Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Find the direction and the magnitude of the total electric field

  1. Jul 21, 2010 #1
    1. The problem statement, all variables and given/known data

    1. Two point-charges of charges +q and +q are held at the corners of an isosceles rectangle
    triangle, as shown in figure below. The absolute value of q is 1.414 μC. The distance d as shown in the figure is 0.5 m. The angle at A is 90. The gravitational forces are negligible.

    a) Find the direction and the magnitude of the total electric field at
    the apex A. (Use the coordinate system shown in figure by x and y.)

    I don't have the drawing up but if you can imagine a triangle with the top angle equaling 90 degrees and the two bottom angles having charges of +q. A line descending from point A along the y axis dissects the bottom portion of the triangle, divides it in, each side being distance d.

    I know, using Summation that Etotal = E1 + E2 and that E1 and E2 are the same because they have the same charge.

    I know the equation for the magnitude of an electric field is E=k(q/r^2)

    To find r I used the equation .5/sin 45 = x/sin 90, ending with an r value of .707m

    So I know that E = 9x10^9 ((1.414 x 10^-6)/(.707^2))

    Using that equation I end with a value of 25,459V/m for E1. Using my previous equation, I find that Etotal = 25,459 + 25,459 = 50,918 v/m.

    That is incorrect, can someone tell me where I slipped up? Thanks.
  2. jcsd
  3. Jul 21, 2010 #2


    User Avatar

    Staff: Mentor

    First, "isosceles rectangle triangle" is a funny typo. Second, I really did try, but without a figure, your text is too hard for me to follow and check your math. Can you please post a figure so we can check your vector math?
  4. Jul 21, 2010 #3
  5. Jul 21, 2010 #4


    User Avatar

    Staff: Mentor

    That's a big help. Now show us your calc for the summation of the y components of the E field. The x components cancel obviously.
  6. Jul 21, 2010 #5
    Oh wow, I totally ignored the Y component. I realized the X would cancel out.

    Assuming that my other calculation of E = 9x10^9 ((1.414 x 10^-6)/(.707^2)) is correct, then E1 and E2 would be cos45(25,459 + 25,459) or 36004 V/m

    Does that look correct?

  7. Jul 21, 2010 #6
    I have to go to bed soon, but if anyone can help me out, I'd be very grateful!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook