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Homework Help: Electric Field Strength problem.

  1. Dec 13, 2011 #1
    1. The problem statement, all variables and given/known data
    2 point charges are situated 45nm apart in a vacuum. Charge A is +8μC and charge B is -25μC. X is a point 8nm from A.

    Calculate the electric field strength at point X only due to A (Ans is 1.2 x 10^9 N/C)
    Calculate the overall electric field strength at point X due to A and B. (Ans is 1.32 x 10^9 N/C)

    2. Relevant equations
    E = [itex]\frac{1}{4\pi\epsilon}[/itex] X [itex]\frac{Q}{d^{2}}[/itex]

    3. The attempt at a solution
    Electric field strength due to A.
    [itex]\frac{1}{4\pi(8.9X10^-12)}[/itex] X [itex]\frac{8X10^-6}{(8X10^-9)^{2}}[/itex]

    = 1.117X10^21
    = 1.12X10^21 N/C

    Electric Field Strength due to B.
    E = [itex]\frac{1}{4\pi(8.9X10^-12)}[/itex] X [itex]\frac{25X10^-6}{(37X10^9)^{2}}[/itex]

    = 1.632X10^20
    = 1.63X10^20 N/C

    1.12X10^21 + 1.63X0^20 = 1.283X10^21 N/C

    Where am I going wrong?
    I would appreciate any help.
  2. jcsd
  3. Dec 13, 2011 #2


    User Avatar

    Staff: Mentor

    It doesn't look like you've done anything wrong. The "answers" given seem to be well out of whack in terms of order of magnitude.
  4. Dec 13, 2011 #3
    I don't see anything wrong with a and b either but the combined field is a vector sum and one of the charges is negative and the other, positive.
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