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Neutral Point between point charges

  1. Dec 26, 2014 #1
    1. The problem statement, all variables and given/known data
    How can I find the nuetral point between two charges or more ? Example:

    (-Q3)................(0.8m)........................(Q2).......(0.3m)......(P).........(0.3m)...........(Q1)
    Q1=2nC ,,, Q2=3nC,,, Q3=-4C ,,,
    q: Determine in which direction must be Q3 moved to make P neutral? Calculate the distance which P will be moved.


    2. Relevant equations
    E=K q / r^2

    3. The attempt at a solution
    what i did i found E of q1 which is 200N/C ,, and q2 = 300N/C
    And then stuck....
     
  2. jcsd
  3. Dec 26, 2014 #2

    gneill

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    Staff: Mentor

    Electric fields are vector quantities, having both magnitude and direction. So what directions do you assign to the two magnitudes that you've calculated? What's their sum?
     
  4. Dec 26, 2014 #3
    Their sum is 100N/C since they are repulsive , No idea about direction XD
     
  5. Dec 26, 2014 #4

    gneill

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    See if you can find a picture of the electric field surrounding a positive charge, and for a negative charge (your text or the web should have this). What direction are the arrows representing the fields directed in each case?
     
  6. Dec 26, 2014 #5
    No directions are shown !
     

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  7. Dec 26, 2014 #6

    gneill

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    That's just the problem statement. You're expected to know the directions to assign to the fields produced by positive and negative charges. See if you can find another image showing the fields surrounding charges. I found several in a few seconds by googling "electric field".
     
  8. Dec 26, 2014 #7
    ahh i thought you were meaning from the question itself ... XD
    definitely for positive its outwards , while negative its inwards
     
  9. Dec 26, 2014 #8

    gneill

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    Okay! So now you can assign a direction to the sum of the fields that you calculated earlier, right?
     
  10. Dec 26, 2014 #9
    well , this is the point im not getting and need help
    What i know is if there's repulsion we subtract , but if there's attraction we add
     
  11. Dec 26, 2014 #10

    gneill

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    At point P sketch in the arrows representing the field direction from the two positive charges (along the line that all the charges lie). Suppose you choose the direction to the right to be the +x-axis direction. Now, those arrows (vectors) have the magnitudes that you calculated previously. What's the resultant of adding them?
     
  12. Dec 30, 2014 #11
    Respecting to P , the direction to right is positive , so E2 - E1 - E3 = 0 ,,, that means E3=E2-E1 which results into 100N/C ...
    Now I got the point ! .. So whenever I have two or more charges in a straight line i should make the same concept ?
    Thanks a lot
     
  13. Dec 30, 2014 #12

    gneill

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    Yes, that's the idea. You set up coordinate axes and determine both the magnitude and directions of the fields due to each charge before you start adding or subtracting them as the situation warrants. You will find that the same concept applies even if the charges are not along a straight line, in which case you'll deal will all the components of the vectors (time to review your trigonometry!).
     
  14. Jan 1, 2015 #13
    Alright , Thanks A Lot ! :D
     
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