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How is negative ion acceleration possible?

  1. Sep 16, 2010 #1
    I was wondering at the remarkable fact that when an atom has an extra electron it is then possible to apply forces to it to accelerate it. What I find remarkable is the fact that even a heavy ion like mercury where the nucleus is many orders of magnitude more massive than the one electron that is able to cause the acceleration. One could conclude that when the forces to accelerate the ion are applied, it rips off the electron(s) and the electrons go in the direction of acceleration and the now positive nucleus is then accelerated in the opposite direction.

    For example, in the case of a cyclotron, a speeding mercury ion (-) encounters the magnetic field and the electrons get pulled to the right, say, and the bare nucleus would then be pulled to the left. In real life this does not happen (or does it to a small extent?) but what prevents it from happening generally?


  2. jcsd
  3. Sep 16, 2010 #2


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    The electrons are still bound to the nucleus, so as long as your accelerating force doesnt exceed the binding energy then you would be able to move the whole atom without stripping the electron off. I'm assuming that the binding energy of that extra electron is quite low compared to the others, but still high enough to enable the acceleration of the atom.
  4. Sep 16, 2010 #3


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    The electric field of the nucleus, at the position of the electron, is large compared with the electric field of the accelerator.
  5. Sep 17, 2010 #4

    Vanadium 50

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    Drakkith has it right. One way to think of it is that you are pushing the electrons in one direction, and because the electrons attract the nucleus, the nucleus gets dragged along with it. So long as you don't deionize the ion, this works.

    The same thing happens macroscopically - think about a box with a handle duct-taped on. If I pull the handle softly, the box moves. If I jerk it, the handle comes off.
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