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Homework Help: Charge on a particle to balance its weight

  1. Feb 9, 2013 #1
    1. What must the charge of a 1.45g particle be for it to remain stationary when placed in a downward-directed electric field of magnitude 650N/C?

    2. Relevant equations
    [tex]E = \frac{F}{q}[/tex]

    3. The attempt at a solution

    So the field is pointing downwards. E fields point in the direction a positive charge would take, so the charge must be negative to stay balanced. Gravity is also pointing downwards.

    So I take the above formula, and I get F = Eq = ma
    [tex]q = \frac{(1.45*10^{-3})*g}{650} = 1.488*10^{-16} C[/tex]
    (Using g = [itex]6.67*10^{-11}[/itex]) However the answer given is [itex]-21.9\mu C[/itex]

    Not sure what I'm doing wrong/missing here?
  2. jcsd
  3. Feb 9, 2013 #2


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    Homework Helper

    hey man welcome to physicsforums :)
    Why are you using g=6.67*10^-11 ?

    Edit: or, what units are these?
  4. Feb 9, 2013 #3
    I'm trying to balance the weight of the particle (mg) with the force from the electric field (Eq).

    Thanks for the welcome ;D

    Edit: the original particle is 1.45grams, so I use the [itex]10^{-3}[/itex] to convert that to kg. E is in N/C, and g, well I don't know :P Whatever the units of the gravitational constant are
  5. Feb 9, 2013 #4
    Ohhhhh. I see what I've done! Doing dimensional analysis shows I have the wrong units; thanks for that.

    I have to use 9.81 instead of the gravitational constant -_-

    Thanks :P

    Edit: Yeah, thanks ap123 :P I certainly won't make THAT mistake again ;D
  6. Feb 9, 2013 #5
    g is the acceleration due to gravity, not the gravitational constant, ie g should be 9.80m/s2

    Edit: OK - you've got it :)
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