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Basic Energy Transfer Question

  1. Jul 26, 2008 #1


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    Hi All,

    I have a question and hopefully this is the right board for it. First let me explain that I am a Chemist/Biologist, not a Physicist, and haven’t touched even physical chemistry since my degree many, many years ago. However I have recently found myself reading up on my fundamental physics and am hoping someone can clarify something for me.

    Now before my question I’d like to briefly run through my current understanding so that if I have got something wrong it can be cleared up. Otherwise I might misunderstand any answers if I’ve got my basics wrong :)! Ok, deep breath…..

    1) There are two types of energy: Kinetic and Potential
    2) Potential energy is sort of ‘energy available for use’
    3) Kinetic energy is sort of ‘energy in use’ - motion
    4) While practically speaking energy can be divided into all sorts of types like Thermal, Electrical, Chemical etc… If you go down to a small enough scale it’s all ultimately kinetic energy, ie the movement of objects, be they tennis balls, molecules or electrons.
    5) Energy can never be lost, only transferred
    6) There are two energy transfer processes: Work and Heat
    7) Work is the transfer of ‘organised motion’, ie no change in entropy
    8) Heat is the transfer of ‘chaotic motion’, ie there is an entropy change
    9) Temperature is a measure of the total atomic/ molecular motion of a substance (translational, vibrational, rotational movement) and on a macroscopic scale defines the direction of energy transfer via heat.
    10) Absolute zero is when a substance has no motion at all, no kinetic energy
    11) Electromagnetic Radiation is perpendicular oscillating electrical and magnet fields, which have both wave and particulate properties.
    12) Electromagnetic Radiation has both energy and momentum, but no mass.
    13) When defining all this energy and motion you have to have some reference point which it is relative too (such as some arbitrary point on Earth)

    Now, finally, my rather simple question: When Electromagnetic Radiation strikes something and transfers energy is it a heat process? That’s it (for now………)
  2. jcsd
  3. Jul 26, 2008 #2


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    Energy transfers between thermodynamic systems are often labeled as either work or heat. However, it is generally recognized that the exchange of electromagnetic radiation cannot be neatly categorized as one or the other. Exceptions are blackbody radiation, which is one of the three traditional forms of heat transfer, and ideal radiation which can be categorized as work by the Caratheodory definition. (Mungan, "Radiation thermodynamics with applications to lasing and fluorescent cooling," Am J Phys 73(4), 2005)

    I've seen a monochromatic, polarized laser beam described as work, since there is only one possible microstate. Blackbody radiation is the opposite, as entropy is maximized. You may also be interested in seeing Kelly, "Thermodynamics of blackbody radiation," Am J Phys 49(8), 1981, and Landsberg and Tonge, "Thermodynamic energy conversion efficiencies," J Appl Phys 51(7), 1980.

    Note that like all real processes, the conversion of EM radiation to thermal energy is an irreversible process.
  4. Jul 28, 2008 #3

    Andy Resnick

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    I've been struggling with this question recently as well. Here's my take on it:

    First, your points 1-13 are sort-of correct. Those sort of statements are made when people attempt to "ground" thermodynamics in statistical mechanics. It's important to realize that thermodynamics does not require the existence of atoms to be a correct theory- there is no need to use poorly-defined terms like "organized motion" or "chaotic motion", or to even use that mental picture.

    When electromagnetic radiation interacts with an object, three things can happen- absorption, transmission, or reflection. While all three involve the interaction of material and light, only one involves the transfer of energy from the light to the object- absorption.
    Reflection involves transfer of momentum. If you like, scattering is an elastic collision while absorption is an inelastic collision.

    Radiative heat transfer is generally taken to mean processes involving the absorption of light and the resultant heating of the material object. I don't know if anyone has done a thermodynamic analysis of say, Mie scattering- for example, to answer if that process is isoentropic?
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