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Homework Help: Coulombs law and electric fields

  1. Nov 19, 2007 #1
    Hey, just wanted to check my method.
    1)What is the distance between 2 protons experiencing a mutually repelling force of magnitude 4*10-11?

    so for this, we have to use the formula for coulombs law--F=kq1q2/r^2
    we know F and k. And q1=q2= 1.602*10-19. hence we can find r.

    2)Two point charges, 4*10-5 C and -1.8*10-5 C are placed 24 cm apart. What is the force on a third small charge of -2.5*10-6 C, if it is placed on the line joining the other two midway between the originally given pair of charges.

    For this what i did was:
    Find Fq3q1. The force for this will be in the left direction.
    then i found Fq3q2. Force for this will be in the right direction.

    Then i found the sum of the forces, and that is the force on q3.

    Note: q3 is in the middle of q1 and q2. distance between q3 and q1 is 0.12 m. the same is tru for the distance between q3 and q2.

    Sounds right?
  2. jcsd
  3. Nov 19, 2007 #2
    your Fq3q2 should be in your left direction, and what do you mean by sum of forces (vector sum or algebraic sum)?
  4. Nov 19, 2007 #3
    how will Fq3q2 will in the left direction. q3 is in between q1 and q2.so Fq3q1 is in the left direction and Fq3q2 is in the right direction.

    by sum of forces i mean the vector sum
  5. Nov 19, 2007 #4
    Unlike charges attract, Like charges repel, so ... (you got it?)
  6. Nov 19, 2007 #5
    ahh..k..got it..thnkz..one more question..three small, negatively charged spheres are located at the vertices of an equilateral triangle.The magnitudes of the charges are equal. Sketch the electical field in the region around this charge distribution, including the space inside the triangle.

    ok..so all i know is that density of field lines will be more whr the field is greater..
  7. Nov 19, 2007 #6
    ah.. too hard..
    I'll try and come back to you as soon as I get something of it.
    Last edited: Nov 19, 2007
  8. Nov 19, 2007 #7
  9. Nov 20, 2007 #8


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    Electric field lines will converge to these three negative charges from outside. They are indipendent of each other beacuse field lines repell each other. There will be no field lines in the space inside the triange because if you keep a positive charge any where inside the triangle it will be at rest due to Lami's theorem.
  10. Nov 20, 2007 #9
  11. Nov 20, 2007 #10
    umm..still unclear on where to place the positive test charge. I know that charge flows from postive to negative...
  12. Nov 20, 2007 #11
    help anyone..?
  13. Nov 20, 2007 #12
    The applet (in the link) shows the electric field lines in the area around the equilateral triangle (you have to make a triangle yourself, though). When you place a test charge anywhere on the plane, the arrow shows the direction in which the test charge will move. (hope this helps)
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