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Finding force produced by a Magnetic Field on a Proton

  1. Aug 6, 2013 #1
    1. The problem statement, all variables and given/known data
    A Proton moves with a velocity of 5x10^6 m/s in the +y direction. What is the force (magnitude and direction) on the proton if a magnetic field of 2.12Ti + 2.12Tj is applied.


    2. Relevant equations
    - F = |q|vBsinθ
    - Right hand rule to find direction

    3. The attempt at a solution

    I think that I'm overlooking some math for this problem, and I'm require to use some more trig or something. I would really appreciate it if anyone could help me point out if I did something wrong, and how to go about fixing my mistake.

    In the i direction, I had the magnitude of the force equal to (1.6x10^-19)(5x10^6)(2.12)(sin90) which equals 1.696 x 10^-12. I set the magnitude of the force in the j direction equal to 0, because the sin of the angles between the V and the B is equal to 0, in what I think sets that whole force equal to 0.

    Then using the right hand rule, I get the final force equal to - 1.696 x 10^-12N k
     
  2. jcsd
  3. Aug 6, 2013 #2

    HallsofIvy

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    Staff Emeritus
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    Why are you taking [itex]\theta[/itex] to be 90 degrees? Shouldn't you use the angle between the velocity vector of the proton and the magnetic field vector, which would appear to be 45 degrees?
     
  4. Aug 7, 2013 #3
    Wow, I clearly wasn't thinking straight when I did that problem.. 45degrees makes complete sense. No idea why I thought I attempted to split up the magnetic field components.

    Oh well, thank you for your help!
     
  5. Aug 7, 2013 #4

    gneill

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    Staff: Mentor

    Note that since you are given the v and B vector components, you could consider using the vector form of the equation and just do the cross product: ## F = q \vec{v} \times \vec{B}## .
     
  6. Aug 7, 2013 #5
    Unfortunately, I do not know how to do cross products yet.
     
  7. Aug 7, 2013 #6

    gneill

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    Ah. That's a shame. It avoids having to work out the angles between vectors, which can be annoying if they're 3D. How about determinants? Have you learned how to compute the determinant of a 3x3 matrix?
     
  8. Aug 7, 2013 #7
    No I haven't learned to do that either. I'm taking calculus 2 right now, and I'm assuming most of that stuff is taught in Calc 3?
     
  9. Aug 7, 2013 #8

    gneill

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    I think I first came across them in pre-calculus and linear algebra. But if you haven't seen them yet, you'll just have to carry on the way you're going (or take a detour and read up on cross products and determinants).
     
  10. Aug 7, 2013 #9
    Wait, didn't MikeBriganti get the problem correct the first time?? The component of the magnetic vector in the j direction will have no effect???
     
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