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Homework Help: Conductor question

  1. Feb 27, 2010 #1
    1. The problem statement, all variables and given/known data
    The potential of a metal sphere of diameter 2 cm is 10^4 V relative to the ground.
    a) What is the surface charge density?
    b) How many electrons were removed from the sphere?
    c) What is the electric field strength at the surface?

    2. Relevant equations

    3. The attempt at a solution
    a) so first I converted the diameter into the radius (0.01 m). Then I considered the formula
    Q (total charge)= 4 X pi X R^2 X sigma. The sigma is the surface charge density. So then since i have the electric potential and the radius, I replaced Q in the first equation with the electric potential. In this case, VR/k =Q where k=9.0X10^9...and when I solved it, I got 8.84X10^-6 C/m^2...but does this seem right because I have a gut feeling that it isn't...

    b) for the next question, i solved for Q (the total charge) and then converted the Joules into electrons by dividing the total charge by 1.602X10^-19 C...again I'm not sure if my method makes sense...

    c) for the last one, I used E= kq/r^2..and I basically plugged in the numbers but i'm not sure if placing 0.01 m for the radius is the right move....

    Any help would be appreciated!
  2. jcsd
  3. Feb 27, 2010 #2


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    Homework Helper

    It's right. The static charge that builds up on everyday objects is extremely small, on the order of millionths or billionths of a coloumb. Even a cloud-to-ground lightning strike delivers only 300 coloumbs.

    Well, you didn't convert the joules to electrons; you converted the coloumbs to electrons. Your method is right.

    Yup, that's how you do it.
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