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Simple question about heat

  1. Dec 22, 2011 #1
    is heat simply the mechanical motion of atoms and molecules, or is it the absorption of infrared radiation?

    heat is transported from the sun to the earth by radiation, but is often described as the rapid oscillation of atoms or molecues. when i touch something that is hot, am i feeling the motion of the molecules on the surface of the object, or am i feeling IR eminating from the surface of the object?

    if it is due to radiation, why is infrared the major carrier, as opposed to higher energy radiation? perhaps because atoms and molecules are most likely to be able to absorb and re-emit these frequencies? if it is mechanical motion, why wouldnt higher energy radiation cause more agitation of atoms and molecules than lower energy radiation?

  2. jcsd
  3. Dec 22, 2011 #2
    Heat is kinetic energy. Radiation has to be absorbed in order for it to heat up an object. A gamma ray photon will impart far more energy to an atom or molecule than an infrared photon it just has to be absorbed first.
    Last edited: Dec 22, 2011
  4. Dec 23, 2011 #3
    I think you are confusing "heat" with internal energy. Substances have internal energy due to the motion of atoms, molecules, electrons, photons, etc., but that energy can be transported to the surroundings as either heat transfer or work transfer. In thermodynamics, we have to be careful to talk about "heat transfer" and "work transfer" and not "stored heat" or "stored work" because the latter are not viable concepts in general. For example, we can add heat to the gas in a cylinder, or do work on it, and the stored energy does not come with a label that says "this is stored heat" and "this is stored work" -- it's all just internal energy.

    In essence, heat transfer is energy exchanged between a system and its surroundings because of a temperature difference, while work transfer is energy exchanged through a generalized force. Generalized forces include tensile and compressive forces, torques, pressure, and even electric fields.

    I should add that internal energy can include potential energy, also, in addition to kinetic energy. For example, if you transfer energy to a crystalline solid, the atoms oscillate about their equilibrium positions, having both kinetic energy and elastic potential energy, and the amount of energy stored reflects both kinetic and potential terms.

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