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Thermal radiation

  1. Dec 10, 2011 #1
    Hi there. Here is the deal. In my lab work, I've done experiments concerning on heat conduction. Now my professor had corrected my inform, and asked me a question for which I have no answer. I consulted for my work the book "heat conduction in solids" by H.S. Carslaw.

    The book says at the introduction:
    "When different parts of a body are at different temperatures heat flows from the hotter parts to the cooler. There are three distinct methods by which this transference of heat takes place: (i) Conduction, in which the heat passes through the substance of the body itself, (ii) Convection, in which heat is transferred by relative motion of portions of the heated body, and (ill) Radiation, in which heat is transferred direct between distant portions of the body by electromagnetic radiation.
    In liquids and gases convection and radiation are of paramount importance, but in solids convection is altogether absent and radiation usually negligible. In this book we shall consider Conduction of heat only, and usually speak of the body as solid, though in crtain circumstances the results will be valid for liquids or gases. In this chapter the general theory of conduction of heat is developed; the subsequent ohapters are devoted to special problems and methods.
    "

    I've subtracted from it the bold part. And my professor asked my, why is it negligible? and I don't know how to answer this, I've been investigating, and I get to the black body, and Planck's law, it doesn't seem like the answer I'm looking for this work, it actually seems to be like a topic for an entire different work. So I don't know what to do, or how to justify this sentence. I think that it is negligible for the temperatures we've been working in the lab, but for temperatures near the fusion point the radiation would be more and more significant with increasing temperature.

    Perhaps you can help to understand a bit more on thermal radiation, or point me some work or book on this topic, so I can justify correctly what I said in my inform.

    Thats all.

    Bye there.
     
  2. jcsd
  3. Dec 10, 2011 #2
    Hi There,
    I will firstly say I am no expert in this field but your statement:

    It is my understanding that in a solid, the atoms are arrange in a lattice with tight bonds. Therefore when considering the three heat transfer mechanisms, conduction, convection and radiation. Convection is characterized by fluid motion transporting hotter parts of the fluid along gravity induced pressure gradients , therefore in a solid where the lattice is rigid, convection heat transfer will be negligible.

    The radiation contribution to heat transfer is in general much smaller than the other terms (I believe this can be shown but I forget how!) However it is when other objects are not in contact with the "hot" object that the radiation term becomes large (such as a radiator). This is because the electromagnetic radiation does not suffer the same losses with distance as conduction.

    Therefore the only term with a significant contribution in the conduction.
     
  4. Dec 11, 2011 #3
    This is an interesting post, I would also like to know the answer. What about lasers burning holes in solids?
     
  5. Dec 11, 2011 #4
    I didn't mention it, but the part I must justify is why the radiation is negligible. I think it's negligible under certain condition that I should specify more clearly, conditions that my experimental work fits (the slab bar with different boundary conditions, I used an electric soldering to heat one extreme).

    For example, in the soldering it self, radiation doesn't seem that negligible, one can "feel it" putting a finger close to it. I think that not necessarily the object must be "separated" from the source of heat to consider radiation, but perhaps you're right, I'm not sure. What I'm trying to say is that one could consider in the experiment looses in heat due to radiation, and the bar was isolated (so radiation could be important under other conditions). For example, a heated iron red colored it's radiating an important amount of heat I think, and it looks like it radiates more and more while you're heating it, and the extreme case would be when you have a plasma, but then it's not a solid anymore, but I think it illustrates an increasing radiation with temperature. However, as closer you are to the melting point radiation becomes more important for the iron case I think, but this is not something general, for example water doesn't behave this way.

    Thanks for your answers, I'd really like to discuss a bit this thing.

    Bye and sorry for my awful english.
     
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