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Homework Help: Electric Forces and fields question

  1. Jan 5, 2012 #1
    Two charges are placed at an equal distance x from point P, as shown in the following diagram. Charge A is placed directly west of P and B is placed directly north of P.

    1)If A is given a charge of –Q and B is given a charge of +Q, what is the direction of the resulting electric field at point P?
    A) southeast
    B) northwest
    C) northeast
    D) southwest
    Incorrect. Your answer=B, Correct answer=D

    I need an explanation on how it's done, especially on how you get the angle which it travels and stuff. Thanks

    Attached Files:

  2. jcsd
  3. Jan 5, 2012 #2

    There is the attachment for the diagram. :)
  4. Jan 5, 2012 #3
    Don't forget the field lines are curved as shown by this diagram.


    Now just imagine that diagram rotated to suit your question
  5. Jan 5, 2012 #4
    There I rotated it for you to make it easier to see.

    Attached Files:

  6. Jan 5, 2012 #5
    Erm, I still don't see the connection? The + attracts the - charge, and i guess i have to use pythagorean theorem, but how do i calculate which direction it is deflected?

    I need a explanation, i totally suck at this topic..
  7. Jan 5, 2012 #6
  8. Jan 5, 2012 #7


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    Figure the electric field at P due to B and the electric field at P due to A and add them as vectors.
  9. Jan 5, 2012 #8
    Well it's a repellent force from the north and an attractive force from the west, ergo, the added force vector is toward the southwest.
  10. Jan 6, 2012 #9
    Erm, how do I go about the vector addition?

    I have lEl= kQ/x^2 for the field of B and lEl=k(-Q)/x^2 for field of A. and shouldnt it be southeast? Why do you say it's southwest?
  11. Jan 6, 2012 #10


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    You are just giving the magnitude of the field. Electric fields have a direction. Those two fields have the same magnitude |kQ/x^2|. What's the direction of each of the vectors?
  12. Jan 7, 2012 #11
    How do I figure out the direction? The diagram arrows point towards in both ways?
  13. Jan 7, 2012 #12


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    Forget the dipole diagrams !

    What is the direction of the Electric Field at P due to the -Q charge at point A ?

    What is the direction of the Electric Field at P due to the +Q charge at point B ?

    How do the magnitude of these two fields compare?

    Add the two fields vector-wise.
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