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Electric Potential, spherical shell: Multiple Choice

  1. Sep 11, 2009 #1
    A non-conducting, spherical shell centered at the origin has radius r1 and total charge -Q, where Q is a positive value. (DO NOT ASSUME THE POTENTIAL AT INFINTIY IS ZERO!)

    http://i662.photobucket.com/albums/uu347/TwinGemini14/ELECpotential.gif


    1) The electric potential just outside the surface of the shell is __________ the electric potential infinitely far from the shell.

    A) higher than
    B) lower than
    C) equal to

    ------
    This problem stumped me. I figured that the electric potential outside the sphere was...
    V = -Q/(4*pi*EpsilonNot*r)

    So as r grows, the potential actually gets higher.
    So the answer is B.
    -----
    2) If a negative charge outside the shell is moved closer to the shell, the potential energy of the system will __________. If a positive charge outside the shell is moved closer to the shell, the potential energy of the system will __________.

    A) decrease, increase
    B) increase, stay the same
    C) increase, decrease

    -----
    Since the force and displacement vectors are acting in opposite directions, the total work decreases. PE = -W, so then that means that the potential energy increases. The opposite is true with the positive charge.

    So the answer is C.
    --------------------
    Can somebody please confirm these answers? I would like to know if I messed up some logic along the way. Thanks in advance!
     
  2. jcsd
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