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Applying Newton's 2nd law to single charged particle

  1. Nov 27, 2006 #1
    How much are we justified in applying Newton's 2nd law in predicting trajectories of a single charged particle?What will be the answer for a number of charges?Of course, we have cyclotron motion at hand, but still...as we know, sub atomic level physics needs quantum mechanical consideration and linear superposition is liable to question here...So my question...
     
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  3. Nov 27, 2006 #2

    dextercioby

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    Classical electromagnetism and Newton's laws don't go well hand in hand, since electromagnetism requires a relativistic description of massive point charges. Accelerator physics is in the realm of quantum physics, of course.

    Daniel.
     
  4. Nov 27, 2006 #3
    I wasn't aware that combining Maxwell's equations with the Lorentz force law led to any glaring contradictions in the non-relativistic energy regime. Could you elaborate a bit, dexter? I'm curious as to what we see.
     
  5. Nov 27, 2006 #4

    ZapperZ

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    Actually, accelerator physics is predominantly classical. Particle trajectory software such as PARMELA use purely classical mechanics and E&M.

    ..... unless you are confusing "accelerator physics" with "high energy physics".

    Zz.
     
  6. Nov 27, 2006 #5

    dextercioby

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    They don't. It's just that the theory is not unitary, so it can't offer a complete description of electromagnetic phenomena at classical level.

    Daniel.
     
  7. Nov 27, 2006 #6

    dextercioby

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    Perhaps. :blushing: So is accelerator physics not about HEP ?

    Daniel.
     
  8. Nov 27, 2006 #7

    ZapperZ

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    Correct.

    Accelerators are used in a number of different applications. The accelerated particle can be funnel into a synchrotron storage ring, for example. Or it can be used in a medical device to generate x-ray either for diagnostic or treatment. Having it as part of a particle collider facility is simply one example of its use. The physicists who work in accelerator physics very often do not know much about high energy/particle physics.

    The APS publishes a FREE journal dedicated to accelerator/beam physics called Physical Review Special Topic - Accelerator and Beams (PRSTAB).

    http://prst-ab.aps.org/

    This is in addition to the section in PRL on Plasma and Beam.

    You'll find the type of subject matter that is covered in there that are part of this area of study. You won't see any high energy physics papers.

    Zz.
     
  9. Nov 27, 2006 #8
    ................The discussion seems to go beyond my point in this thread.What I asked was that when we calculate circular/cycloidal trajectories of a charged particle using Newton's 2nd law, does it mean a physical reality or just mathematical analysis?
     
  10. Nov 27, 2006 #9
    In physics aren't mathematical calculations and physical reality presumed to be the same thing if the theory being used is correct?
     
  11. Nov 27, 2006 #10
    Yes, but to a certain extent and with a level of understanding.You may refer to the beginning Jackson's 2nd chapter where he writes,"...such idealizations as point charges or electric fields at a point must be viewed as mathematical constructs that permit a description of the phenomena at the macroscopic level, but that may fail to have meaning microscopically".
    When Griffiths makes a calulation of the helical or cycloidal trajectory, it raises question whether we are justified in equating ma with the Lorentz force, being applied on a single isolated particle...Chrged particle radiates EM energy-should we not take these facts into account? Rather I think to make some applications of Lorentz force, these idealizations are adopted...They are not reality.
     
  12. Nov 27, 2006 #11

    ZapperZ

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    There is a difference between "idealization" and "simplification".

    When you ignore air resistance in simple free-fall measurement, that is a simplification. When you require more precise results and realize the simplification isn't sufficient, you ADD more relevant factors that influences the dynamics. You keep doing this until your model matches the results as accurately as you require.

    The same thing here. So far, we haven't seen any 'size effect' of the electron that has been manifested in various experiments. Thus, the simplification to a point charge works! Until there's something relevant, either experimentally or theoretically, that requires a more indepth consideration of an electron size, then the point-charge model works perfectly fine for our use.

    The same principle applies to the helical path of charge particles.

    Zz.
     
  13. Nov 27, 2006 #12
    So, what about the radiation of the chargged particle in cyclotron motion?Classically,it needs to have radiated...
     
  14. Nov 28, 2006 #13

    ZapperZ

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    What makes you think it doesn't? Cyclotrons and synchrotrons USE the radiation generated by such motion.

    Zz.
     
  15. Nov 28, 2006 #14
    If I remeber correctly, I believe that even electrons in a cathode ray tube travel so fast that the Newtonian approximation is no longer accurate, and you need to include relativistic dialation terms.
     
  16. Nov 28, 2006 #15

    ZapperZ

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    I don't think this has anything to do with classical vs. relativistic. It has more to do with classical vs. quantum mechanical. In particular, it is the commonly asked question on why "classical" motion of charged particles will radiate if they move in a circular/accelerating path, while it does radiate when it is in "orbit" in an atom.

    We have already addressed this issue in the FAQ thread in the General Physics forum, because we get this type of question very often.

    Zz.
     
  17. Nov 28, 2006 #16

    jtbell

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    Yes, in principle we need to account for this radiation. In practice, however, the effects are negligible in many situations, including most simple textbook exercises.
     
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