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B What is the photon direction?

  1. Jul 3, 2016 #1
    If a accelerated electron 10keV fly through a decelerating electric field that is set by 10KV high voltage, then after exit, the electron all kinetic energy is lost and become 0 velocity. Of course, braking radiation will happen.
    My question are:
    1. what is the direction of radiation photon? perpendicular to the electron moving direction? or opposite to?
    2. how many photons will be generated? in same energy or in a distribution?
    3. how the angular momentum conserve?
    Assume in vacuum condition.
    I think easy to setup experiment to get the answer, so maybe many guys know.
    thanks
     
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  3. Jul 4, 2016 #2

    Simon Bridge

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    look up "bremsstrahlung radiation".
     
  4. Jul 4, 2016 #3
    I did, but all textbook only deal with electron passing around nuclei. All figures show you electron deflected by nucleus, and photon seems fly away tangentially. I doubt the direction really tangentially?
    My interest is the bremsstrahlung of electron only straight line braking, not circular with nucleus interaction.
    It looks so simple experiment, nobody did it?
     
  5. Jul 4, 2016 #4
    When an electron jumps from high orbit to low orbit, nobody really knows which point and which direction the photon fly away.
    I am disappointed with the science community.:cry:
     
  6. Jul 4, 2016 #5

    Vanadium 50

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    That's fair. After all, I am disappointed in the crackpot community.
     
  7. Jul 4, 2016 #6
    The free electron laser use multiple magnetic pairs to wiggle electron beams. By studying the illustration figures, it seems the photon tangential with the curviest point of the wiggling beam.
    Why free electron laser not use straight line electron beam? linear bremsstrahlung not work? I guess photon direction opposite to electron moving in linear case, so the produced photon from the leading electron will block the way of following electron. that is why free electron laser not like pure linear beam.
     
  8. Jul 4, 2016 #7
    Even the crackpot community no tell the photon direction in atom occurring electron orbit jump.
    what a fuzzy science!
     
  9. Jul 4, 2016 #8

    mfb

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    The effect is completely negligible. As in, your electric fields are about 15+ orders of magnitude too weak to have a reasonable chance to get any radiation.
    It has been shown that you cannot know it. That is a remarkable achievement, proving that you cannot know something. Why are you disappointed?
    That is not a very scientific approach. Those figures are always sketches, don't try to get proper scientific statements just from them.
     
  10. Jul 4, 2016 #9
    the original energy of electron is 10KeV, after braking to stop, it lost the same energy. If the braking radiation is completely negligible, where the lost energy gone? No need energy conserve here?
     
  11. Jul 4, 2016 #10

    jtbell

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    Consider an analogous situation: You throw a ball straight upwards. When it leaves your hand, it has a certain amount of kinetic energy. As it rises through the earth's gravitational field, it slows down and comes to a stop. What happened to the kinetic energy?

    Continuing further, what happens next? Does the ball remain stopped?
     
  12. Jul 4, 2016 #11
    Of course, the ball gained potential energy, and will fall down then, but for the stopped electron, where is the potential energy?
     
  13. Jul 4, 2016 #12

    jtbell

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    Where is the potential energy of the ball?

    How are the situations with the ball and the electron different?
     
  14. Jul 4, 2016 #13
    inside the ball
     
  15. Jul 4, 2016 #14

    mfb

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    Not really. It is in the system - and if you want to locate it, the best approach is "in the gravitational field".

    Same for the electron. By moving the electron towards the anode, you increase the negative charge at the electrode, increasing the field strength. The kinetic energy of the electron gets converted to potential energy of the electron, increasing the field strength a tiny bit.
     
  16. Jul 4, 2016 #15

    jtbell

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    Why not inside the electron, then? As the electron slows down in the electric field, it loses kinetic energy and gains electric potential energy. It comes to a stop, and then starts to move again, in the opposite direction, gaining kinetic energy and losing potential energy.

    Actually, in both cases, "inside the object" is wrong. Potential energy is not a property of the object alone, but of the system composed of the object plus whatever it is interacting with.

    [mfb beat me to this point while I was typing]

    In the case of the ball, the system is the ball plus the earth. In the case of the electron, the system is the electron plus whatever apparatus is producing the electric field that is being used to slow it down.

    But this doesn't affect the fact that the outcome in both situations is similar, and can be analyzed using conservation of energy.
     
  17. Jul 4, 2016 #16
    Imagine this scenario:
    If a car slide down a hill, you jump to the middle way and brake it by using your arms.
    As per your interpretation, you do not lose any energy, but luckily gain energy. This is obviously counter common sense.
     
    Last edited: Jul 4, 2016
  18. Jul 4, 2016 #17

    mfb

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    Losing energy where relative to what?
    Letting a car move down the hill converts gravitational potential energy to kinetic energy (and usually a bit of heat). Stopping the car with brakes converts kinetic energy to heat.
     
  19. Jul 4, 2016 #18
    Imagine this scenario:
    The initial energy of the electron 10KeV, go through a decelerating field of 9KV electrode plates. After exit, the remaining energy 1KeV and constantly run by inertia, now no longer in-between the 10KV field, how can it accelerate in back direction just like the falling fall?

    you brake it by your arms pushback.

    right, it is converting to heat, not strengthen the gravitation field.

    the decelerate electric field also offer work to stop the electron, so the field lose energy, so it seems the best expectation is that all lost energy convert to photons

    OK, don't mind how a little bit of the bremsstrahlung effect, as long as you recognize it does exist in linear breaking, then we can discuss the direct of photon.
     
    Last edited by a moderator: Jul 4, 2016
  20. Jul 4, 2016 #19

    mfb

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    You are taking the analogy too far, but you can imagine something thrown upwards at above the escape velocity: it won't come back, and fly away forever.
    Both happens, We just don't care about the change of the gravitational field because it has no practical relevance in any situation.
    No it does not lose energy.
    This is nonsense.
    Mainly orthogonal to the acceleration direction - in the electron rest frame, for relativistic particles you have to transform this back to the lab frame, where the direction gets more collimated forwards.
     
  21. Jul 4, 2016 #20
    +
    Almost perfect answer. mfb is the #1 scientist here!
    Only the statement "No it does not lose energy" is in little question, as my analogue, if you use your strong body to block a rolling down unmanned car, you have to pay work.
     
  22. Jul 5, 2016 #21

    mfb

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    That becomes a biology question. You need to expend energy even if you just hold an object in front of you without moving it, this energy goes into heating your muscles. Our muscles are not well designed to hold - the individual fibers either contract or not contract. If you hold something, your muscles are actually constantly switching between those two modes, which needs energy.
     
  23. Jul 5, 2016 #22

    Simon Bridge

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    ... there is still a field between the charge and the plates after the charge has exited from between them. The "zero field" you see in texts is an approximation only valid for short distances.
    The 9keV lost in your example is stored in the electric field.

    Try the following puzzle from
    https://www.lhup.edu/~dsimanek/museum/advanced.htm
    Imagine two oppositely charged plates placed very close to each other. Some elementary analysis reveals that there is an electric field between the plates, and essentially no field just outside the plates. Drill a small hole in the center of the plates and drop in a charged particle. (An ordinary electron will do.) The particle will be accelerated by the electric field and pop out the hole in the other plate. Now set up a magnetic field outside the plates that causes the particle to move in a circle back to the first hole. It drops in as before, is accelerated again, and so on forever, eventually attaining arbitrary velocities.​

    In the car rolling downhill example - energy is lost from the gravitational field and goes, ultimately, to thermal: another form of kinetic energy.
    If the car were being pushed uphill, some of the work initially stored in a wide range of biological processes and structures ... this energy gets converted to heat, kinetic energy in the car, and gravitational potential energy. The last just says it gets stored in the gravitational field.

    You have specifically asked about the angle of radiation from an accelerating charge ... from reading your comments, I think I have a better idea of what you need to see. I'll tackle that in the next post.
     
  24. Jul 5, 2016 #23

    Simon Bridge

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    You have specifically asked about the angle of radiation from an accelerating charge ... from reading your comments, I think I have a better idea of what you need to see.
    Basically you are mixing up wave and photon models for understanding radiation from accelerating particles.
    The wave version goes like this:
    http://www.tapir.caltech.edu/~teviet/Waves/empulse.html
    ... here, the accelerated charge radiates uniformly in all directions.

    I take it your problem is relating this image to the photon model.
     
  25. Jul 6, 2016 #24

    vanhees71

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    Well, science is about an accurate quantitative description of nature and not necessarily pleasing your prejudices. There's also some danger that you'll learn something new when reading about science or participate in a forum where scientists are present. If you don't like this, just don't use either of these sources.

    Anyway, first of all you should realize that according to modern quantum theory nothing ever jumps. The "quantum jump" is a buzzword used by economists but not a term used in proper science. There are radiative transitions in atoms, nuclei, or any other bound states of charged particles. Quantum theory delivers an accurate description about the probability distribution for the direction of an emitted photon (usually you find this in dipole approximation in atomic physics). There's nothing more to know about it, because nature behaves as she does, and to the best of our knowledge this behavior is described at very high accuracy by quantum theory (in the case of atomic transitions specifically by QED).
     
  26. Jul 8, 2016 #25

    ChrisVer

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    I have looked through the whole stuff, and to be honest I am confused with what is actually being discussed here.... can you clarify your question? Because all this roller-coaster stuff makes it rather unclear.
    Is it what happens to the lost energy of the electron while it was decelerated by the electric field?
     
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