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Heat transfer by infrared radiation (the very basics)

  1. Sep 8, 2015 #1
    Q. Regards infrared radiation, conduction and convection, is infrared radiation the starting point, the fundamental source of heat energy (thermal energy), and the fundamental source of the other two?

    I’m thinking that the Sun predates the Earth by 30 million or so years so the first heat energy put into the Earth was infrared radiation from the Sun. This in turn has caused, over the aeons, much conduction and convection but are those two forms of heat transfer by-products of the more basic form, ie, radiation?

    I’m also thinking in terms of convection in a living room. Convection (the movement of air of varying density circulating the room?) is the main way in which the room is heated but the starting point for the process is radiation (from the radiator), yes?
     
  2. jcsd
  3. Sep 8, 2015 #2
    From bbcbitesize (re infrared radiation);

    "Because no particles are involved, radiation can even work through the vacuum of space. This is why we can still feel the heat of the Sun even though it is 150 million km away from the Earth."

    Q. I’ve heard physicists talk about ‘the vacuum of space’ not really being a vacuum, ie, it’s not the empty space we once thought it to be, rather it’s actually a ‘bubbling brew of virtual particles popping in and out of existence at every moment’; would heat transfer by infrared radiation be possible in an actual vacuum (an actual empty space), or rather across an actual vacuum (in an actual vacuum there would be nothing to transfer the heat from and to, yes?) ?
     
  4. Sep 8, 2015 #3
    Q. In an episode of Auf Wiedersehen Barry and Moxy were sunbathing with tin foil placed beneath their chins in the hope of reflecting the sunshine on to their faces; does this work?

    Q. And why are people who are trying to keep warm (eg, when a swimming pool has been evacuated for some reason and the bathers have to hang around outside) sometimes covered with large sheets of (what looks like) aluminium foil? Wouldn’t that reflect any heat energy when the purpose would be to absorb it, no?
     
  5. Sep 8, 2015 #4
    "Infra-red radiation (also called thermal radiation)
    transfers heat between all objects."


    Q. Including air? As well as molecules of air being heated by, to a small extent, conduction, and circulating due to convection; do those molecules also radiate thermal energy?

    Q. Do atoms emit infrared radiation?

    Q. Do subatomic particles emit it?
     
  6. Sep 8, 2015 #5

    Drakkith

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    No. None of them are more fundamental than the others. Remember that thermal energy is the energy an object has that is stored in the random vibrations and other motions of its composite particles. So a hot gas has thermal energy because the gas molecules are bouncing around, vibrating, rotating, etc.

    Again, no. Radiation is not more basic or less basic than either of the other two. Also, the Sun serves as the main source of heat here on the Earth's surface, but plenty of heat still comes from inside the Earth.

    No, both radiation and convection are involved (and conduction too I'm sure). There is no 'starting point'. The radiator heats the room via all three methods.

    Don't even worry about virtual particles or anything else having to do with quantum physics right now. You need to learn classical physics first.

    Of course.

    It reflects the IR radiation from their bodies back onto their bodies, where it is absorbed.

    Yes.

    Yes and no. What you're talking about is known as black body radiation. It is the radiation emitted by an object by virtue of being above absolute zero. This radiation is simply standard EM radiation with a well known spectrum that changes depending on how hot the object is. Hotter objects emit more radiation at higher frequencies than cooler objects do. The Sun, by being at nearly 6,000 kelvin, emits strongly in the visible light region of the spectrum, but it also emits some amount of every frequency up to that point too (and a little bit beyond the visible range). This includes radio waves, microwaves, IR radiation, and a small amount of UV radiation. An object at 20,000 kelvin, perhaps a blue supergiant star, will emit LOTS of UV radiation. In fact, it's peak emitted wavelength is 144 nanometers, which is well into the UV part of the EM spectrum.

    Now, a single atom or molecule cannot emit EM radiation that follows this well known spectrum. It may emit specific frequencies that fall within the IR band, but it will not emit a wide band of frequencies like a hot object does.

    No, for the exact same reasons I just listed.
     
  7. Sep 8, 2015 #6

    davenn

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    Yes it would. IR is just another form of electromagnetic radiation. just higher frequency than radio waves and a little lower in frequency than visible light.
    As such, it doesn't need a medium to travel through

    try it .....don't blind yourself with the brightness

    aluminised mylar foil has a shiny side and a dull side ... it serves 2 purposes
    These things are commonly called survival blankets. they can be used in one of 2 ways.
    1) to keep a person warm, the shiny side goes towards the body to reflect heat back to the body. The dull side facing out will absorb some heat and also transfer that to the body.
    2) If the person is overheating, the shiny side can face outwards so the person doesn't absorb any more heat from the surrooundings


    air is a very poor conductor of heat, and convection is the main carrier of heat in it. IR for the most part passes straight through it

    yes ... that's the source of IR radiation ( see the next answer for clarification)

    The increasing and decreasing energy levels of the electrons ( within the atom) are the source if IR radiation

    Other sub-atomic particles as a source, I'm not sure .... some one else may answer?


    Dave
     
    Last edited: Sep 8, 2015
  8. Sep 8, 2015 #7

    davenn

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  9. Sep 8, 2015 #8

    Drakkith

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    I should probably clarify that I meant that single subatomic particles do not emit thermal radiation. Groups of them can though.
     
  10. Sep 8, 2015 #9

    Andy Resnick

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    I vote no- these three mechanisms of heat transfer are all handled differently. For radiation, the starting point is the radiative transfer equation (radiometry):
    https://en.wikipedia.org/wiki/Radiative_transfer

    For thermal conduction, it's typically a diffusion-type equation:
    https://en.wikipedia.org/wiki/Heat_equation

    And for convection, you must introduce fluid motion as heat is advected by the fluid- a starting point are the mass, momentum, and energy balance equations.

    Similarly for sources of heat: the earth is heated both by the sun (radiation), but also from internal radioactive decay and the 'primordial heat':

    https://en.wikipedia.org/wiki/Earth's_internal_heat_budget
     
  11. Sep 16, 2015 #10
    Thanks for your reply.

    “Remember that thermal energy is the energy an object has that is stored in the random vibrations and other motions of its composite particles.”

    For energy to be stored in the particles of an object is something I can begin to get my head around but for it to be stored in the vibrations of these particles is something that puzzles me. Particles are concrete things, yes? They are objects, yes? But a vibration is something that a thing does, it is not a concrete thing itself, no? A vibration is more abstract, no? This is similar to my puzzlement over particles, eg, an electron, *having* negative charge; does it *have* negative charge? Or *is it* negative charge?

    “…the Sun serves as the main source of heat here on the Earth's surface, but plenty of heat still comes from inside the Earth.”

    And that energy that’s inside the Earth didn’t come from the Sun? Where did that come from?

    “No, both radiation and convection are involved (and conduction too I'm sure). There is no 'starting point'. The radiator heats the room via all three methods.”

    But if we go back to when the radiator was first installed one of the three types of thermal energy transfer must have taken place first, no? We install the system and switch it on. The boiler heats the water. The hot water in the radiator heats the surface of the radiator and the surface then radiates heat energy into the air in the room, no?

    “Of course (re sunbathers using foil to reflect the sunlight onto their faces).”

    So they get the direct sunlight from the Sun on their face and then they *add* to this by getting the sunlight that would otherwise hit their chest/shoulder area? Would it be significant? Or negligible?

    “It reflects the IR radiation from their bodies back onto their bodies, where it is absorbed.”

    So they miss out on the heat that is being reflected but this is negligible in comparison with the heat that they would lose if the heat leaving their bodies was not reflected back into their bodies? Why don’t wee see more aluminium foil clothing around in the winter? Does it have to be directly next to the skin?

    “The Sun, by being at nearly 6,000 kelvin, emits strongly in the visible light region of the spectrum, but it also emits some amount of every frequency up to that point too (and a little bit beyond the visible range). This includes radio waves, microwaves, IR radiation, and a small amount of UV radiation.”

    But not x-rays nor gamma rays? Also, I’ve been looking at images of the EM spectrum and most have 10^3 m next to radio waves re their wavelength. I assumed that they were therefore all around that length (a km or so) but have since read that they range from as short as 1mm to as long as a km or so; is that correct?

    “144 nanometers”

    I am finding the discovery of these very small (and very large) measurements / quantities a profound experience. Nanometres, picometres, femtoseconds etc; does this mean that a metre (and a second) can be divided ad infinitum? Take a millimetre. A nanometre is one millionth of a mm, yes? So the mm can be divided into (at least) a million parts. Can we keep dividing it without end?

    And a femtosecond is 10^-15 seconds, one quadrillionth of a second. That is profound. A second consisting of a thousand trillion parts / instances. So, a thousand trillion events (at least) can occur during one second? Can we divide the second ad infinitum also?

    “It's not about what's possible, it's about what's probable.”

    Very thought-provoking quote. At the risk of sounding pedantic, does this really mean it’s about what is highly probable (if something is possible it is probable, no (even if it has an extremely low probability)? Also, determining what is possible is a vast task, yes?
     
  12. Sep 16, 2015 #11

    Drakkith

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    Hi Cliff.

    I'm having some trouble with quoting text at the moment, so I'll have to get back to you at a later point (otherwise I'll never be able to keep track of your questions).

    Sorry!

    P.S. I'm sure it's a problem with my browser, but I'd like to blame @Greg Bernhardt anyways.:-p
     
  13. Sep 16, 2015 #12

    davenn

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    Cliff
    you really need to learn to quote properly ... its real easy to do

    its hard to tell who you are quoting ??? ... I recognise some of my own text


    think of the energy in sound waves ,,, they are created by the vibrations of say a speaker cone or say an explosion

    no it didn't, its the remaining heat from when the earth and the other planets condensed and formed out of the gas clouds
    the gravitational collapse of the gas and other material clouds would generate the heat

    yes correct ... is there a question / mis-understanding ?

    a significant addition drastically decreases the time till you sunburn

    cuz its not fashionable


    The sun is an intense emitter of X-rays and a considerable amount of gamma rays too
    here's a link on solar gamma rays ...... http://today.slac.stanford.edu/feature/gammaraysfromthesun.asp

    The sun emits energy right across the EM spectrum from wavelengths of many 10's of metres to those of micrometre length

    IR,, visible light, UV, x-rays and gamma rays are ALL substantially shorter in wavelength than 1mm


    don't think so ... do some googling on the Planck Length


    I will let Drakkith respond to his signature


    cheers
    Dave
     
  14. Sep 17, 2015 #13
    Thanks. Very helpful.
     
  15. Sep 17, 2015 #14
    Sorry, davenn, I've messed up the above reply. How do I insert responses to the various quotes within your post (the way you have done with your last reply to me)?
     
  16. Sep 17, 2015 #15

    Drakkith

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    Highlight the text you want to quote and a small popup will appear. Then hit reply in the popup to immediately quote that text in the reply text box. Or you can hit quote to add that text to a quote que. You can add multiple quotes to the quote que and when you are ready to reply, just scroll down and click Insert Quotes.
     
  17. Sep 25, 2015 #16

    CWatters

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    The earth started out hot. So hot that for a long time it radaited/lost far more heat to space than it gained from the sun. What do you think the balance is like today?
     
  18. Sep 25, 2015 #17

    CWatters

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    It depends on the heat source. A hot water filled "radiator" emits heat by radiation, conduction and convection but not necessarily in equal amounts. Electric bar fires (incandescent type) emit most of their heat by radiation.
     
  19. Sep 25, 2015 #18
    Cwatters, you said, “The earth started out hot. So hot that for a long time it radiated/lost far more heat to space than it gained from the sun. What do you think the balance is like today?”

    Given that you’ve said that, ‘for a long time…’ it sounds like you’re implying that it’s no longer the case so my guess is that the Earth now gains more heat from the Sun than it loses to space.

    Q. What was the source of the Earth’s original heat? The singularity re the big bang? Was that the source of all the heat in the universe today?
     
  20. Sep 25, 2015 #19

    davenn

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    I answered that way back in post #12
     
  21. Sep 27, 2015 #20

    CWatters

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    No there is more to it. There is more than one heat source involved. There are nuclear reactions going on in earth's core that generate new heat. As I understand it the planet as a whole is still cooling very slowly. I read somewhere that its only lost about 30% of its original heat so far.

    The temperature is only falling slowly because the planet core is well insulated. One source says the planet looses only 50TW which is very small.
     
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