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Homework Help: This Is A Challenging One

  1. Oct 6, 2004 #1
    This Is A Challenging One....

    A proton and an electron have the same kinetic energyuopn entering a region of constant magnetic field
    What is the ratio of the radii of their circular paths?

    I used MeVe squared= MpVp squared
    but i couldn't get the ratios from these....
  2. jcsd
  3. Oct 6, 2004 #2


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    Homework Helper

    You need to use the fact that the gyroradius varies directly with the momentum of the particle.
  4. Oct 6, 2004 #3
    Force due to magnetic field : F = qvB, with v perpendicular to F
    Force due to circular motion : F = ??? (the most elementary form...)

    Mix all of this together using Newton's 3rd.
  5. Oct 7, 2004 #4
    curved path

    use the fact that in circular motion, F=(mv^2)/r.

    after a bit of fiddling that gives you the ratios of the circles:

    r(p) = radius for proton path
    r(e) = radius of electron path
    m(e) = mass of electron
    m(p) = mass of proton
    v = velocity of particle
    B = mag field strength
    V = Voltage across plates

    r(e) : r(p) = - (m(e)v^2)/BV : (m(p)v^2)/BV
  6. Oct 7, 2004 #5
    You are heading wrong way, to calculate take lorentz force in account and try it again
    use relation,
    (mv^2)/2 = qvb
    m mass
    v velocity
    q charge
    b magnetic field.
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