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Compute lorentz force from charge

  1. Oct 22, 2011 #1
    1. The problem statement, all variables and given/known data

    A raindrop carrying a charge of 25x10-12 coulombs falls at a velocity of 5 m/s between the poles of a permanent magnet. It passes through a 50 mm long region in which the magnetic field density of 0.9 T is directed horizontally.

    1) determine the lorentz force of the raindrop

    Joined, diagram of how i interpreted the text

    2. Relevant equations

    F=q*(vector v x vector B)=qvb in magnitude

    3. The attempt at a solution

    1) by applying the formula I got F=25.10-2*5*0.9=1.125 N

    However I do not understand question 2. For me the horizontal velocity component is equal to zero (going down) so it doesn't make sense to me.

    Can someone explain it to me ? Thanks

    Attached Files:

  2. jcsd
  3. Oct 22, 2011 #2
    sorry F=1.125.10-10
  4. Oct 22, 2011 #3


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    Won't the force cause the drop to accelerate? What will happen to its velocity?
  5. Oct 23, 2011 #4
    the velocity will increase and change slightly direction i think. But even though there is not any horizontal component , right ? I really cannot see it...
  6. Oct 23, 2011 #5


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    Try drawing it from the point of view where the magnetic field points into or out of the page. Which way does the force point?
  7. Oct 23, 2011 #6

    I like Serena

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    Hi tifa8! :smile:

    The raindrop is accelerated horizontally by the Lorentz Force F.
    To find the horizontal acceleration you need to estimate the mass of the raindrop (F=ma).
    To find the horizontal component of the velocity you need to calculate the time the magnetic field exerts its influence (v=at).
  8. Oct 23, 2011 #7
    Oh ok thanks. Now I can visualize the problem. Thank you I like serena and vela :)
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