Verification of Conservation of Energy

  1. How is conservation of energy experimentally verified if some energy is dissipated, for example, as heat in air friction or friction with a road?
  2. jcsd
  3. Vanadium 50

    Vanadium 50 18,460
    Staff Emeritus
    Science Advisor
    Education Advisor

    By doing other, more controlled experiments. Making a bad measurement is not part of the scientific method.
  4. Ok. I guess I don't understand the extent to which experiments can be controlled. How would one measure energy lost via friction with a road?
  5. russ_watters

    Staff: Mentor

    In a lab environment, with a dnyo or sort of road treadmill.
  6. Nugatory

    Staff: Mentor

    The energy "lost" to friction shows up as heat somewhere in the system. Thus, we need to design the experimental apparatus in such a way that any heat leaving the exoerimental apparatus is detected and counted; usually we do this by very carefully monitoring the temperature.

    For example, a propeller turning in a container of water doesn't appear to conserve energy. We keep on adding energy to it by turning the shaft, yet its kinetic energy remains stubbornly fixed as it keeps turning at the same speed. But if we also measure the temperature of the water in the container (and insulate the container well enough that it doesn't leak heat out into the environment) we'll find the "missing" energy in the increasing heat of the water.
  7. Ok, but won't there ALWAYS be some amount of energy dissipation? Not just theoretically, but an observed discrepancy between energy in and energy out? It seems sloppy and pointless to propose conservation of energy as an unqualified physical fact, rather than stating that energy in and energy out tend to differ by some experimentally observed amount, or to at least state the law as practically (with regards to the measurement precision of day-to-day life) true but technically off.
  8. Nugatory

    Staff: Mentor

    When you read the papers published by experimentalists, you will find that they do exactly as you are suggesting: They do not say "We have proven X to be true", they say "Our results match X to within the bounds of experimental error", and you will often find a series of papers in which ever narrower error bounds are reported.

    Because every credible experiment that has ever been done finds no violations of energy conservation greater than experimental error, I am justified in approaching a physics problem as if conservation of energy is exact - the answer that I calculate will match a measurement of the actual value to within the measurement error so there is no way that my calculation can be objectively wrong - and that's what makes energy conservation a "law".

    And as a practical proposition, the past few centuries have generated so much experimental support for energy conservation, and have so narrowed the bounds of experimental error, that arguing that energy conservation is not exact would be as perverse as arguing the hypothesis that pigs sprout wings and fly around when they aren't being observed - we can't prove it's not that way, but that doesn't mean anyone believes that it could be that way.
    Last edited: Jun 7, 2015 and DaleSpam like this.
  9. Yes. Too bad such precision isn't widely reflected in physics education, theory, and general discourse.

    On another note, I suppose that it's just another one of those arbitrary and therefore unintuitive (to me) conceptual things about physics that what we've been discussing here is considered to be a kind of experimental error. To my mind, the term "experimental error" would only properly apply to imperfect experimental procedures or equipment — such errors do not exist/have no meaning except in the context of experiments. As there exists no known substance or procedure with which to construct the completely closed experimental system that is necessary to account for absolutely all of the energy produced and expended in a given experiment, I'm inclined to think of unmeasured dissipated energy as inherent to the natural world rather than a facet of experimentation (and thus not properly called an experimental error), particularly since the phenomenon (non-existence of closed systems) obtains regardless of whether or not anyone even attempts any experiments.

    And so I reasoned, if the impossibility of accounting for absolutely all of the energy involved in a given energy transaction is an inherent facet of nature (not an error), and physics is to reflect the operation of the natural world, then physical laws ought not imply a physical state of affairs that is literally impossible to verify. However, I think that I can see how, if unmeasured dissipated energy is regarded as a fault in experiment, one can conclude that energy conservation constitutes a physical law: by taking it as the limit as precision of measurement approaches infinity.

    A proposition to whom, or for what? No one in this thread has argued any such thing. Nor do I see how the flying pigs example is in any way analogous to anything posted. I certainly haven't suggested that ability to prove or disprove conservation of energy implies anything about belief in conservation of energy.
  10. I think the short answer to your original question is good experimental design. You design the experiment to eliminate the need to measure things where possible and if you can't do that ;you design it so you can measure the rest as accurately as possible.
  11. Strictly, physical laws are not verified - they survive attempts to falsify them. Despite ever greater accuracy of measurement, conservation of energy (or mass-energy) survives and makes accurate predictions. A really exciting example was the successful prediction of the existence of the (anti-) neutrino. Though laws cannot be verified, they certainly may be vindicated in such ways.
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