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Homework Help: Electric potential

  1. Sep 24, 2010 #1
    1. The problem statement, all variables and given/known data

    What are (a) the charge (in C) and (b) the charge density on the surface of a conducting sphere of radius 0.21 m whose potential is 290 V (with V = 0 at infinity)?

    2. Relevant equations

    v = kq/r
    [tex]\sigma[/tex] = q/SA
    SA of a sphere = 4[tex]\pi[/tex]r^2

    3. The attempt at a solution
    so this is what i did
    i used the first equation and solved for q = Vr/k
    i plugged in the numbers
    v= 290, r is the same as the radius because its only the surface r = 0.21,
    k = 1/(4[tex]\pi\epsilon[/tex])
    I got 5.5e11
    the right answer is 6.8E-9

    Then for the charge density,
    i put the numbers in too
    but this is also wrong for sure because it uses my q value
    I got 9.9e11
    the right answer is 1.2e-8

    I don't know how i got that far off :frown:
     
  2. jcsd
  3. Sep 24, 2010 #2

    ehild

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    Homework Helper

    How did you get that value for q?

    ehild
     
  4. Sep 24, 2010 #3
    yeh sorry i realized i divided by 1/k istead of multiplying by k/1 :S
     
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