1. Not finding help here? Sign up for a free 30min 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!

Electric field

  1. Jun 21, 2003 #1
    I'm revising for my exam and got stuck in the following questions. Please help.

    1) An isolated spherical conductor of radius r is charged to a potential V. The total electrical energy stored is _______.
    This is an MC question and the answer is V2(2*pi*εo*r)
    I only know energy stored = Qq/(4*pi*εo*r) and V=Q/(4*pi*εo*r)

    2)A positively-charged metal sphere A of radius a is joined by a conducting wire to an uncharged metal sphere B of radius b placed far away from the first sphere. THe ratio of the surface charged density on sphere A to that on sphere B is ______.
    The answer is b/a.

    Well, I know surface area of A :surface area of B = a^2:b^2. How can I get the answer b/a ?

    Thanks in advance.
  2. jcsd
  3. Jun 22, 2003 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    In the first problem you have an incorrect formula: this is an "isolated" conductor so you do NOT have two charges, q and Q.
    Since you have only one charge, Q, you need to use
    energy stored = Q2/(4*pi*epsilon*r).

    You also know V=Q/(4*pi*epsilon*r) so Q= V/(4*pi*epsilon*r) and Q2= V2/(4*pi*epsilon*r)2. Substitute that into the first equation and then simplify.
  4. Jun 25, 2003 #3
    That means the answer isn't V^2(2*pi*eplison*r) but V^2(4*pi*eplison*r). I'm pretty sure I was given a wrong answer.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Electric field