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Heat Radiation by Groups of Atoms

  1. Jul 7, 2005 #1
    Does anybody have any thoughts on why atoms that are warm need to radiate heat in order to stop vibrating or bond back together?

    The obvious example is the thermos. That is a thermos has the hot liquid contained in a glass container with a mirrored surface towards the inside and then a vacuum around that.

    Why do the vibrating atoms need a reflective surface? Is this so that they can see how pretty they are? Why does the vacuum transport heat energy at a slower rate and not a faster rate than having air around the glass? You would think the bouncing air would add heat not take more away?

    And ultimately, why can't the atoms rebond or reduce vibration without the need to radiate heat?
    You see planets form from asteroids being drawn together.
    You would think atoms would also draw together in the same fashion.
    Why is heat radiated as part of the process of cooling down?

    In a thermos no external heat is supplied. How does the liquid stay hot?
    It does cool down eventually but only by radiating the heat over a long time.
  2. jcsd
  3. Jul 7, 2005 #2


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    The atoms don't need the reflective surface. Heat is carried by electromagnetic radiation (infrared). The reflective surface reflects this radiaton back into the thermos.

    The vacuum space greatly diminished heat loss due to conduction. Combining this with the minimization of radiation loss (above) leads to the desired result.
    Last edited: Jul 7, 2005
  4. Jul 7, 2005 #3
    But once the heat is transferred to the atoms by making them move faster doesn't the infrared radiation cease to exist like light is supposed to when it is absorbed by atoms?

    Are atoms inherantly colder than vacuum? Why would atoms steal heat by bouncing against the back of the glass?
  5. Jul 7, 2005 #4


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    Energy is transferred by both radiation and convection simultaneously and separately. You see a fire glow red and feel the heat because the radiation goes through the air without being absorbed (caveat: refraction) before it gets to you. Besides, like you said, a thermos has a vacuum around it, so there isn't anything to absorb the infrared radiation anyway.
  6. Jul 8, 2005 #5
    Thank you for your answer Russ.

    So the answer is that atoms absorb infrared radiation. By that I assume you mean the air atoms would go faster and the mirror atoms would slow down if air surrounded the internal container. Is that what you mean?

    That is good but doesn't fully answer my question. Why should the liquid atoms which are moving fast and are hot, why should they release infrared radiation when they slow their vibration?

    From my current understanding heat is absorbed by making the atoms go faster; and not by changing the orbit of an electron like light does. So it is not as though an electron returning to a less excited state will magically generate the infrared heat because the heat is not represented by a more excited electron. Instead the infrared heat is represented by faster moving atoms.

    So is science saying that atoms as a whole behave in a similar fashion to electrons and the atoms magically produce infrared heat when they change to a less excited state like electrons magically produce light when they return to a less excited state?

    Does this mean that we will cool down slower in space in the shade then we will in a cold room?
  7. Jul 9, 2005 #6
    Can somebody please explain the process to me?
  8. Aug 7, 2005 #7
    Heat is not radiated. It is conducted from one substance to another that are close enough to be in "thermal contact". Heated atoms produce em radiation, but radiation is a less effective method of transmitting heat, as those who have been in rooms heated by steam radiators(without a fan system) know. Heat lamps are too hot to touch but a person can hold a hand in front of it(distance depending on how hot the lamp is).

    Conduction heat transfer between two substances depends upon the difference in temperature of the two substances, as well as characteristics of the two substances. Conversion of heat into radiation is determined solely by the temperature and characteristics of the substance emitting the radiation. The amount of energy transmitted as radiation increases with the Kelvin temperature raised to the fourth power. An additional factor is that the energy disperses as the radiation moves away from the source with the energy received by an object of specified size declining with the square of the distance from the source of the radiation.

    A thermos bottle blocks heat transfer by being a poor conductor of heat between the interior and exterior and by blocking radiation to the outside.
  9. Aug 8, 2005 #8


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    I'm an engineer, not a physicist, so while I can do heat transfer, I'm not real clear on the mechanism for producing the ir radiation.

    edit: d'oh, of course I know - BLACK BODY RADIATION. Monday morning brain fart. :grumpy:
    We will cool slower in space, but the reason is that at the frequencies/temperatures of humans, convection is more efficient than conduction. There are some heaters that use radiation, though...
    Last edited: Aug 8, 2005
  10. Aug 8, 2005 #9


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    Those two sentences contradict each other. Radiation is a form of heat energy.
    Well, not really. Just like with conduction, it depends on the delta-t between the source and the sink. A car gets cooler (several degrees cooler than ambient) on a clear night than a cloudy night because of the higher delta-t between car and space than between car and sky. Yes, you can treat the car and the sky as separate entities, each radiating toward each other, but either way, you have to look at both.
  11. Aug 10, 2005 #10
    That is too broad a definition of heat energy. Radiation can exist separately from heat because radiation can pass through a vacuum, but heat cannot. Heat can pass through a solid directly or be passed to another substance in contact with it. Energy transfer to that same substance at some distance must first be converted to radiation that can be absorbed by the other substance rather being reflected by it or passing through it unaffected. Water which has a high coefficient of heat will transfer more heat to a colder substance, or absorb more heat from a warmer substance, than other substances do, but water is a lousy radiator and is transparent to much radiation.

    On a cloudy night the clouds prevent the "warm" air from rising holding it closer to the ground. This may also increase confinement of the air which lowers the coefficient of heat reducing the amount of heat energy air would need to absorb to rise in temperature. Clouds also reflect radiation back to the ground particularly in the visible range from street lights, etc. which may or may not have a heating affect after passing through car windows and hitting the car's interior. On a cloudy night the dew point will be closer to the air temperature( a factor in cloud formation) and the dew point is the lowest temperature the air at ground level can normally fall to.

    On a clear night air rises as it warms allowing colder air to fall to the ground. Ground level temperatures can be several degrees cooler than air at the level where temperature is normally measured. Which is why plants may suffer frost damage even if the official air temperature is above freezing.

    Radiation by vehicles also depends on color. Dark colors radiate more than light colors so the temperature will fall to the dew point faster.
  12. Aug 10, 2005 #11


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    Please give us a precise definition of the term "heat" .
    What do you mean by "high coefficient of heat"?

  13. Aug 10, 2005 #12

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    You seem to be using a nonstandard definition of heat. "Heat" is the transfer of energy between two objects due to a difference in temperature. Thermal radiation is definitely a form of heat transfer, as is conduction. (Don't confuse the internal energy of a body with "heat"; heat is a transfer of energy.)
  14. Aug 13, 2005 #13
    The classic physics definition of "heat", the noun, is "the motion of atoms". All motion ceases at absolute zero. Substances have heat regardless of whether they transfer it or not.

    You are using the term as a verb "to heat" -- "to transfer energy from one substance to another". If "heat" is what is being transferred than it must also be a noun.

    Incidentally, "substance" would be a better word than "object" because "object" implies a solid, but "substance" could refer to a liquid or gas. In order for EM radiation to "heat" anything the source must be relatively close to what is being heated. Many stars produce radiation with much higher energy levels than the sun, but they are too far away from the earth to heat it. Radiation can pass through substances, such as the atmosphere, without heating them or be reflected without heating whatever causes the reflection.
  15. Aug 13, 2005 #14
    Perhaps I should have used "specific heat" instead, but the two terms seem to be stored in my brain as referring to the same thing. It refers to the relative amount of heat energy required to change the temperature of a substance. For water at 15 C it is defined as 1.0000. Air is substantially lower(around 0.20),but it varies with the degree to which the air is "confined" and air temperature.

    Water must absorb, or transmit, more heat than air to reach the temperature of another substance which is why cold water draws more heat energy from the human body than air of the same temperature.
  16. Aug 13, 2005 #15

    Doc Al

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    Modern terminology distinguishes heat from internal energy. Heat is a transfer of energy associated with a temperature difference. You could take a restrictive view of heat as the transfer of energy due to collisions between molecules. If that's your point I would support it. (Many textbooks do not do that, but I actually prefer that restrictive definition.)

    But "heat" is not internal energy. One can increase the internal energy of a substance without "heat" being involved.

    "Heat flow" is an old-fashioned and inaccurate phrase, an artifact of our common language patterns, which were developed millennia before our scientific understanding of the world. I'll try to be more careful. "Heat" is best used as a noun describing a method of energy transfer.

    As you wish. But "object" can also be a generic term: A glass of water can be an "object" as can a balloon full of gas.

    If you wish to use a restrictive definition of heat (as due to collisions between molecules), then only conduction would be specific to heat transfer. So what?
  17. Aug 13, 2005 #16


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    Then you have a major explanation to do on how the "heat" from the sun reaches the earth, unless you are insisting that there are atoms and molecules from there, travelling VERY fast (at the speed c) to transfer the heat.

  18. Aug 13, 2005 #17


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    There are generally three different ways heat transfer occurs:

    So radiation too, is a form of heat.
  19. Aug 16, 2005 #18
    Sounds like physics is moving into the twilight zone. What exactly does temperature measure then if not heat? I agree that heat is not internal energy. Internal energy of an atom would seem to be more a form of potential energy. Heat energy as the motion of atoms is a form of kinetic energy which would be more of an external form of energy because it involves interaction with other atoms.
  20. Aug 16, 2005 #19
    Heat energy can be converted into radiation. Radiation(particularly from a hot source) can be converted into heat energy.
  21. Aug 16, 2005 #20
    That assumes there is some significant amount of energy in the radiation. Most naturally occuring radiation has very little such energy. I am extremely skeptical of the idea that low energy radiation can heat anything.

    There is a fourth form of heat energy transfer. The evaporation of water. Water at its boiling point carries 540 calories per gram into the air. At lower temperatures more energy is required to break the surface tension of the water. That energy is released as heat when the water vapor condenses.
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