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Resolving Vectors for Electric Charges

  1. Jan 14, 2010 #1
    1. The problem statement, all variables and given/known data
    Hi there, I am having alot of trouble with this problem. Any help much appriciated.

    Thanks inadvance.

    Two point charges, each of 4x10-6C are placed 1m away at points A and B respectively. Calculate the electric field strength at a point P, which is a distance of 1m from both A and B in the cases when:

    i) both charges are positive
    ii) one is positive and one is negative


    2. Relevant equations
    E=kQ/r2


    3. The attempt at a solution
    Ok well I know that I need to work out a resultant, however I am not sure whether to use sin or cos, and even then how to use them to get the correct answer!

    Thanks again for any input

    Mike
     
  2. jcsd
  3. Jan 14, 2010 #2
    When you separate a single vector into two perpendicular components, the three lines form a triangle, yes? Then you can easily apply your trigonometric relations easily with reference to the diagram. E sin theta would give the component opposite to the angle, while E cos theta gives the component adjacent to the angle used.
     
  4. Jan 14, 2010 #3
    Oh I see, so since the x components would cancel, you would just double the y component which is cos30*the elctric field strength calculated

    i.e. 6.28104NC-1

    Thanks alot, do you know if the answer I have calculated is correct?
     
  5. Jan 14, 2010 #4
    That's for part i) I presume?
    My answer appears to differ from yours from the 3rd significant digit; what value are you using for k?
     
  6. Jan 14, 2010 #5
    Hello, I used 8.99x109 for k.

    Thanks again!

    Mike
     
  7. Jan 14, 2010 #6
    Hmm...press your calculator again, you appear to have read some digits wrongly in the answer you provided above.
     
  8. Jan 14, 2010 #7
    You are quite right!

    I got 6.2284x104 this time!
     
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