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Thread: Accelerating Electrons

  1. #1 Accelerating Electrons 
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    If every time you accelerate an electron, it then emits a photon and slows back down again, then how would an electron ever gain speed and keep it? Wouldn't they have to absorb some energy and not re-emit it?

    I'm sure it makes sense somehow.


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  3. #2 Re: Accelerating Electrons 
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    Quote Originally Posted by kojax
    If every time you accelerate an electron, it then emits a photon and slows back down again, then how would an electron ever gain speed and keep it? Wouldn't they have to absorb some energy and not re-emit it?

    I'm sure it makes sense somehow.
    In a slightly different form this is the question that resulted in the Bohr model of the hydrogen atom.

    Classically an accelerated electron will emit electromagnetic energy. In the simple model of the hydrogen atom, the electron is orbiting the nucleus in a circular orbit, which requires acceleration as the direction of the velocity vector is constantly changing. So, classically the electron should be constantly losing energy and therefore spiraling into the nucleus. Thus one would expect the atom to be unstable, but of course it is not.

    The solution is quantum mechanics. An electron in this situation can only emit a photon of certain energies, and does not emit a photon unless it has enough energy to do so, energy above the ground state.

    So, the classical picture in which any acceleration of an electron results in some electromagnetic radiation has to modified somewhat when one considers the more exact models of quantum electrodynamics.


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  4. #3 Re: Accelerating Electrons 
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    Quote Originally Posted by kojax
    If every time you accelerate an electron, it then emits a photon and slows back down again, then how would an electron ever gain speed and keep it? Wouldn't they have to absorb some energy and not re-emit it?

    I'm sure it makes sense somehow.
    It just depends on the type of field that is causing it to accelerate.

    if the field causing it to accelerate is due to charge then only some of the energy will be emitted as light.

    If the field causing it to accelerate is due to an electromagnetic wave (light) then it will be (to a first approximation) entirely re-emitted.
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  5. #4 Re: Accelerating Electrons 
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    Quote Originally Posted by granpa
    Quote Originally Posted by kojax
    If every time you accelerate an electron, it then emits a photon and slows back down again, then how would an electron ever gain speed and keep it? Wouldn't they have to absorb some energy and not re-emit it?

    I'm sure it makes sense somehow.
    It just depends on the type of field that is causing it to accelerate.

    if the field causing it to accelerate is due to nearby charges then only some of the energy will be emitted as light.

    If the field causing it to accelerate is due to an electromagnetic wave then it will be (to a first approximation) entirely re-emitted.
    If that were correct then the electons in atoms would be emitting all the time and the atom would be unstable. The field that holds the electron bound to the nucleus is 100% elelctromagnetic, and the electrons are constantly accelerating, which is necessary in order to remain bound.

    Moreover, a "field due to nearby charges" is also 100% electromagnetic. If the charges are moving then the field will be time-varying, which is a "wave". In any case if the field accelerates the charge then it will radiate.

    The word "nearby" is superfluous.

    In short, your explanation is wrong.
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  6. #5  
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    By the way ... acceleration does not cause radiation. It's another thing.
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  7. #6  
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    Sorry. It's not scientific community's view that acceleration does not cause radiation,
    just jartsa and R. Feynman have that view.
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  8. #7  
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    right, cause that wasn't already said...
    Wise men speak because they have something to say; Fools, because they have to say something.
    -Plato

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  9. #8  
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    If the energy emitted is equal to the energy required to create the acceleration, then wouldn't a proton have to emit a greater amount of energy than an electron would if it were accelerated by the same amount?

    Protons have greater mass, so it takes more work to accelerate them, but they have the same magnitude of charge. So, I'm curious which of those two considerations wins out.


    Quote Originally Posted by DrRocket
    Quote Originally Posted by kojax
    If every time you accelerate an electron, it then emits a photon and slows back down again, then how would an electron ever gain speed and keep it? Wouldn't they have to absorb some energy and not re-emit it?

    I'm sure it makes sense somehow.
    In a slightly different form this is the question that resulted in the Bohr model of the hydrogen atom.

    Classically an accelerated electron will emit electromagnetic energy. In the simple model of the hydrogen atom, the electron is orbiting the nucleus in a circular orbit, which requires acceleration as the direction of the velocity vector is constantly changing. So, classically the electron should be constantly losing energy and therefore spiraling into the nucleus. Thus one would expect the atom to be unstable, but of course it is not.

    The solution is quantum mechanics. An electron in this situation can only emit a photon of certain energies, and does not emit a photon unless it has enough energy to do so, energy above the ground state.

    So, the classical picture in which any acceleration of an electron results in some electromagnetic radiation has to modified somewhat when one considers the more exact models of quantum electrodynamics.
    Doesn't the fact that the electron is imparting an equal and opposite force on the proton make any difference to this? I would think the proton would emit a canceling EM wave most of the time.

    Or... maybe I have that backwards, and it would be expected to reinforce instead?
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  10. #9  
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    Quote Originally Posted by kojax
    If the energy emitted is equal to the energy required to create the acceleration, then wouldn't a proton have to emit a greater amount of energy than an electron would if it were accelerated by the same amount?
    The energy emitted is not equal to the energy creating the acceleration.

    If it takes x joules of energy to accelerate a neutral particle up to a given velocity, that same x joules when applied to an equally massed, but charged particle will result in a lower final velocity of the particle. Part of the energy goes into accelerating the particle while the rest is bled off by the radiation produced by the accelerating particle.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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  11. #10  
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    Quote Originally Posted by Janus
    Quote Originally Posted by kojax
    If the energy emitted is equal to the energy required to create the acceleration, then wouldn't a proton have to emit a greater amount of energy than an electron would if it were accelerated by the same amount?
    The energy emitted is not equal to the energy creating the acceleration.

    If it takes x joules of energy to accelerate a neutral particle up to a given velocity, that same x joules when applied to an equally massed, but charged particle will result in a lower final velocity of the particle. Part of the energy goes into accelerating the particle while the rest is bled off by the radiation produced by the accelerating particle.
    Right. And this accounts for an electromagnetic component to the mass of the electron and proton. There is a nice elementary discussion of this in the Feynman Lectures on Physics.

    More generally the phenomena is known as the Abraham-Lorentz force or the Abraham-Lorentz-Dirac force.

    http://en.wikipedia.org/wiki/Abraham-Lorentz_force

    http://en.wikipedia.org/wiki/Abraham...tz-Dirac_force
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