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Heat energy and thermal energy

  1. May 30, 2012 #1


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    My editor friend is questioning the following passage in a book. Does this sound right?

    In particular, they seem to be making a distinction between "heat energy" and "thermal energy".
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  3. May 30, 2012 #2


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    I think that "heat energy" as a term is just wrong. It's okay to think of a system as containing a certain amount of internal energy (or thermal energy), U, because U is a state variable. E.g., for an ideal gas, it depends only on temperature. If the system's temperature changes by a certain amount, then the internal energy will change by a corresponding amount, and this is independent of the path taken (through phase space) to get there.

    In contrast, it's incorrect to think of a system as "containing" a certain "amount" of heat, because the heat is not a state variable. The amount of heat transferred to or from a system (even during the same temperature change) depends on the path taken to get from one state to the other. So heat should only be thought of as what is involved in the *transfer* of energy from one system to another, it should not be thought of as existing in a specified quantity within the system.

    That, at least, is my understanding, based on what I remember from thermodynamics.
  4. May 31, 2012 #3

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    It sounds a bit weird, but literally it is true.

    Thermal energy is a type of internal energy (symbol U).
    We say indeed that "heat" (symbol Q) is transferred from one body to another.

    Conservation of energy dictates that the change in internal energy equals the amount of heat transferred (in the absence of other energy conversions such as "work").

    The explanation you have, looks like a formulation of the first 2 laws of thermodynamics in words.
    The first law says that energy is conserved.
    The second law says that heat moves from warm to cold (entropy can only increase) .
    Last edited: May 31, 2012
  5. May 31, 2012 #4


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    This last statement: "Heat is the movement of thermal energy from warmer places to cooler places." seems to be an attempt to define heat. I think it's not accurate, or at least, misleading. Heat is not the movement of thermal energy. Heat may exist without any movement to a cooler body.
  6. May 31, 2012 #5
    "Heat is the movement.." is actually correct.
  7. May 31, 2012 #6
    Now I am getting little confused too. Unit of heat is Joule. That means there is no need of transfer.

    Wikipedia says "Because in physics it is by definition a transfer of energy". (http://en.wikipedia.org/wiki/Heat)

    Hope someone will clear the doubt.
    Last edited: May 31, 2012
  8. May 31, 2012 #7

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    What's the confusion?

    When heat is transferred from 1 system to another, the internal energy of the first decreases by the "heat", and the internal energy of the second increases by the same "heat".
    The unit of heat is indeed joule.
  9. May 31, 2012 #8
    Would this be for a primary school science book?

    I think discussing the first and second laws and the kinetic theory is a bit over the top in that case.

    I would simply remove the first sentence, and leave that for the teacher to add for the benefit of the brighter pupils.

    The rest is adequate for purpose - it is true but not the whole truth.
  10. Jun 1, 2012 #9
    Here is confusion...
    Heat is by definition a transfer of energy. That is what some sources say.
    At the same time, unit of heat is Joule, that is basic unit, does not include anything related to 'transfer', I mean, 'rate of transfer' or 'distance of transfer' etc. Hence, Bobbywhy's comment looks fair enough. I gave negative reply to Bobbywhy's comment, but now I think the comment is actually valid. There is no need of any transfer.
    Wondering why so many sources have Heat definition that includes transfer word.
  11. Jun 1, 2012 #10
    "It moves away from your body as heat" is misleading. We can say 'Heat moves away from your body' (but even that depends on surrounding temperature).

    Correct statement for following part is "Heat is thermal energy that moves spontaneously from warmer places to cooler places"
  12. Jun 1, 2012 #11
    Exactly so q = ΔU - w (depending upon your sign convention)

    The important point is that q is the heat energy exchanged between the system and its surroundings in thermodynamic process.
    This is just the same as w is the work energy exchanged.
    U is the quantity, already correctly noted as the property accounting for the energy of the system, not the exchange.

    So this statement will introduce a falsehood to be caried on to more advanced studies.
  13. Jun 1, 2012 #12
    Heat and work are not properties of a substance, but results of a process - i.e., an event of some time duration. Internal energy (which really should be called thermodynamic internal energy) is a property of a substance, and is basically the sum total of all energy of a substance (i.e., relative to some reference state) in some thermodynamic system that it is available to be transferred as heat to some other thermodynamic system with which there is some avenue of heat transfer, due solely to temperature difference with that other system. All these parameters are in terms of energy (joules.)

    The 1st Law of Thermodynamics states that for any process, the net heat transferred into a system minus any work done by that system (or plus any work done into that system) is equal to the change in the net internal energy of that system.

    δQ - δW = δU

    BTW, the man Joule made the discovery that (absent heat transfer) the work done on a system is equal to the change in internal energy of that system.

    I'm wondering if this question has come about because of Sadi Carnot's 216th birthday.
    Last edited: Jun 1, 2012
  14. Jun 1, 2012 #13


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