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Speed of heat radiation

  1. Aug 15, 2007 #1
    What's the speed of heat radiation in vacuum?

    And how does the heat propagation (in vacuum and through a conductor) happen? Similar to the particle theory and wave theory for the light, is there any theory for the heat energy?
  2. jcsd
  3. Aug 15, 2007 #2


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    You may get a better answer in the Solid State forum. But here's my two cents:

    Electrons are generally responsible for heat conduction. The phonons (vibrations of the nuclei) also play a part, depending on the temperature of the medium. It depends on the specific conductor you're using. At some point, in a vacuum, I would think the electrons have to be excited enough to spit out photons, which are Electromagnetic radiation... which in a vacuum should go on forever at the speed of light theoretically.

  4. Aug 16, 2007 #3
    In an ideal vacuum you have no electrons, there is only electromagnetic radiation that travels at the speed of light. In a material, heat generally moves diffusively - see http://en.wikipedia.org/wiki/Heat_diffusion for some of the basic concepts and equations.
  5. Aug 16, 2007 #4
    But it is vacuum in between all the atomic particles, right? So, heat should conduct through the vacuum inside the material at the speed of light, right?
  6. Aug 16, 2007 #5


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    In a vacuum, heat moves by radiation alone; there is no conduction.

    In a solid (or an otherwise matter-occupied space, such as water or air), conduction comes into play. This is the physical transfer of kinetic energy from atom to adjacent atom.

    The third form is convection, which happens only in materials where the atoms are free to move about. This is the physical movement of atoms with kinetic energy from place to place (distinct from conduction)
    Last edited: Aug 16, 2007
  7. Aug 16, 2007 #6


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    Between the atoms it does. In simple terms an atom vibrates, the electromagnetic waves from this vibration travel at the speed of light to the next atom which begins to vibrate. This vibration then creates and elctromanetic wave to the next atom and so on ...
    It is the finite time needed for the atom to begin vibrating which slows the propagation of heat and also means that the speed depends on the nature of the material.
  8. Aug 17, 2007 #7
    I just read this URL.

    I am not convinced that the atoms and molecules move when conduction happens. Will it move, or as Pythagorean said, it just vibrates? It should be just vibration. Or there wouldn't be any matter left after it conducted heat, right?

    Another question, when we heat water, how did it become less denser? I would imagine, it is because the heat caused the inter-atomic attraction to get weaker, and because of that, the atoms/molecules moved away slightly, and that caused the substance to be less denser.

    If this is true, here's my question. How did the inter atomic force got weaker with temparature? I would imagine, inter-atomic forces are fundamentally is the resultant of protonic/electronic repulsion/attraction. Does heat cause a reduction in charge in the atomic particle? No, right? Electrons and protons have the same charge, regardless of the temperature, right?
    Last edited: Aug 17, 2007
  9. Aug 17, 2007 #8


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    In a solid, they'll vibrate, since they're confined, but in a less dense medium such as water or air, they are not so confined and will move around like billiard balls.

    If atoms simply bounce off their neighbors, transferring kinetic energy, it's conduction. If the atoms are able to travel a distance from source to recipient, it's convection.

    No. The heat imbues the molecules with kinetic energy, causing them to bounce off each other more vigorously, pushing them farther apart.

    It didn't. It gets weaker with distance.
  10. Aug 17, 2007 #9
    If the atom transferred the heat energy to kinetic energy, there should not be anymore heat to conduct/convect, right? but, why is the conductor/convector still hot?
  11. Aug 17, 2007 #10


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    1) Heat energy is kinetic energy.
    2) Any matter that is above absolute zero (that is to say, all of it) has kinetic energy and thus heat energy
    3) "Why is the conductor still hot?" This sounds like a macroscopic observation i.e. when you touch it. If it has more heat/kinetic energy than you, it will continue to transfer that heat until the you and it are in equilibrium.
  12. Aug 17, 2007 #11
    May be I did not clearly state my question.
    Let me elaborate:

    Heat is an electromagnetic wave, and in the EM spectra, it is the Infrared wave. The wave hits a conductor, the EM infrared wave causes the atom in the conductor to move/vibrate. The atom got the energy to move around, from the infrared wave. My question is, if the atom already used the heat energy for the particle movement, how could it still have the heat energy (by the law of conservation of energy)? The EM wave should die right there, right?
  13. Aug 18, 2007 #12


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    an EM wave contains several photons. Given that the photons are on the same order as the given material, electrons in the material will absorb photons, and step up an energy state... but then the electrons can simply emit the photon and step back down a state. They don't absorb the whole EM wave, and they don't "like" being in high-energy states as much as they "like" their lower states, so they tend to calm back down after being excited.
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