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What is absolute temperature?

  1. Dec 4, 2004 #1


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    What is absolute temperature? The aboslute value of a temperture value?
  2. jcsd
  3. Dec 4, 2004 #2
    Not sure in what context you are asking the questions but possible answers are :

    1. The temperature in degrees Kelvin (vs C or F)
    2. A number that is proportional to the thermal energy of a substance
    3. (?)
  4. Dec 4, 2004 #3


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    I think its the first one, though it could go either way.
  5. Dec 5, 2004 #4
    Ok, the first thing you got to know is that "absolute temperature" exactly means "temperature in degrees K (Kelvin)".
  6. Dec 5, 2004 #5
    The absolute temperature scale is in Kelvin as others have said. On this scale, 0 K is the coldest anything can ever get. It's the same scale as celcius (centigrade) except you have to add 273.

    So if it is 61 F in your garden, that's about 16 C or 289 K
  7. Dec 5, 2004 #6


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    Actually it's both #1 and #2. While #1 only tells you the name of a unit, #2 is more important, as it gives you a physical meaning : The absolute temperature is a number that is proportional to the total kinetic energy of the atoms/molecules in a closed system.

    The Absolute scale can be defined by any two points, say 0K (where all classically calculated molecular motion stops) and 273K (where water freezes at 1 atm) and a linear interpolation/extrapolation.
    Last edited: Dec 5, 2004
  8. Dec 5, 2004 #7
    For the sake of completeness, space can also contain thermal energy, even though it is not a "substance", and so that is how it can be said that interstellar space (background radiation) is at about 2-3 K. If I understand correctly, this energy is basically all in photon (microwave) form.
  9. Dec 6, 2004 #8


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    As others have said, Absolute or Thermodynamic Temperature is measured in Kelvin scale. If you want a definition, apart of that given by the Kinetic Theory, you may as well take a look at the First Principle:

    [tex] dU=TdS-PdV[/tex]

    So that:

    [tex] T=\frac{\partial U}{\partial S}\Big)_{V} [/tex]

    Now is when a physicist should tell us if this derivative can be negative or not. I don't really know.
  10. Dec 6, 2004 #9
    Yes it can be....But then again that is very exotic. For example in some spin-systems (i mean many atoms and we only look at spin spin interactions) absolute NEGATIVE temperatures can arise. These temperatures are no really negative, but they need to be looked at as bigger then infinity....

    The conditions for this to occur are for example that the spin-spin relaxation time is little compared to the spin lattice relaxation time. This means that the spins mutually interact long before thermal degrees of freedom come into play...

  11. Dec 6, 2004 #10


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    So in your opinion, would it be possible to build a device (such as a heat engine) that takes advantage of negative temperatures? All of us know what happens with Classic Thermodynamics when you substitute a T<0 in the formulae. :surprised

    For engineers, it would be a funny thing to design and test such device... :!!)
  12. Dec 6, 2004 #11
    Temperature can be defined with reference to the propeties of some standard substance. Like, in terms of expansion of mercury as in the ordinary thermometer, expansion of some gas, resistance of a metal.

    But then, there is another way in which one can define temperature. In terms of kinetic energy of the gas molecules. When defined in this way, the temperature is said to be the absolute temperature as it doesn't depend upon some other reference.

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