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Homework Help: Eletrostatics--electric field

  1. Apr 9, 2015 #1
    1. The problem statement, all variables and given/known data

    A spherically symmetric charge distribution has net positive charge Q distributed within a radius of R.
    Its electric potential V as a function of the distance r from the center of the sphere is given by the following.
    [tex]V(r)=\frac{kQ}{R}( -2+3{\frac{r^2}{R^2}})[/tex]for r<R
    [tex]V(r)=\frac{kQ}{R}[/tex] for r>R
    https://www.physicsforums.com/file:///page5image8000 [Broken] https://www.physicsforums.com/file:///page5image8160 [Broken]

    https://www.physicsforums.com/file:///page5image9232 [Broken]Express all algebraic answers in terms of the given quantities and fundamental constants.

    1. (a) For the following regions, indicate the direction of the electric field E(r) and derive an expression for its magnitude.

      i. r < R____ Radially inward ____ Radially outward

      ii. r > R____ Radially inward ____ Radially outward

      The answer to (i), i.e., r<R is radially inward.
    2. Relevant equations

    3. The attempt at a solution
    How can the field due to a positive charge be radially inward?
    For (ii), it's radially outward, which is fairly straightforward, because field lines will originate radially from the sphere, but inside, INWARD??
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Apr 9, 2015 #2


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    Staff: Mentor

    The problem states that the distribution has a NET positive charge. It doesn't say that it is positive throughout. Look at the given potential function...
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