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Sensation of heat, physical explanation?

  1. Apr 30, 2016 #1
    I've read some articles online saying that the temperature is a measure of average kinetic energy of molecules so as the temperature increases, more molecules will bump into each other.

    However, I still don't quite understand why we feel hot when more molecules bump into each other. Is high temperature allowing more "conduction" to take place between molecules of my own with that of others(i.e. air)? if that is the case, how do we define the "heat content" for a single molecule since heat is defined to be the total kinetic energy of molecules..

    I am just having a hard time grasping the mechanism of heat conduction.
  2. jcsd
  3. Apr 30, 2016 #2


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    You may like to read about the physiology of thermoreceptors. I think this is still an area where many questions are still open. WikiP ahs a brief summary in Mechanisms of transduction in its article on thermoreceptors.
    The crucial point, IMO, is that temperature is important in chemical reactions, affecting both eqilibrium and rate of reaction. In chemistry it seems fairly obvious (at least to me) that the energy of molecules and the frequency of their interactions should affect reactions. I think temperature also affects conformation of complex molecules (such as proteins) and that alters their chemical properties.

    I don't think we do define the heat content of a single molecule. A single molecule clearly has kinetic energy of translation, rotation and vibration and there is a numerical equivalence between that mechanical energy and heat.
  4. Apr 30, 2016 #3
    Hmm.. Then could you explain why we feel hot when molecules bump more into each other? Is the sensation of being hot the same as feeling a pain due to the momentum change of molecules(impulse imparted onto our body) ?
  5. Apr 30, 2016 #4


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    Actually, I can't!
    The general principle for neuroreceptors is that there is a balance of ions on either side of a membrane. When that balance is changed, the electric potential across the membrane changes and, if sufficient, this changes the selective permeability of the membrane in a positive feedback manner resulting in complete depolarisation. After a brief interval normal permeability is restored and the membrane potential is restored.
    This process can be triggered electrically, for example by the small currents generated by adjacent depolarisation, or chemically by neuro transmitters which change the ion permeability of the membrane. Other chemicals can also cause membrane changes directly or indirectly via reactions resulting in the release (or removal) of neurotransmitters. Light in the eye causes a series of reactions leading to cahnges in neurotransmitter concentration.
    I do not know the mechanism involved in thermoreceptors. It is postulated that some ion channels in the nerve membranes are particularly temperature dependant and change the permeability with small temperature changes. I can only speculate that this is related to some change in the conformation of proteins in the ion channels. This may be due to a molecule having more than one stable form with different internal energy. Changing temperature could provide the energy to cause a shift between the two forms.

    You seem to liken the impact of molecules to the impact of macroscopic objects. I think individual molecules are much too small for that. Their action, I think, is via the statistical mechanism of temperature. As the average energy of molecules increases, more molecules have the energy to participate in some reactions and equilibria shift in favour of molecules with higher internal energies.

    (Any chemists around please check and correct! I started off with a comment about physiological psychology and seem to have got drawn way out of my depth here! I just wanted to say that chemistry is the mechanism by which heat interacts with our body.)
  6. Apr 30, 2016 #5
    Thank you so much for the reply ! :) I think I need to dig into more books to understand the mechanism
  7. Apr 30, 2016 #6
    Try this on for size. Maybe will help for context.

    In a previous life I was working with a Nd:YAG laser for welding. The tech came in to calibrate the laser to make sure I was getting my full allotment of watts for which I paid. The focusing lens was removed. While helping, I accidentally passed my hand in front of the path of the emitted coherent beam of light. The beam was about 40mm in diameter and about 500 watts. What I felt was a warm spot move across my hand like I was passing my hand close to a light bulb (one of the old incandescent types). I'm no physicist, just a dumb mechanical engineer. But I figure all those photons hitting my hand transferred their kinetic energy & momentum to my hand. This transfer of momentum manifested itself as a change from kinetic energy to heat energy and the temperature increased on my hand.

    Some years later another interesting event. There are things called radiant heaters. They typically hang up in the rafters of warehouses and shop facilities. A gas burner is used to heat air, which heats a tube or duct. The duct is coated with a material that turns the tube into a black body radiator. The black body radiator emits infrared energy. The mechanism for heating is when the IR energy hits an object, ....see above.
  8. Apr 30, 2016 #7
    I think this is because thermal energy indeed IS a sum of internal kinetic energies(rotational, translational, vibrational, etc).. So I guess in a way, the kinetic energy of a molecule(microscopic scale) can be thought of as thermal energy. I think I will be free of doubt if someone can explain the mechanism in which kinetic energy of molecules is felt as being hot to us. Thanks!
  9. May 1, 2016 #8


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    As I said, I'm way out of my comfort zone here, but
    I don't think "hitting" and momentum come into absorption of electromagnetic radiation to any significant degree.
    Even photons, which have zero rest mass, have momentum.
    Their energy is E=hf and their momentum p=h/λ and since fλ=c the speed of light, I get p=E/c
    So for your beam of 500 J per second, ##p = {\frac{500}{3\times10^8}=1.7\times10^-6Nsec\ per\ sec}##
    Not enough to feel as a force I'd have thought and not enough to do noticeable mechanical work on the molecules.

    Absorption of photons depends a lot on the frequency. As you say, it seems infrared is the key. Their photon energies match the differences in vibrational energy levels of many molecules. So they can be absorbed and increase the vibration of the molecules. (I would speculate their electric field is oscillating at a similar frequency to the molecular vibrations and having just looked up a few values, IR is between 3x10^11Hz and 4x10^14 Hz, molecular vibrations around 10^12Hz to 10^14Hz)

    For visible light, with higher frequency, I believe absorption is usually by raising electrons to a higher quantum PE state within a molecule. When these electrons decay back to lower levels, if they do so in smaller steps, the radiation emitted would be of lower frequency (IR) which can then be absorbed in vibrations of molecules.

    The warm spot you feel is from absorbing the energy of the light beam, but I think it is the electronic effect rather than mechanical impact which is warming your molecules. On the other hand I do find people talking about the energy of a photon being kinetic energy, so maybe I'm all wrong and the photon simply bangs into a molecule, makes it vibrate and translate faster, and all the electronic effects are just side effects of the molecule being shaken up? Maybe it doesn't make any difference what you say? There seems to be a lot in the quantum world where people say, that's just the way it is - don't ask questions!
    Last edited: May 1, 2016
  10. May 2, 2016 #9
    Yup, maybe that's viable. The energy contained in the wavelength / frequency gets dissipated & transformed.
  11. May 5, 2016 #10


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    It's possible to put both hands into the same bucket of water and for it to feel hot to one hand and cold to the other. Try the experiment at the end of here..

    In general the human hand is a very bad judge of temperature. Anything over about 50C feels too hot to touch.
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