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Charge on a sphere

  1. Jan 29, 2013 #1
    1. The problem statement, all variables and given/known data

    see attachment for pic of diagram and question

    b) what is the magnitude of the charge
    c) what is the number of excess or deficit electrons?

    m=(2.6*10^-15)kg
    Δv=265.4V
    r=0.005m

    2. Relevant equations

    q=Ne
    where (q=charge), (e=electron), (N=# of excess/deficit electrons)

    ε=ΔV/r

    FE = qε

    3. The attempt at a solution

    FE = Fg
    qε=mg
    q(ΔV/r)=mg
    q=mgr/ΔV

    q=(2.6*10^-15)(9.8)(0.005)/265.4
    q=4.8*10^-19

    The thing is, I tried checking this number and I got some weird values:

    ε=kq/r^2
    ε=0.0001728

    FE=qε
    FE=8.2944*10^-23

    FE=Fg
    8.2944*10^-23 = mg
    8.2944*10^-23 = 2.548*10^-14

    as you can see... not equal at all.
    help?








    oh, and assuming B) is correct, here's the answer to C):

    q=Ne
    N=q/e
    N=(4.8*10^-19)/(1.6*10^-19)
    N=3

    3 deficit electrons, since charge = (+)
     

    Attached Files:

    Last edited: Jan 29, 2013
  2. jcsd
  3. Jan 29, 2013 #2

    mfb

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    Staff: Mentor

    I don't see any attachment.

    That is related to the electric field from the charged object itself, not the electric field from your setup. If you would work with units, you would see that it does not have the correct units.
     
  4. Jan 29, 2013 #3
    Ah.
    What are the units on each and can you tell me of a way to check my answer? I don't see any other method (using different equations) using the set of equations I have.
     
    Last edited: Jan 29, 2013
  5. Jan 29, 2013 #4

    mfb

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    Staff: Mentor

    There is no alternative way to check your answer, just the formula you used to calculate the value.
    Well, the nice result of "exactly" 3 electrons is an indirect way to check the result - a value of 2.5 would be weird.

    I think you should know/determine this yourself.
     
  6. Jan 29, 2013 #5
    well, i know this one is NC

    the other... uhh.. I'm guessing is V/m
    which I've never seen before (wasn't used in the course)
     
  7. Jan 29, 2013 #6

    mfb

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    Staff: Mentor

    V/m is the electric field strength.
    If you mean N/C: Ok, then the units do not show the problem, as N/C = NV/J = V/m
     
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