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Homework Help: Trying to find electric field

  1. Feb 18, 2013 #1
    The Problem:
    The electric potential in a certain region is
    V = αx2y2+βz2(x2-γ)+δy3z
    where α = 5 V/m4, β = 4.5 V/m4, γ =6.8 m2, and δ = 7.1 V/m4.
    What is the y component of the electric field Ey at (1.3 m, −3 m, 4.4 m)?
    Answer in units of V/m

    My Attempt:

    I took the partial derivative with respect to y and got
    Ey = 10x2y + 4.5z2(x2-6.8) + 21.3y2z

    (But I think I may have done that wrong....)

    I then plugged in my values for x, y, and z, and got 347.5968 as my answer, which was wrong. Any help would be appreciated! :)
  2. jcsd
  3. Feb 18, 2013 #2


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    Can you show how you got your middle term with the factor of 4.5? The middle term of V doesn't seem to depend on y.

    Also, the components of E are not given by the partial derivatives of V (but close!)
  4. Feb 18, 2013 #3
    Thanks for the response,
    I knew I must have messed something up in that derivative. So it should be Ey = 10x2y+21.3y2z.

    But if not the partial derivatives, then I am confused as to what I am supposed to be looking for.
  5. Feb 18, 2013 #4


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    See here and note if there's something you left out.
  6. Feb 18, 2013 #5
    Ooooh I see. Got it! Thanks so much!
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