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Heat and Thermal Energy

  1. Apr 10, 2012 #1
    Postulate a parcel of air within the larger atmosphere. The air above the parcel is cooler than the parcel, while the air below it is warmer. The parcel is emitting photons to both the overlying and the underlying layers of air and simultaneously absorbing photons from both layers. Both sets of photons that are absorbed by the parcel help to warm the parcel.

    My understanding of the thermodynamic definition of heat is that absorbed photons from the lower warmer layer represent heat; while the absorbed photons from the upper cooler layer merely represent thermal energy. Am I correct in this understanding, or am I missing something?
  2. jcsd
  3. Apr 10, 2012 #2


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    You are overlooking the fact that if something can absorb photons, it also emits photons. The middle layer will absorb more photons from lower layer than it emits, but it will emit more photons than it absorbs from upper layer. The net flow of heat will be from lower to middle layer and from middle to upper.
  4. Apr 10, 2012 #3
    My second sentence indicates that I am aware that matter both emits and absorbs photons. I am also well aware of the net heat flow in the system postulated. My question is on terminology. Restated, this question is: Is the photon from the warmer layer properly termed "heat" while the photon from the cooler layer properly termed "thermal energy" even though the two photons may be otherwise indistinguishable?
  5. Apr 11, 2012 #4
    In both cases it could be called Heat/Thermal energy transfer. In classical thermodynamics Energy can be transferred by two mechanisms: doing work and transferring heat (which is the transfer of Thermal energy). As far as I know Heat and Thermal energy are synonyms. *of course there might exist an esoteric distinction in some subfield of science.

  6. Apr 12, 2012 #5
    All my sources give the thermodynamic definition of heat as that form of energy that is being transferred from a warmer substance to a cooler one solely because of the difference in temperatures. This means that energy transferred from a cooler substance is not heat. It is thermal energy. Moreover, it is not heat before transfer nor after transfer. It is only heat while it is in transit.

    Just to be sure, I did a quick google on "heat" before posting this. This position is borne out by the online sources (with slight changes in wording from one source to another).
  7. Apr 12, 2012 #6


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    Could you provide one of those sources please. You are overcomplicating this by doing a microscopic analysis of a macroscopic phenomena: the two terms are synonomous, minus the minor grammatical quibble betwern engineers and physicists associated with the word "heat".

    The same problem exists if you overanalyze evaporation.
    Last edited: Apr 12, 2012
  8. Apr 12, 2012 #7
    You should understand that any net (macroscopic) Heat or Thermal Energy transfer is from "Hot" to "Cold" (the so called parcel receives radiation energy but it also radiates it away as a function on its temperature). So I claim there is no difference in your definition of Heat and Thermal energy. Here is another nomenclature point for you: the "Heat capacity" of a body is also called interchangeably as "Thermal capacity", what a pickle huh? :)


  9. Apr 12, 2012 #8
    Gladly. See http://en.wikipedia.org/wiki/Thermal_energy

    “Thermal energy is distinct from heat. Thermal energy is a state function, a property of a system, while heat, in the strict use in physics, is characteristic only of a process, i.e. it is absorbed or produced as an energy exchange, always as a result of a temperature difference. It is not a static property of matter. Matter does not contain heat, but rather thermal energy. Heat is thermal energy in the process of transfer or conversion across a boundary of one region of matter to another, as a result of a temperature difference. In engineering, the terms "heat" and "heat transfer" are thus used interchangeably, since heat is always understood to be in the process of transfer. The energy transferred by heat is called by other terms (such as thermal energy or latent energy) when this energy is no longer in net transfer, and has become static.”
  10. Apr 12, 2012 #9

    In classical thermodynamics the state function is the Internal Energy which is modulated by Work done "to" and "by" as well as Heat, transferred "to" and "from". Not all Internal Energy is Thermal Energy (i.e. is not proportional to temperature) e.g. the Energy that is stored in chemical bonds which when broken might transfer Heat to another body. That Wiki article looks like it was tailored to some field of science, may be some subset of condensed matter physics (thermodynamics).
    I gather it would be best if you ask your teacher what he implies by the two terms. Actually I fail to understand how this nomenclature nuance affects the understanding of thermodynamics. I wish you good luck with thermodynamics and I hope you will not lose the proverbial forest ;)


    Last edited: Apr 12, 2012
  11. Apr 12, 2012 #10


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