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!

Charged balls and force problem

  1. Jan 25, 2007 #1
    1. The problem statement, all variables and given/known data
    Three balls, with charges of +4q, -2q, -q, are equally spaced along a line. The spacing between the balls is r. We can arrange the balls in three different ways: (1) +4q, -2q, -q; (2) -2q, +4q, -q; (3) +4q, -q, -2q.
    (a) Which ball experiences the largest magnitude net force in each of the three cases?
    (b) Express your answers below in terms of k, Q, and r. Calculate the magnitude and direction of the force applied to the ball that has the -q charge in each of the three cases.
    (c) In which case does the ball of charge +4q experience the largest force?


    2. Relevant equations
    k=9x10^9
    magnitude of the force between charges: F=k(q1)(q2)/(r)^2 and F=qE
    magnitude of the electric field: E=kq/r^2


    3. The attempt at a solution

    (a) Case 1: The -2q charge has the largest force because the -q charge experiences a large force to the right and a small to the left, the -2q charge experiences a large force to the right and a large force to the right, and the +4q charge has a large force to the right and a small force to the right.
    Case 2: The -q charge and the -2q charge have equal magnitude but opposite direction net forces.
    Case 3: The -q charge has the largest force because the -2q charge has a large force to the left and a small force to the right and the +4q has a large force to the left and a small force to the left.
    I'm just learning how to do this so I'm not sure if these are correct. Please let me know so I can move on to part (b). Thanks!
     
  2. jcsd
  3. Jan 27, 2007 #2
    Can anyone tell me if this is correct? I posted a few days ago but no one has responded.
     
  4. Jan 27, 2007 #3
    the first 2 cases are correct. The third one isn't.

    The -q feels a repulsive force of magnitude 2 in the -x direction due to the -2q , and a attractive force of magnitude 4 in the -x direction due to the +4q: total magnitude = 6 in -ve x direction.

    The +4q feels a an attractive force of magnitude 4 in the +x direction due to the -q charge and an attractive force of 2 (2*4/(2)^2=2) in the +ve x direction due to the -2q charge. Total magnitude 4+2=6.

    Hence the +4q and -q feel the same magnitude of force, in opposite directions.

    (Note, these aren't the absolute magnitudes, just the relative magnitudes.)
     
  5. Jan 28, 2007 #4
    Ok thanks
    So for part (c), the ball with the +4q charge would experience the largest force in Case 1 because of the large attraction to -2q and the smaller attraction to -q.
    Now for part (b) I know I use the equation F=k(q1)(q2)/r^2, so do i do it for each of the surrounding balls and then add them together to get the net force? Thanks!
     
  6. Jan 28, 2007 #5
    That is correct
     
  7. Jan 28, 2007 #6
    yess thank you

    I think I also figured out part (b) but it is a lot to write out and I don't know how to make the formulas look nice so I'm not sure if I should even try to write them down so if you don't understand how i'm writing this I apologize...
    Case 1: F= -[(k*2q*q)/(r)^2] + [(k*4q*q)/(2r)^2]
    Case 2: F= +[(k*4q*q)/(r)^2] - [(k*2q*q)/(2r)^2]
    Case 3: F= +[(k*4q*q)/(r)^2] - [(k*2q*q)/(r)^2]
    Hopefully that's right!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Charged balls and force problem
  1. Force on a charge (Replies: 10)

  2. Force and Charges (Replies: 2)

Loading...