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Homework Help: Magnetic field, energy, and mass

  1. Feb 27, 2012 #1
    1. The problem statement, all variables and given/known data

    A 4.0 µC particle with a kinetic energy of 0.50 J enters a uniform magnetic field of magnitude 0.20 T. In the field, the particle moves in a circular path of radius 2.0 m. Find the mass of the particle.

    a. 2.2*10-12kg

    *b. 5.12*10-12kg

    c. 6.1*10-12kg

    d. 4.7*10-12kg

    e. 10.2*10-12kg

    2. Relevant equations

    E=1/2mv^2
    r=mv/qB


    3. The attempt at a solution

    .5=1/2mv^2
    1/m=v^2
    v=1/√m

    r=mv/qB
    qBr=m/√m (substituting from above)
    qBr=√m
    (qBr)^2=m
    m=(4x10^-6C*2m*.2T)^2
    m=2.56x10^-12

    The answer key says it it 5.12x10^-12...what am I doing wrong?
     
  2. jcsd
  3. Feb 27, 2012 #2

    gneill

    User Avatar

    Staff: Mentor

    Your calculation and result look okay. Could be the answer key is wrong.
     
  4. Feb 27, 2012 #3
    Thanks!
     
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