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Electric Force and Superposition..Help

  1. May 15, 2016 #1
    1. The problem statement, all variables and given/known data
    Two equal positive charges, both q = 2.0 × 10−6 C, interact with a third charge, Q = 4.0 × 10−6 C, as shown in the figure above. What is the magnitude of the electric force on Q? (kC = 8.99 × 109 N•m2/C2)
    Screen Shot 2016-05-15 at 5.22.23 PM.png



    2. Relevant equations
    I'm confused about assigning signs when I get to the part of finding the X and Y components of each vector towards Q
    I don't know if the values for the components I found are correctly signed... I believe are all positive



    3. The attempt at a solution
    I find that the magnitude of both vectors since they are the same is O.29N
    The X component of one vector is 0.23N and the Y component is 0.17N I think.
     
  2. jcsd
  3. May 15, 2016 #2

    gneill

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    Staff: Mentor

    It can help if you sketch in the force vectors on your diagram. Both of the 'q' charges will produce a force vector on the 'Q' charge. If you examine the vectors that you sketch then you should be able to tell from them the appropriate signs of their components.
     
  4. May 15, 2016 #3
    I did but according to my sketch all four would be positive. I'm doing something wrong I'm not sure what it is because one of them is supposed to be negative, one of the y components
     
  5. May 15, 2016 #4

    gneill

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    Staff: Mentor

    Can you post your diagram with the force vectors?
     
  6. May 15, 2016 #5
    this is what i drew IMG_0201.JPG
     
  7. May 15, 2016 #6

    gneill

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    Staff: Mentor

    Okay. So looking at your two force vectors I see one pointing into quadrant 1 and the other into quadrant 4. You should be able to assign the signs of the vector components just by looking at the diagram. Take a close look at the y-components...
     
  8. May 15, 2016 #7
    So it would be like this?
    Q2 going to Q both components would be positive
    Q1 going to Q the X component would be positive (cosine is positive in the 4th quadrant) and Y component would be negative( sine is negative in the 4th quadrant)
     
  9. May 15, 2016 #8

    gneill

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    Staff: Mentor

    Yup.
     
  10. May 15, 2016 #9
    Thank you!!
     
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