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Homework Help: Finding the electric field at a point midway between two charges

  1. Feb 26, 2009 #1
    Find the electric field at a point midway between the two charges of +30.0 x 10^-9 C and +60.0 x 10^-9 C separated by a distance of 30.0cm

    My work:

    E= Kq / r^2

    E= (9 x 10^9 NxM^2/C^2)(30.0 x 10^-9 C)/ (0.30m)^2

    i get 3000 N/C

    E= (9x10^9 NxM^2/C^2)(60 x 10^-9 C)/(0.30m)^2

    i get 6000 N/C

    i added the two together and got 9000N/C, but its telling me the answer is "12.0 x 10^3N/C toward 30 nC"

    i am confused on what i am doing wrong. please help
  2. jcsd
  3. Feb 26, 2009 #2


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    Homework Helper

    I believe your r value is wrong. The distance from a charge to the midpoint is NOT 0.30m. 0.30m is the distance from one charge to the other. Also, electric field is a vector quantity, so directions matter when you sum up contributions from multiple sources.
  4. Feb 26, 2009 #3
    I completely understand it now, thank you. =D
    Last edited: Feb 26, 2009
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