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The Conservation of Energy

  1. May 3, 2009 #1
    Energy cannot be created or destroyed in an isolated system.

    Can energy be created or destroyed in an open system? Do open systems exist? Where does energy come from?

    I'd appreciate it if someone could tell me more about this.
  2. jcsd
  3. May 3, 2009 #2


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    I'm not sure why this is puzzling. An "open" system simply means that you allow energy transfer in and out of the system. Example: you HEAT an object, where the object is the "system".

    Thus, by definition, the amount of energy of that system isn't a constant.

  4. May 3, 2009 #3
    I think I might more simply say that energy can't be created or destroyed. As ZapperZ pointed out, if a system is not isolated, then energy can be brought into it from the outside. Or it can leak out. But it's not created out of nothing.

    This of course doesn't answer your question about where energy originally comes from. I think that would require a much greater command of cosmology than I have.

    Since you're curious, I thought I might add a couple of more points about energy.

    In my understanding, energy is conserved (remains constant over time) as long as the equations of motion of the system have no explicit time dependence. In other words, energy conservation is not a fundamental "law" of the universe. Rather, it's a consequence of the observation that there's no preferred origin for time. In a system where energy is conserved, you can start your clock at zero any time you want and it won't matter.

    Given that, one can imagine systems where energy is not conserved. The classical example often given is an oscillating mass on a spring that's being driven by some external (typically periodic) force. Here the choice of zero time does matter. Basically, the longer that driving force is "on", the more energy gets pumped into the oscillating mass.

    Finally, the amount of energy in a system is actually quite arbitrary. The kinetic energy of a particle depends on which inertial reference frame the observer chooses. The potential energy of a system is well defined only up to an arbitrary constant. This makes energy quite a different sort of beast than, say, electric charge. The amount of electric charge in a (closed) system is not only conserved, it's invariant. That means charge doesn't vary with the speed of the observer, unlike energy.
  5. May 3, 2009 #4
    Thanks, now I get it. That clears things up a lot.

    Thanks for that, too. I don't have a scientific background but I think energy is really interesting.

    I wonder where I might be able to learn more about this sort of thing. If you or anyone could recommend any books or websites that someone without much knowledge of science could understand I'd appreciate it.
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