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Finding the electric field at 3 points due to 2 identical charges.

  1. Jan 22, 2012 #1
    3 points (a,b and c) and two identical positive charges. How do you find MAGNITUDE of the electric field at the 3 points? I just need to rank them, don't need numbers.

    all points are on the x-axis:
    A (x=0); charge +Q(x=1); B (x=3); c(x=4); charge+Q(x=5)

    (Picture attached in the 3rd post)

    2. Relevant equations

    E= (1/4pie) (q/r^2)

    3. The attempt at a solution
    I just focused on the r^2 since everything in the formula is the same.
    for point A: 1^2+ 5^2 = 26
    Point B: 2^2+2^2 = 8
    Point C: 1^2 + 3+2 = 10

    so would B then have the greatest strength while A has the least?
     
    Last edited: Jan 22, 2012
  2. jcsd
  3. Jan 22, 2012 #2

    SammyS

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    r2 is in the denominator. You need to add or subtract 1/12 and 1/52, 1/22 and 1/22, etc.

    Also you haven't taken into account the direction of the E field from each charge.
     
  4. Jan 22, 2012 #3
    Picture attached
     

    Attached Files:

  5. Jan 22, 2012 #4
    Yes, r^2 is the denominator, that's why the smallest r^2 will have the strongest field. Can you please explain what you mean I need to add or subtract?
     
  6. Jan 22, 2012 #5
    In that case, would there be no electric field strength at B? since the directions are opposite, so they cancel each other out?
     
  7. Jan 22, 2012 #6

    SammyS

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    Yes.

    As for using 1/r, not r2:
    1/1+1/52 = 25/25 + 1/25 = 26/25

    1/22 + 1/22 = 1/4 + 1/4 = 1/2   (Yes, I know these should be subtracted, but this is just to illustrate the point.)​
     
  8. Jan 22, 2012 #7
    In this case, after doing the calculations, would a have the smallest electric field strength since it is -26/25, and then after that would be B with 0 and finally, C?
     
    Last edited: Jan 22, 2012
  9. Jan 22, 2012 #8
    would you just assume the left is negative while if arrows pointing to the right is positive?
    Thanks
     
  10. Jan 22, 2012 #9

    SammyS

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    What do you get for electric field at point C ?

    Look at the problem statement. It asks about the "MAGNITUDE of the electric field". (That capitalization was in your original post.)

    As with number lines, it's fairly standard to consider a vector pointing to the right to be positive, and left is negative.
     
  11. Jan 22, 2012 #10
    Oh !!! thank you so much I finally got it. thank you for your patience. appreciated!
     
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