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Homework Help: Magnetic Fields and electron

  1. Feb 15, 2007 #1
    1. The problem statement, all variables and given/known data
    An electron is moving through a magnetic field whose magnitude is 9.10x10^-4 T. The electron experiences only a magnetic force and has an acceleration of magnitude 3.60x10^14 m/s^2. At a certain instant, it has a speed of 6.30x10^6 m/s. Determine the angle (less than 90°) between the electron's velocity and the magnetic field.

    2. Relevant equations
    Magnetic Force
    F = qvBsin(theta)


    3. The attempt at a solution

    F=ma --> mass of electron x acceleration = 3.27x10^-16N

    F = qvBsin(theta)
    3.27x10^-16N / [(charge of electron) x (6.30x10^6 m/s) x (9.10x10^-4 T)] = sin (theta)

    solving like this would make sin theta = 3.56 x 10^11, which is not reasonable at all....What am i doing wrong?
  2. jcsd
  3. Feb 15, 2007 #2


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    Gold Member

    I didnt punch in the numbers, but you need to solve for theta not sine theta (assuming the last step you did was the finial step). Example in a right triangle, arcsine(opp/hyp) = theta.
  4. Feb 15, 2007 #3
    i understand what you are saying, however when i use the numbers i have calculated in the arcsine (opp/hyp) = theta equation, i am getting numbers that dont correspond to reasonable angles. This leads me to believe i am screwing up something in the F=ma or F=qvBsin(theta) department. in other words arcsine(F/qvB) should equal theta...right?
    Last edited: Feb 15, 2007
  5. Feb 15, 2007 #4


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    Science Advisor
    Homework Helper

    If it's any encouragement, I plugged your numbers in and got a perfectly reasonable value of sin(theta). Your theory is fine, your plugging is a problem.
  6. Feb 15, 2007 #5
    ok, i'll check it out again
  7. Feb 15, 2007 #6
    amazing...i finally got it! what in the world was i doing to get such wacky #'s. Thanks guys.
    i just realized i was putting in the wrong exponent for the charge of the electron. DOH!
    Last edited: Feb 15, 2007
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