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Homework Help: Excess electrons or protons on drop.

  1. Jan 15, 2008 #1
    1. The problem statement, all variables and given/known data

    A small drop of water is suspended motionless in air by a uniform electric field that is directed upward and has a magnitude of 8450 N/C. The mass of the water drop is 3.80 10-9 kg.

    (b) How many excess electrons or protons reside on the drop?

    2. Relevant equations

    3. The attempt at a solution

    i'm not sure what i'm doing wrong, but i get about 4.1071 x 10^-12 C

    and for the drops i get 2.754 x 10-31

    not sure why its wrong?

    i used this for formula, see image: http://img180.imageshack.us/img180/6734/80350082pv3.png [Broken]
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Jan 16, 2008 #2


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    Staff Emeritus
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    Gold Member

    I'm not sure what you're asking here really. I presume the image is the answer to the question and you're wondering why yours is different. From the 2.754x10-31 it appears you've multiplied the charge on the drop by that on a proton.
  4. Jan 16, 2008 #3
    well the image has different values to be calculated, and the image was just another example to show what was going on,

    the problem i calculated was different, but it should still be calculated the same.

    and it should be: 2.75e+07 but not sure why???
  5. Jan 16, 2008 #4


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    Homework Helper

    I'm getting slightly different numbers than you, but are you sure you're dividing by the electron charge and not multiplying? That's the only way I can see you getting an answer with that magnitude.
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