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Thread: Wanting to understand photon emission from an accelerated electron

  1. #1 Wanting to understand photon emission from an accelerated electron 
    Time Lord
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    I know that when a charged particle is accelerated, it emits a photon and then slows down again. However clearly not all accelerations result in the particle returning to its original velocity, or it would not be possible to impart a sustained momentum to them.

    What I'm wondering is, what happens if you accelerate an electron up to a speed, and then decelerate it again? Does it require the same energy to decelerate it as what it took to accelerate it? I'm thinking if it emitted photons during the acceleration stage, then it must have lost energy in doing so, and so now it's traveling slower than it should for the amount of energy that was expended to get it to that speed. Would that mean that when we go to slow it down, it takes less energy, or does it also emit photons when we go to slow it down, canceling the effect?


    I'm just trying to understand what it means kinetically, for an electron to emit a photon. It must be losing energy in the process somehow, but I'm not clear how.


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  3. #2  
    Time Lord
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    What if an electron were being bounced back and forth by two magnetic fields (in a vacuum)? Would the electron's velocity gradually reduce between the trips back and forth, or would it ultimately stay the same, just like a ball bouncing back and forth?


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    Moderator Moderator Markus Hanke's Avatar
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    Quote Originally Posted by kojax View Post
    I'm just trying to understand what it means kinetically, for an electron to emit a photon. It must be losing energy in the process somehow, but I'm not clear how.
    A photon carries a certain amount of energy, which is directly associated with its frequency. So, the emitted photon's energy is exactly the energy that the electron will be missing after the emission. I am not too certain where else your question is going ?
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    Moderator Moderator Markus Hanke's Avatar
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    Quote Originally Posted by kojax View Post
    What if an electron were being bounced back and forth by two magnetic fields (in a vacuum)? Would the electron's velocity gradually reduce between the trips back and forth, or would it ultimately stay the same, just like a ball bouncing back and forth?
    It would stay the same, provided that there are no other interactions taking place.
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    Time Lord
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    Thanks very much for taking interest to examine this problem. I'm confused by the contradictory conclusions. Admittedly they are the same ones I arrived at as well, but they contradict.

    If the electron is losing energy shouldn't that be causing it to slow down? Where is outside energy coming from to keep it going?

    On the other hand, when we decelerate the electron, I presume that would also cause it to emit a photon also, and speed up again (because deceleration in one direction is acceleration in another). But continually emitting photons in this manner must reduce the energy of the system somewhere.
    Some clocks are only right twice a day, but they are still right when they are right.
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  7. #6  
    Moderator Moderator Janus's Avatar
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    Quote Originally Posted by kojax View Post
    What if an electron were being bounced back and forth by two magnetic fields (in a vacuum)? Would the electron's velocity gradually reduce between the trips back and forth, or would it ultimately stay the same, just like a ball bouncing back and forth?
    Electrons don't bounce back and forth between magnetic fields. An electron passing between two magnetic poles will curve, but not in the direction of either pole. For example if the South pole was in towards the screen and the North pole was out away from the screen, then an electron crossing the screen from left to right would curve upwards on the screen.

    If you shoot and electron between large magnets you can get it to make a complete circle. (this fact became the basis behind the beta-tron, an early particle accelerator.)

    If you want to bounce an electron back and forth, you would need (negatively)charged plates.

    In either case, if you shot an electron between two poles or two charged plates so that the electron was more or less constantly accelerating, it would radiate and lose kinetic energy.

    This fact led to one of the major problems for classical physics. With the Bohr model of the atom, you had the electrons circling the nucleus like little planets around a tiny sun. However, circular motion is accelerated motion, so these electrons would radiate away energy and after a very short time fall into the nucleus. The event of quantum mechanics solved this problem by restricting these electron to discrete energy levels. If there wasn't an available lower energy for the electron to fall into, it couldn't radiate away any energy.


    There is an actual term for the radiation emitted by an electron (any other charged particle) when it is slowed down, it is called "Bremsstrahlung" (braking radiation). It is seen when you slam particles into a target in accelerator experiments.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
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