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Homework Help: Find the flux through the equilateral triangle with corners at

  1. Sep 12, 2014 #1
    1. The problem statement, all variables and given/known data
    Find the flux through the equilateral triangle with corners at the points (1m,0,0), (0,1m,0), and (0,0,1m) in x,y,z space (measured in meters) for an electric field with magnitude E=6N/C pointing
    • (a) in the z direction,
    • (b) parallel to the line y = x.

    2. Relevant equations
    Flux = EA;
    Surface area of an equilateral triangle = (sqrt(3)/4)(a^2), where a = side of the triangle
    Distance formula = sqrt(((x2-x1)^2) + (y2-y1)^2)

    3. The attempt at a solution

    I approached this using the equation:

    Flux = EA, where "E" is 6 N/C.

    I tried solving for A by using the formula for the equilateral triangle area: ((sqrt(3)*s^2)/4), where s
    is the side of the triangle. I solved for the side using the distance formula.

    Now, with all things in consideration, I plugged everything in:

    A=((sqrt(3)*(sqrt(2))^2)/4); s=sqrt(2);

    A=(sqrt(3)/2) or .867

    Flux = 3sqrt(3) or 5.1962

    Now, I put this answer for question (a), and still got the wrong answer. What am I doing incorrectly?

    Thank you kindly for the help.

  2. jcsd
  3. Sep 12, 2014 #2


    User Avatar
    Homework Helper
    Gold Member
    2017 Award

    Hello, and welcome to PF!

    This equation is valid only if the electric field is oriented perpendicular to the area surface. You should have covered how this equation is modified to handle other cases.
  4. Sep 12, 2014 #3
    Find the area vector of the triangle. Remember that it should be normal to the triangle's surface and have magnitude equal to the triangle's area.
  5. Sep 13, 2014 #4
    Go outside on a sunny day and hold your text book out and look down at its shadow. How can you orientate the book to maximize the area of the shadow? To minimize it?
    Can you think of a mathematical relation that will allow you to calculate the area of the shadow given the orientation of the book wrt the sun?
    How does the area of the shadow relate to the flux through a surface as in your problem?
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