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What is heat?

  1. May 11, 2007 #1


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    The common definition of heat is "the energy transfered from one body to another one as result of a temperature difference"
    This energy isn't "in" any of the bodys, rather it's "energy on the move" between the two. But if energy is the potential to do work, then heat is the potential of what to do work? (for example, KE is the potential of the body that's moving to do work.)
    I think that it's the potential of a system of bodies with different temperatures to do work. Is that right?
  2. jcsd
  3. May 11, 2007 #2
    Problem is that you should not define energy as "potential to do work". Energy is a measure of a quantity that is constant for the system over time (relating to time-symmetry of the underlying physics). For "potential to do work", study thermodynamics. But generally, you should think of heat as the energy contained in the random motion of atoms composing a material, and temperature is then the heat of a material divided by the material's capacity for storing heat.
  4. May 11, 2007 #3
    i think heat energy is rapid kinetic energy. if you touch a boiling hot kettle the rapid vibrations of heat from the kettle's atomic structure will vibrate your finger causing it to split, depending on the amount of time. my very primitive theory on heat.
  5. May 11, 2007 #4


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    I'm afraid that is a common misconception. What you have defined there is thermal energy; whereas heat, is the transfer of thermal energy through some medium. One cannot store heat since it is the transfer of energy.

    We can however, define some thermodynamic potentials which can describe the amount of "potential energy", or potential to do work in a system that is subject to one or more constraints.
    Last edited: May 11, 2007
  6. May 11, 2007 #5
    Fair call. Personally though, I can only imagine historical reasons for defining heat so. Is there a particular advantage to maintaining the distinction between "heat" and "thermal energy"?
  7. May 11, 2007 #6


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    Yes, usually we can consider the thermal energy as the sum of the kinetic energies of the individual molecules. Internal energy we consider as the sum of the total kinetic and potential energies between the individual molecules. Now, let us consider the first law of thermodynamics;

    [tex]\bar{d} U = \bar{d} Q + \bar{d}W[/tex]

    Where Q is the heat transferred to the system and W is the work done on the system. Hence, we can transfer energy to the system by either doing work on it, or transfering heat to it, both these action raise the internal energy of the system. You can read more here http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heat.html
  8. May 11, 2007 #7
    The internal energy of an object can change in various ways, and loss or gain of heat is only one of these ways.

    When we talk about gravitational or electrical potential energy, we are categorizing the 'types of energy' by the which force, or mechanism, has a potential to do work.

    Heat is 'categorized' according to the fact that the mechanism responsible is the thermal interaction of molecules.

    Some other ways that the internal energy of an object can change besides heat: change in pressure, change in volume, change in the number of particles.
  9. May 12, 2007 #8
    So is heat defined by the ejection of one or more IR photons?
  10. May 12, 2007 #9
    No, pallidin. For the standard definition of heat see hyperphysics. We don't normally speak of IR photons (because the wavelength is so large, the particle nature is not commonly observed), but more importantly, radiated heat is not always infra-red (some is blue.. and I doubt infra-red light is always heat), and heat can also be conducted (or even convected) without involving light at all.
  11. May 12, 2007 #10


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    I never knew that, Hoot. Thanks for the educational boost. :smile:
  12. May 13, 2007 #11


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    All part of the service :biggrin:
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