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How is heat transfered .?

  1. Nov 1, 2006 #1
    how is heat transfered .....?

    I could not give myself satisfying explanation for this:

    When a photon(s) hit my skin (from sun) why do i feel warmth? Simplier, why does sunlight warms my skin?

    I know that photon warms atmosphere when it gets absorbed by atom/molecule. This absorbtion must increase kinetic energy of that elements. (thats how temperature is defined).

    Is this how it happens on my skin? The photon increases vib/rot energy of my atoms?

    To ask more explicitly: It is not the kinetic energy of photon that matters, but the frequency, right?

    If some one has some clear explanation how photons warm my skin, i would be glad to get educated.

  2. jcsd
  3. Nov 2, 2006 #2


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    Science Advisor

    You are correct in that the absorption of the photon by atoms in your skin is what makes your skin feel warmer. As for energy vs. frequency; they are itnerdependant in photons. Higher-frequency photons are more energetic, and therefore transfer more energy to the skin when absorbed.
  4. Nov 2, 2006 #3
    Thank you for answer,
    To make sure: its not kinetic energy of the photon that determines how energetic the photon gets, right. Its E=hv , thats determines the amount of heat delivered?

    But why do we use infrared to heat stuff up, and not shorter wavelength light?

    And one subquestion; if a photon is too energetic so it knocks electrons out of the atom, is it still heating the atom?
  5. Nov 2, 2006 #4
    Just a side point -- the atmosphere gets very little heat directly from the sun. Most of the energy is absorbed by the ground/water/trees/etc -- then radiated to the atmosphere at lower wavelengths where it can be absorbed by the atmosheric contents. This is one of the reasons mountains are colder --
  6. Nov 2, 2006 #5


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    The next shorter section of the spectrum is visible light - you'd be blinded if you looked at your space heater.

    The sun heats you up with ir, visible, and uv.
  7. Nov 2, 2006 #6
    deep IR wavelengths are more readily absorbed across a broad spectrum due to vibrational and rotational energy bands.

    The energy in a single photon is hv, but the total energy in light is hv*# of photons

    You said the atmosphere absorbs little radiation, and the earth is heated by the ground instead (which is heated by sunlight), then why are large plateau basins like Tibet not as hot as lower elevations,... There is a problem with that logic...
  8. Nov 2, 2006 #7
    So on sunny day, is it the IR wavelengths or the vis/uv wavelengths that make me feel warm? [i know IR wavelegnths from sun is very little of the total output].

    Im not sure why large plateau of tibet are not as hot as lower elevations. It might be due to partly due to reflection and part due to dynamics of atmosphere. But indeed, from temperature profile of earth we see that the hottest spot on earth is ground. (neglecting very high altitudes of 200km and higher, where temp is hard to define due to low density). This means that most of the solar rad reaches the ground.
  9. Nov 2, 2006 #8
    I was offering a simple supplemental explaination to the original question. Of course there are atmospheric dynamics as well -- Being higher up -- the density goes down and thus the chances of a IR photon being absorbed by a molecule goes down. Also, the heat from below rising expands as it rises, thus cooling as well.
  10. Nov 2, 2006 #9
    This is a bad thing. That ionised atom (or molecule) will likely form a chemical bond with whatever next thing it encounters, and the product is not likely to be beneficial to the living cell. Especially if it was originally supposed to have been part of a DNA molecule, say the part that tells the cell when not to divide. Hence, to avoid sunburn and skin cancer, we endeavor to limit UV exposure.
  11. Nov 3, 2006 #10
    great thread
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