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Homework Help: Full sphere enrgy

  1. Aug 7, 2009 #1
    1. The problem statement, all variables and given/known data


    the question is :
    what is the energy needed to build a full sphere, radii R, and with density a*r

    3. The attempt at a solution

    i did it this way, i assum that i have a little ball radii r, and i want to bring an intificimal sphere width dr, and use W=Kq1dq2\r

    q1= (a*4PIr^4)\3

    and i intigrate it from 0 to R, and my solution equals to (4a^2PIR^7)\21

    and it is not as the solution says....

    am i doing it right?

  2. jcsd
  3. Aug 7, 2009 #2


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    Science Advisor
    Homework Helper

    Hi en.yokhai! :smile:

    (have a pi: π and try using the X2 and X2 tags just above the Reply box :wink:)
    I don't think you can use 4/3 πr3 for q1

    the density varies, so you have to integrate (over spherical shells) to find the mass. :wink:
  4. Aug 7, 2009 #3


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    Staff: Mentor

    Sorry for being dense guys (my density varies somewhat too...). What is the force that this work is being done against? Electric field? Gravitational field? I don't quite get it from the problem as stated so far.
  5. Aug 8, 2009 #4
    electric field, so the mass is not important...the charge is,namely the density
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