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Thread: History of the theory of energy

  1. #1 History of the theory of energy 
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    I have recently been admiring the theory of conservation of energy.

    Feynman had this to say in 1961:

    "There is a fact, or if you wish, a law, governing natural phenomena that are known to date. There is no known exception to this law—it is exact so far as we know. The law is called conservation of energy; it states that there is a certain quantity, which we call energy that does not change in manifold changes which nature undergoes. That is a most abstract idea, because it is a mathematical principle; it says that there is a numerical quantity, which does not change when something happens. It is not a description of a mechanism, or anything concrete; it is just a strange fact that we can calculate some number, and when we finish watching nature go through her tricks and calculate the number again, it is the same."

    I made this list of all of the forms that I could think of, that energy can take:

    kinetic
    elastic
    raised weight (gravitational potential)
    chemical
    atomic
    compressed gas
    heat
    phase change
    magnetic field
    static electricity
    radiation
    sound waves
    ocean waves

    Any of those forms can be converted to any of the others; sometime machinery is required.

    A related concept is work, a force pushing through a distance. Energy can be converted into work, but there is no storage. The work done ends up stored in one of the above forms.

    I'm not sure how to think about the energy in an electrical transmission line. The line is used to deliver energy from a source to a sink. But, in between, what do we have?


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  3. #2  
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    If it's AC electricity, the energy is in the form of an EM wave, or EM radiation at a low frequency. It's no different than if you beamed the energy through glass in the form of a concentrated laser beam, except it's a lower frequency traveling through a wire.

    If it's DC electricity, that is a little bit harder to envision. The moving electrons would be where the energy is stored. Not a whole lot different from kinetic energy stored in a moving baseball. Except, they're electrons, so I'm not sure the energy of their motion is considered to be "kinetic" in nature. There's probably another term that's more accurate for that.


    Some clocks are only right twice a day, but they are still right when they are right.
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  4. #3  
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    Your types of energy( I think they can be condensed to three types all together ) omits one very important type. Potential energy is associated with fields such as an EM field. In the case you describe the voltage, wether alternating or direct, is the field which gives rise to the potential energy which is soon translated into kinetic energy of the charge carriers.
    So we have energy due to a field, known as potential. Energy due to motion known as kinetic. And the last is an inherent energy that all bodies with rest mass have, with no specific name although you could call it just mass and use the che conversion factor c^2.
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  5. #4  
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    Migl, I listed static electricity as one of my types, and the magnetic field as another, and gravitational potential energy as another. Those are all different. They require different devices to use them, they require different mathematical model to describe them. The same can be said for all thirteen forms in my list.

    The energy of the rest mass is accounted for under atomic energy, AKA nuclear energy.
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  6. #5  
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    Quote Originally Posted by MitchellTimin View Post
    I'm not sure how to think about the energy in an electrical transmission line. The line is used to deliver energy from a source to a sink. But, in between, what do we have?
    Ideally, we would not store energy in the transmission line, but in reality we do. The transmission line has inductance in series, and capacitance in parallel with the load, both of which store energy.
    Characteristic impedance - Wikipedia, the free encyclopedia
    Inductor - Wikipedia, the free encyclopedia
    Capacitance - Wikipedia, the free encyclopedia

    If you need a way to think of it, think of a mechanical analogy. If you were pulling a load with a weightless, infinitely stiff rope, that would be like a transmission line with no capacitance or inductance. But a real rope would have mass which stores kinetic energy and would be stretchy and so would store elastic energy.
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  7. #6  
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    A transmission line can be treated as a contained longitudinal wave propagation. Many wavelengths away from the ends it is not forced, but self propagating. At such a point

    1. while the voltage is passing through zero the current is at its peak, so at this time the energy is entirely induced magnetic energy with no electrical energy.
    2. when the current is at its positive or negative peak, the current is passing through zero, so the induced magnetic energy is zero, and all the energy is contained within the electric field across the conductors.
    3. In between there is a mix of electrical and magnetic field energies, but the sum of these energies is a constant for a continuous wave.

    The transmission line is similar to a free electromagnetic wave in that the electrical and magnetic energies alternate. It is different in that the transmission line carries a longitudinal wave rather than a transverse one, so the propagation speed can never reach the speed of light.
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  8. #7  
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    No they are not all different types and do not require different mathematics to handle them. Potential energy and kinetic energy are handled just fine by either the Lagrangian or Hamiltonian formulations. The energy of an object has nothing to do with the mathematics used to describe the field acting on it.

    The 'atomic' energy you describe is merely binding energy where a small percentage of mass is either added or subtracted depending on wether you are below 26 ( iron ) on the periodic table, or above. You can also have total energy conversion as in pair creation or annihilation of particle/ antiparticle pairs.
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