release energy through radiative transfer and do not absorb energy from their surroundings. As they lose energy, they slow kinetically (conservation at play!) and 'cool' as measured by a temperature sensor.

The paragraph quoted by tuffel999 really just explains the interaction between molecules and a temperature sensor. It does not indicate how the molecules are cooled. In the case of our astronaut removing his helmet, it is due to radiative transfer.

Grand Vizier
2004-Sep-16, 05:45 AM
The atoms/molecules at the surface release energy through radiative transfer and do not absorb energy from their surroundings.

1) Their surroundings are not isotropic.

2) So, of course, they absorb energy from some of their surroundings. (It's better to regard the human body as a liquid rather than as a solid), nevertheless all I see here is is a vindication of the earlier position that elements exposed to vacuum are in a 'preferential' position.

This is only a complicated way of saying that you get cold from the outside in :)

I truly don't see thermodynamic complications here.

AGN Fuel
2004-Sep-16, 07:29 AM
The atoms/molecules at the surface release energy through radiative transfer and do not absorb energy from their surroundings.

1) Their surroundings are not isotropic.

2) So, of course, they absorb energy from some of their surroundings. (It's better to regard the human body as a liquid rather than as a solid), nevertheless all I see here is is a vindication of the earlier position that elements exposed to vacuum are in a 'preferential' position.


Apologies - I should have been clearer. When I noted that there was no absorption from the surroundings, I meant from external surroundings (i.e. the vacuum of space), not deeper tissue. However, the transfer of energy from deeper tissue will in turn be radiated away and once gone, is not recoverable - there will over time be a loss of radiant energy from the astronaut into space.


This is only a complicated way of saying that you get cold from the outside in :)

Yes!


I truly don't see thermodynamic complications here.

Well, I think the point is that Tuffel999's explanation does not correctly describe why our astronaut has turned into a frozen fish finger when he removed his spacesuit. The radiative loss of energy causes the reduction in kinetic energy, not the absence of outer layers of atoms to vibrate against.

milli360
2004-Sep-16, 10:48 AM
chiaroscuro25:

[Nice. I'm sure I would needlessly have said something far more complicated. As a first-approx answer that rules.
No, it's not a good first-approximation; it's completely wrong.
Not quite. I would have said something different. Here's tuffel999's original:


Because there is nothing to bump into next to the outermost molecule so it slows and stops.

Your three ways of cooling (advection, conduction, radiation) are represented. The body is not moving per se, so there is no advection, and the "nothing to bump into" refers to the absence of conduction, since it is in a vacuum. The "so it slows and stops" is true in the sense that that is a result of the radiation. But that doesn't say why they slow and stop.

So, rather than being completely wrong, I would say it's right, but it does not answer the question. It just restates the question in a different form. dreyer, in the OP, could have said "why does it slow and stop?" The reason is radiation--but dreyer knew that already, as he mentioned it in the OP. That's probably why the answer made sense to him.

swansont
2004-Sep-16, 02:15 PM
Could you explain in newbie terms why "radiative cooling" happends? What is this black body radiation really? What happens on the molecular level in simple terms (if that's possible).


When the atoms in a material vibrate, they also collide with free or conduction electrons, causing them to accelerate. Unbound charged particles, when accelerated, emit EM radiation. (with high energy charges, this is called Bremsstrahlung, and is a way of making e.g. x-rays)

A "blackbody" is a perfect absorber and emitter of this type of radiation. A blackbody at thermal equilibrium is absorbing energy at the same rate it's radiating it.

dreyer
2004-Sep-16, 02:19 PM
So this energy our astronaut is radiating away, what exactly is it? What kind of energy are we talking about?

dreyer
2004-Sep-16, 02:20 PM
Could you explain in newbie terms why "radiative cooling" happends? What is this black body radiation really? What happens on the molecular level in simple terms (if that's possible).


When the atoms in a material vibrate, they also collide with free or conduction electrons, causing them to accelerate. Unbound charged particles, when accelerated, emit EM radiation. (with high energy charges, this is called Bremsstrahlung, and is a way of making e.g. x-rays)

A "blackbody" is a perfect absorber and emitter of this type of radiation. A blackbody at thermal equilibrium is absorbing energy at the same rate it's radiating it.

Oh! Thanks!

chiaroscuro25
2004-Sep-16, 04:40 PM
Your three ways of cooling (advection, conduction, radiation) are represented. The body is not moving per se, so there is no advection, and the "nothing to bump into" refers to the absence of conduction, since it is in a vacuum. The "so it slows and stops" is true in the sense that that is a result of the radiation. But that doesn't say why they slow and stop.I think you are trying to give him credit for saying something far more sophisticated than he meant. He wasn't talking about the long term consequences of the radiative cooling; what he was describing might be termed "conduction into the vacuum", in violation of conservation of energy.

Two other quick points: 1) Advection requires more than motion; there must also be surrounding material. Indeed, a simple definition of advection might be motion plus conduction. 2) The atom's thermal motion never actually "stops" except at the unattainable temperature of absolute zero. (Even then, there are quantum zero-point oscillations.)


The reason is radiation--but dreyer knew that already, as he mentioned it in the OP.No offense intended to dreyer, but I didn't think he was using the term radiation in its technical sense. Lay people often talk of something radiating heat when they are actually referring to conduction.


When the atoms in a material vibrate, they also collide with free or conduction electrons, causing them to accelerate. Unbound charged particles, when accelerated, emit EM radiation. (with high energy charges, this is called Bremsstrahlung, and is a way of making e.g. x-rays)This is similar to the answer I was contemplating when I was thinking how to answer him later, so I'll basically concur. (I was thinking more in terms of charge separations and dipoles rather than free electrons, but that's getting fairly pedantic.)

logicboy
2004-Sep-16, 07:14 PM
Welcome to the board dreyer

dreyer
2004-Sep-16, 08:10 PM
Thanks! :)