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!

Electrostatics problem with pith balls hung using threads

  1. Mar 24, 2013 #1
    1. The problem statement, all variables and given/known data

    Please open the attachment for the question
    2. Relevant equations

    1)coulomb's law

    3. The attempt at a solution

    I'm at a loss regarding what to do. Upon reading the question (ignoring the figure), I assumed that the charges will form a square and thus the angle between two adjacent threads will depend on the length of the thread. But the solution key given to me said option (d) was correct. Please help.
     
    Last edited: Mar 31, 2013
  2. jcsd
  3. Mar 24, 2013 #2

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    That problem statement is ill. The angle depends on the charges and masses - for negligible charges, it will be nearly zero, which is not covered in any answer.
    Without or with negligible gravity, the masses will arrange like a tetrahedron, and d is correct. But that requires at least one charge to point "upwards" relative to the point where they are attached.
     
  4. Mar 24, 2013 #3
    Without gravity, won't they get arranged as a square in the plane of the ceiling to which the threads are attached?

    Thanks in advance mfb
     
  5. Mar 24, 2013 #4

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    As far as I know, a square is not the ideal arrangement, a tetrahedron is better.
     
  6. Mar 24, 2013 #5
    But with a tetrahedral arrangement, won't one of the threads slacken? Also, could you please explain how the tetrahedral arrangement is favored to the square one where the balls are far apart from each other and thus the system has the lowest potential energy.
     
  7. Mar 24, 2013 #6

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Electrostatic repulsion keeps it there.

    With a tetrahedral arrangement, the angle is somewhere at ~110°, compared to ~90° for the square. On the other hand, you have 3 nearest neighbors instead of 2, so it would need a calculation to compare both arrangements.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Electrostatics problem with pith balls hung using threads
  1. Pith ball Charges (Replies: 4)

  2. Pith Ball's charge (Replies: 12)

  3. Pith balls problem (Replies: 20)

Loading...