1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    Homework Helper

    How did you get that value for q?

  4. Sep 24, 2010 #3
    yeh sorry i realized i divided by 1/k istead of multiplying by k/1 :S
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook