1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Electrostatic force using vectors

  1. Sep 29, 2012 #1
    1. The problem statement, all variables and given/known data

    Charge A is +2C and is located at <4,0,0>
    Charge B is +12.5C and is located at <0,-3,0>

    What is the the Force(AonB)
    What is the magnitude of F(AonB)

    2. Relevant equations

    F = (k)(Qa)(Qb)/(r^2) (r^)

    For reference the answers are

    < -0.8k, 0.6k, 0 > N (F vector)

    and

    1k N (magnitude)


    3. The attempt at a solution

    So I am confused as to how to solve this problem. I don't know if maybe it is worded strangely or what but I am not getting those answers.

    I did a lot of these problems but not with vectors. How do you apply the answer you get into a vector. The equation is given as r(hat) after the regular F equation. Does this mean I distribute my answer into i+j+k? I am assuming the answer to that is no because that makes no sense mathematically.

    This is not a homework problem it is a practice problem. I am trying to learn physics and would really appreciate if someone could explain how you apply your F into vector notation and how this particular problem can be solved so I can learn. I am sure it is something simple that I am just not getting and my teacher didn't explain properly (in class we never did any with vectors).
     
  2. jcsd
  3. Sep 29, 2012 #2

    ehild

    User Avatar
    Homework Helper
    Gold Member

    Yes, you need to write up the force in x,y,z components F=Fxi+Fyj+Fzk, or <Fx,Fy,Fz>

    The vector "r^" is the unit vector pointing from A to B when you want the force exerted by charge at A to the charge at B. The vector pointing to B from A is the difference of the position vectors r=rB-rA=<0,-3,0>-<4,0,0> (or rB-rA=-3j-4i). You need to divide this vector with its magnitude to get the unit vector r^. "r" means the magnitude of the vector r.



    ehild
     
  4. Sep 29, 2012 #3
    Ah that makes a lot more sense now. So the reason I am finding the unit vector is I need a vector that is pointing in the same direction as the Force caused by these two charges.

    Whenever I see r hat from now on that is referring to the unit vector, correct?

    Problem worked out perfectly after that. I really appreciate the help.

    Also just for clarification. If the question had asked for the F (BonA) or F_BA it then the difference would be the unit vector being composed from A - B or <4,-3,0> and of course the magnitude would be the same (5).

    So the F_BA would have been <4/5 ke, -3/5 ke, 0>



    Could you explain why that is the case? As in why would two positive charges cause one to go in the positive X (AonB) and the other in the negative X (BonA) direction? And why are would both have the same Y component? I just want to get a better understanding of what I am doing. I hate to memorize formulas and plug things in I would rather understand the concept.

    Once again thank you very much for the explanation I truly appreciate it.
     
    Last edited: Sep 29, 2012
  5. Sep 29, 2012 #4

    jtbell

    User Avatar

    Staff: Mentor

    Correct.

    Check your arithmetic. :smile:

    As a reality check, remember Newton's Third Law of Motion. The force that B exerts on A must be equal in magnitude and opposite in direction to the force that A exerts on B.
     
  6. Sep 29, 2012 #5
    Ah you're right. Thank you it all makes sense now! I really appreciate both of your help.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Electrostatic force using vectors
  1. Electrostatic Forces (Replies: 1)

Loading...