Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Electrostatic Problem

  1. Aug 24, 2004 #1
    Point charges q1 and q2 are placed in space, with q1 at the origin and q2 a distance r from q1 making a 45 degree angle with the horizontal.
    a) Find the force using unit vectors i and j from q1 to q2
    b) " " from q2 to q1
    c) If q1=q2, what is the magnitude of the force?

    so far i have:

    q2:
    Fx = F cos (theta)
    Fy = F sin (theta) - so F(1on2) = F cos(theta) i + F sin (theta) j

    am I on the right track? and would F(2on1) be -F(1on2)?
     

    Attached Files:

    Last edited: Aug 24, 2004
  2. jcsd
  3. Aug 24, 2004 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Why use F to represent the force? Use Coulomb's law and write "F" in terms of q1, q2, and r. Remember: signs matter.

    Yes.
     
  4. Aug 24, 2004 #3
    so for 1 on 2:
    Fx = k(q1)(q2)cos(theta)/r^2 i
    Fy = k(q1)(q2)sin(theta)/r^2 j
    2 on 1
    Fx = -k(q1)(q2)cos(theta)/r^2 i
    Fy = -k(q1)(q2)sin(theta)/r^2 j

    Part (c) when q1=q2=5 x 10^-6 C and r = 2.0 m

    sub values into:
    F = k (q1)(q2)/r^2 or F = kq^2/r^2

    am i correct?
     
  5. Aug 24, 2004 #4

    Doc Al

    User Avatar

    Staff: Mentor

    You are right!

    You are correct!
    Sounds good to me.

    Edit: I messed up the signs before! You are correct. :smile:
     
    Last edited: Aug 24, 2004
  6. Aug 24, 2004 #5
    wouldn't the force of q1 on q2 be positive on the coordinate system used?
     
  7. Aug 24, 2004 #6
    ok, assuming they attract.
    got it.
     
  8. Aug 24, 2004 #7

    Doc Al

    User Avatar

    Staff: Mentor

    Right. I messed up the signs before. (Funny... I was telling you to be careful of signs and I goofed up! :blush: )

    Good work!
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook