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Homework Help: Magnetic field

  1. Jun 8, 2010 #1
    1. The problem statement, all variables and given/known data

    An ion of charge 1.6 x 10^(-19) C and Kinetic energy 2.0k eV enters perpendicularly into a uniform magnetic field .The ion performs a circular path of radius 4.3 cm .Determine the mass of the ion if the magnetic field intensity is 0.5 T.

    2. Relevant equations



    3. The attempt at a solution

    1/2 mv^2 = 2.0 x 10^3

    mv^2 = 4.0 x 10^3 ---1

    (mv^2)/r=Bev

    mv = (0.5)(1.6 x 10^(-19))(0.043)

    v = [3.44 x 10^(-21) ]/m ---2

    Sub 2 into 1 ,

    m{[3.44 x 10^(-21) ]/m }=4.0 x 10^3

    Solve for m , i got a very small figure which is wrong .

    THe correct answer is 1.84 x 10^(-26) kg

    where did i go wrong ?
     
  2. jcsd
  3. Jun 8, 2010 #2

    tiny-tim

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    hi thereddevils! :smile:

    don't you have to convert electron-volts into something else?
     
  4. Jun 9, 2010 #3
    hi tiny tim ,

    what do i hv to convert that too ? I thought electron volt is joules which means enerygy ?
     
  5. Jun 9, 2010 #4

    tiny-tim

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    hi thereddevils! :smile:

    no, a joule is a coulomb volt, not an electron volt …

    it's the energy if you move something with a coulomb of charge through one volt …

    an electron volt is the energy if you move something with the charge of an electron through one volt …

    joules volts coulombs and so on are SI units, but the electron volt isn't (se the PF Library on electric units ) …

    an electron has 1.602 10-19 coulombs of charge,

    so 1 eV = 1.602 10-19 joules :wink:
     
  6. Jun 9, 2010 #5
    thanks !
     
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