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Homework Help: Point Charge and e-field

  1. May 22, 2010 #1
    1. The problem statement, all variables and given/known data
    Point charge q1= -5.00 is at the origin and point charge q2= +3.00 is on the -x axis at x= 3.00 cm. Point P is on the y-axis at y= 4.00 cm .

    Calculate the electric fields E1 and E2 at point P due to the charges q1 and q2. Express your results in terms of unit vectors.


    2. Relevant equations

    r = sqrt(x2 + y2)
    hat{r} = (vec{r} /r = x hat{i} + y hat{j}) / r
    vec{E} = (k * q)/r2 hat{r}


    3. The attempt at a solution
    E1 will be the vector from q1 to point P
    r = sqrt(02 + .042) = .04 m

    hat{r} = (0 hat{i} + .04 hat{j})/.04 = 0 hat{i} + 1 hat{j}

    E1 = (9*109) * (-5.00*10-9) / .042 (0 hat{i} + 1 hat{j})
    = 0 hat{i} - 2.81*104 hat{j}

    i did the exact same steps to find E2 and
    E2 = 6.47*102 hat{i} + 8.62*102 hat{j}

    Am I doing something wrong? According to the steps I used because my answers are wrong and i even followed the way the steps were in the book.
     
  2. jcsd
  3. May 22, 2010 #2

    Doc Al

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    Staff: Mentor

    Is q2 at x = +3 cm or at x = -3 cm?
     
  4. May 22, 2010 #3
    q2 is at x=+3 cm = 0.03m
     
  5. May 22, 2010 #4

    Doc Al

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    Staff: Mentor

    So how did you end up with a positive i-component?
     
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