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Homework Help: More Magnetism problems

  1. Apr 21, 2005 #1
    I'm confused if I am using the right equations to solve this problem.

    P: A singly charged Li^7 ion has a mass of 1.16x10^-26kg. It is accelerated through a potential difference of 500 Volts and then enters a magnetic field of 0.4 Teslas, moving perpendicular to the field. What is the radius of its path in the magnetic field.


    I had two different solutions to this problem that has to completely different answers. First, I figured I can use the equation V=(kq)/r. Then I solved for the radius. The second way is by using the volts equation: V=PE/q, which I use to solve for the potential energy. I then used this in the kinetic energy equation KE=1/2mv^2 to solve for the velocity. This velocity I then used in the equation r=(mV)/(Bq). I got 2.88x10^-12 meters for the first solution and 3.44x10^13 meters for the second solution. Which one is right is any?
     
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  3. Apr 21, 2005 #2

    OlderDan

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    What is your understanding of V=(kq)/r? How did you use it?
     
  4. Apr 21, 2005 #3
    Well... since I knew how much volts there is, I figured that it is faster to just use that formula where k is the Coulomb constant (8.99x10^9 Nm^2/C^2), then just solve for the radius. Is it possible to use this equation like that?
     
  5. Apr 21, 2005 #4

    OlderDan

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    No. The r in that equation is distance from a charge q and the V is is the electric potential due to that charge. It has nothing to do with the radius of curvature of a path of a particle, and that V is for a completely dirrerent geometry than what you have.
     
  6. Apr 21, 2005 #5
    So is my second solution the right one? or are they both wrong?
     
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