Results 1 to 3 of 3

Math Help - Missing something

  1. #1
    Member
    Joined
    Nov 2005
    Posts
    111

    Missing something

    Hi I think I am missing something in the solution of this problem.
    I have the final solution of the problem but I get something a little bit different:

    A circular disk of uniform surface integral "RHOs" and radius a lies in the x-y plane with center at (0,0,0). Find the potential PHI(z) along the z axis when the potential is Vo is at the center of the disk, using the potential formula.

    the final answer is:

    PHI(z)=(RHOs/2*epsilon0)*[sqrt(a^2+z^2) -z-a]+Vo

    I found:
    PHI(z)=(RHOs/2*epsilon0)*[sqrt(a^2+z^2) -z]+Vo

    I dont know how they get the variable a in bold.
    Please can someone tell me what I get it wrong.
    Thank you.
    B
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Forum Admin topsquark's Avatar
    Joined
    Jan 2006
    From
    Wellsville, NY
    Posts
    9,939
    Thanks
    338
    Awards
    1
    Quote Originally Posted by braddy View Post
    Hi I think I am missing something in the solution of this problem.
    I have the final solution of the problem but I get something a little bit different:

    A circular disk of uniform surface integral "RHOs" and radius a lies in the x-y plane with center at (0,0,0). Find the potential PHI(z) along the z axis when the potential is Vo is at the center of the disk, using the potential formula.

    the final answer is:

    PHI(z)=(RHOs/2*epsilon0)*[sqrt(a^2+z^2) -z-a]+Vo

    I found:
    PHI(z)=(RHOs/2*epsilon0)*[sqrt(a^2+z^2) -z]+Vo

    I dont know how they get the variable a in bold.
    Please can someone tell me what I get it wrong.
    Thank you.
    B
    What the heck does "uniform surface integral RHOs" mean? The Greek letter rho indicates a (volume) charge density, which I suppose I could assume the subscript "s" is meant to indicate. But the problem you are doing appears to be simply for a constant surface charge density (RHOs in your notation). What's this business about uniform surface integrals?

    In any event, for a disk of uniform surface charge density, the electric potential for a point directly above the center of the disk is given by the formula that you derived, not what your given answer is. Now, by resetting the 0 point (ie. redefining the value of V0) we certainly CAN put it in the form that your given answer indicates (since your two solutions only differ by a constant), but I see no value in doing that.

    That's a long winded way of saying I agree with your answer.

    -Dan
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Member
    Joined
    Nov 2005
    Posts
    111
    You assume right about the rho thing!!
    Ok i will stick with my answer. I have some questions though. I will post them in a new post.
    B
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. I'm missing something
    Posted in the Statistics Forum
    Replies: 7
    Last Post: January 28th 2010, 05:45 AM
  2. Another p-value-am I missing anything?
    Posted in the Advanced Statistics Forum
    Replies: 2
    Last Post: December 14th 2009, 09:41 PM
  3. What am i missing here?
    Posted in the Calculus Forum
    Replies: 3
    Last Post: December 5th 2009, 05:57 AM
  4. What am I missing
    Posted in the Algebra Forum
    Replies: 4
    Last Post: August 28th 2008, 07:30 AM
  5. What am I missing here?
    Posted in the Algebra Forum
    Replies: 3
    Last Post: March 18th 2007, 03:57 PM

Search Tags


/mathhelpforum @mathhelpforum