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Why can we only measure energy (enthalpy) change?

  1. Mar 16, 2019 at 4:01 PM #1
    Hello,
    I asked this similar questions on a different thread but it may be more appropriate for the chemistry forum.

    What is the reason we can only measure energy "changes" and not the actual energy value associated with a system? Absolute energies and enthalpy cannot be determined but I am not sure why...

    I guess we can still say that a system has more or less energy than another system but we cannot know the actual energies of each system, only their difference...

    However, when a car moves at a certain speed, we can certainly express the car's kinetic energy without a problem...

    Thanks!
     
  2. jcsd
  3. Mar 16, 2019 at 5:30 PM #2

    Ygggdrasil

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    Yes, that's the essence of the problem. We can measure the energy difference between different states, but there's not really a way to define the absolute energy of the system.

    Are you sure? It makes sense for a car at rest on the side of the road to have a kinetic energy of zero, but is it really motionless, say, relative to the sun?
     
  4. Mar 16, 2019 at 5:36 PM #3
    This is probably a dumb question, but isn't the absolute energy of a mass given by Einstein's equation?
     
  5. Mar 16, 2019 at 8:26 PM #4

    Ygggdrasil

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    I guess the difficulty of measuring absolute energy versus relative energies applies most to potential energy, which is defined by integrating across a conservative force field: $$U(x) =\text{ } –\int_{x_o}^x F(x) \, dx$$
    which requires arbitrary definition of some reference position from which to calculate energy differences.
     
  6. Mar 19, 2019 at 2:41 PM #5
    Thanks. I am still confused. I see how kinetic energy ##KE## and potential energy ##PE## are relative quantities.

    Does that means we can measure the total energy of a system but that value is frame dependent? For instance, if we said that something has zero energy, it would mean ##KE+PE=0##...

    I see how potential energy is about differences: we don't know the initial and final potential energies but we know their difference...

    In chemistry, only enthalpy changes are measurable. A calorimeter is the instrument used. I guess, experimentally, the only possible result happens to be the energy difference and the minuend and the subtrahend are unknown...
     
  7. Mar 19, 2019 at 5:46 PM #6
    Would knowing the answer to this question in any way change the way that you would analyze a practical thermodynamics problem? If the answer is no, why even bother worrying about it? I think that your time is much more valuable than that.
     
  8. Mar 22, 2019 at 8:48 AM #7
    I would say yes... it’s been mentioned already that energy can only be measured relatively, whether it be potential or kinetic, therefore there shouldn’t be a way to measure an absolute energy. It can only be measured per whatever frame you measure it within.
     
  9. Mar 22, 2019 at 2:44 PM #8
    The unknown amount of internal energy comes from classical mechanics. In relativity we can indeed use m·c² as a measure of the internal energy if we define U(m=0):=0 but the practical use is limited.
     
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