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Homework Help: An accelerated iron Ion moving through a mass spectrometer's field

  1. Jun 20, 2013 #1
    1. The problem statement, all variables and given/known data

    A sample of two different iron ions, Fe2+ and Fe3+, are accelerated by the same potential and then sent through the uniform magnetic field of a mass spectrometer.

    a) Which ion moves faster after being accelerated?
    b) Which ion follows the path with the largest radius?

    2. Relevant equations

    Ek = 1/2mv^2
    v= sqrt|(2qV)/m|
    ??v= (qBr)/m??

    Fc= (mv^2)/r
    r = sqrt|(m2V)/(qB)|

    3. The attempt at a solution

    a) Stated that Fe3+ had a stronger charge and a smaller (though negligibly) mass. Related this to the Ek and v= sqrt|(2qV)/m| equations in order to show that it would have a faster velocity. I think I'm alright here...

    b) This is where I have trouble. My gut tells me that the Fe2+ should have the larger radius (when I picture the charge in my head it has a lower Ek and gets deflected more). However, relating the equations is confusing me as I keep getting tripped up by thinking that the larger charge of the Fe3+ ion will cause it to experience more force and thus a wider radius pushing it. Maybe I should be picturing it enter the field from over top? (i.e. (x) and not from the side -->). Also, I want to try and set up r(Fe+3)/r(Fe+2) = ???/??? but can't figure out which equation to use on the right side...

    Help is very much appreciated.
  2. jcsd
  3. Jun 20, 2013 #2

    rude man

    User Avatar
    Homework Helper
    Gold Member

    Equate centripetal force to the force of a moving charge in a magnetic field. What do you get?
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