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Homework Help: Net Electric Field Between Two Point Charges

  1. Jan 24, 2015 #1
    1. The problem statement, all variables and given/known data

    Two point charges q1 = 4.90×10-5 C and q2 = 2.45×10-4 C are separated by a distance d = 0.10 m. Compute their net electric field E (x) as a function of x for the following positive and negative values of x, taking E to be positive when the vector E points to the right and negative when E points to the left.What is E(-0.100)?

    2. Relevant equations

    I tried the following equations, but I couldn't get any to work for any of the values of x:
    E(x<0)= -k [(Q1/x^2)+(Q2/(d-x)^2]
    E(0<x<d)= k [(Q1/x^2)-(Q2/(d-x)^2]
    E(d<x)= k [(Q1/x^2)+(Q2/(d-x)^2]

    3. The attempt at a solution

    E(-0.100)= -9e9 [(4.90e-5/-0.01)+(2.45e4/0.04)] = -1.1e7 N/C

    This is being marked incorrect and I don't know what I'm doing wrong. I've attempted multiple times with no luck.
  2. jcsd
  3. Jan 24, 2015 #2


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    2017 Award

    Does the exercise mention the origin of the x-coordinate ? Your equations place Q1 at x = 0 at Q2 at x = d.

    I expect that is OK. But then I don't understand the minus sign before the 0.01
    and I also don't understand the 2.45e4 if the charge is 2.45e-4 Coulomb !?

    One or more typing / calculator keying errors ?
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