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What is the definition of physical contact?

  1. Dec 7, 2015 #1
    We talk about local effects and surfaces being "in contact" with one another but do we have a vigorous definition for such a state? The objects reach a distance where their repelling EM charges resist and balance against a given force (such as gravitation)...but increased charges would make that distance greater as well.

    If the distance between two objects is not constant in our definition of being in contact then at what point does, for example, conduction no longer occur?
     
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  3. Dec 7, 2015 #2

    mfb

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    There is no sharp dividing line, so every definition would be arbitrary.
    An electric current? Tunnel current just gets smaller and smaller with increasing distance. It is non-zero even if you would classically say "they don't touch each other", this is used in scanning tunnel microscopes.
     
  4. Dec 7, 2015 #3
    Ahh so are you claiming that energy transfer through conduction is just quantum tunneling occurring frequently enough to be observed? The reason I'm bothered by this is that it takes only a very small vacuum gap (in a thermos, for example) to almost completely prevent conduction...although the link provided says that tunneling drops off in the 1-3mm range so I guess it's plausible. I think of conduction as a physical jostling of atoms; a transfer of kinetic energy. I cannot easily picture how the tunneling would work.

    In a purely classical sense it seems that atomic kinetic energy could be transferred over arbitrary distances via the Coulomb force. Is it possible that there isn't a technical difference between radiation and conduction?
     
  5. Dec 7, 2015 #4

    mfb

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    Nanometers (nm), not millimeters. 0.000001 millimeters. Comparable to the size of atoms.

    But I think you are talking about heat conductance here? There tunneling is negligible, you need much closer contact (which you have if you have relevant forces between the objects). On the other hand, thermal radiation can bridge every gap size, and gaps smaller than the wavelength of light lead to some new effects.
    They can be described with photons and phonons, respectively. Similar names, but completely different things.
     
  6. Dec 7, 2015 #5
    Gah, of course! Didn't sound right to me but I have zero experience with these types of experiments.

    Yes, heat conductance. Your response confused me. Let's ignore tunneling and radiation for a moment. When you say "There tunneling is negligible..." what is the THERE referring to? The distance between atoms for conduction to occur? I don't mean to take you in circles here, but I was kind of hoping that there was a standard answer.

    REPHRASE: If we separate two objects by a vacuum do we not proclaim that heat conductance does not occur? But don't their Coulomb forces propagate across the vacuum, and wouldn't those Coulomb forces be manipulated via atomic kinetic energy?
     
  7. Dec 7, 2015 #6

    bcrowell

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    Another nice way of thinking about this is in terms of residual interactions.

    The term is usually used to describe the nuclear interaction between neutrons and protons, which is approximately zero because each object is built out of three quarks, and the color charges of the quarks cancel. But at short distances, the forces are not zero, because the quark-quark interaction depends on distance, and the different quarks are all at different distances from each other. However, this residual force falls off exponentially with distance, so it's negligible beyond about 10^-15 m. "Contact" between nuclei should naturally be defined as coming within this range. For example, if you want to fuse two nuclei, you have to get them this close together.

    The same thing happens with electrical interactions between molecules or between bulk matter and other bulk matter. They're electrically neutral, but you get a residual interaction that has a range of about 10^-10 m. (This also happens to be about the thickness of the electron cloud.)
     
  8. Dec 7, 2015 #7
    Yeah, I was wondering about that! From a distance the atoms could be considered electrically neutral so their kinetic energy "em wave" is negligible. Let me think about this. Thank you
     
  9. Dec 8, 2015 #8
    OK bcrowell I think you nailed it. Once the distance between two molecules becomes somewhat comparable to the distance between a nucleus and an electron then the Coulomb forces become significant and kinetic energy transfer can occur. Thanks so much
     
  10. Dec 8, 2015 #9

    mfb

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    There = thermal conduction (instead of electric).
    If the atoms are really, really close, yes. At that size it is unclear what "vacuum" means. See van-der-Waals force.
     
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