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Electric field=0 for 2 charges

  1. Oct 24, 2009 #1
    1. The problem statement, all variables and given/known data
    a 6uC + charge is at (0,0) and a 1uC + charge is at (0,1m). Where between them is the electric field equal to 0?


    2. Relevant equations
    F=qE
    F=K(q_1)(q_2)/r^2


    3. The attempt at a solution
    F=K(q_1)(q_2)/r^2
    F=(9.0*10^9)(1*10^-6)(6*10^-6)/(1^2)
    F=5.4*10^-5 (The force each exerts upon eachother)

    F=qE
    Since E=0, F=0

    F=0 at:
    0=K(q_1)(q_2)/r^2
    Find r

    0=(9.0*10^9)(1*10^-6)(6*10^-6)/r^2

    And this is where we have a problem
     
  2. jcsd
  3. Oct 24, 2009 #2

    Doc Al

    User Avatar

    Staff: Mentor

    You want to study the field from the two charges at various points, not the force between the two charges. Write an expression for the field at point X from each charge. Add to get the total field. (Careful with signs.)
     
  4. Oct 24, 2009 #3
    Electric field at distance r for each charge
    E=Kq/r^2

    E_1=(9.0*10^9)(6*10^-9)/r^2
    E_1 = 54/r^2

    E_2=(9.0*10^9)(1*10^-9)/r^2
    E_2 = 9/r^2

    E_net = E_1+E_2 = 54/r^2 + 9/r^2
    E_net = (54+9)(1/r^2)
    E_net = 63/r^2
    0 = 63/r^2
     
  5. Oct 24, 2009 #4

    Doc Al

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    Staff: Mentor

    If a point is a distance X from charge #1, how far is it from charge #2?

    Instead of calling the distance r, call it X. Don't plug in numbers right away. (And μC = 10^-6, not 10^-9.)

    What direction is the field?

    Instead of r, write the distance in terms of X and the distance between the charges.

    What direction is the field?

    Note that different directions will get different signs.
     
  6. Oct 24, 2009 #5
    Thanks, figured it out.
     
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