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: Electric forces

  1. Sep 3, 2008 #1
    1. The problem statement, all variables and given/known data

    Two small metallic spheres, each of mass
    0.14 g are suspended as pendulums by light
    strings from a common point as shown. The
    spheres are given the same electric charge, and
    it is found that the two come to equilibrium
    when each string is at an angle of 7◦ with the
    vertical.
    The value of Coulomb’s constant is
    8.98755 × 109 N · m2/C2 and the acceleration
    of gravity is 9.81 m/s2 .
    If each string is 13.7 cm long, find the mag-
    nitude of the charge on each sphere.
    Answer in units of nC.


    2. Relevant equations


    y=Tcos(theta)=mg
    x=Tsin(theta)=Fe

    3. The attempt at a solution

    Tcos(7)=(.14g)(9.81)
    T=1.38371

    1.38371sin(7)=.168632

    q=sqrt((.1683632)(2*(.137sin7)^2))/8.98755x10^9
    q=1.44639e-7
     
  2. jcsd
  3. Sep 3, 2008 #2

    LowlyPion

    User Avatar
    Homework Helper



    Your method looks fine.

    I haven't checked the math.
    Have you been careful to make sure that the units of grams and cm's is properly matched to the units of g and Coulomb's constant?

    Your answer needs to be in nC.
     
  4. Sep 3, 2008 #3
    I changed it no nC and that is 1.44641e2 nC. It's still not right. :(
     
  5. Sep 3, 2008 #4
    sorry i meant *to
     
  6. Sep 3, 2008 #5

    LowlyPion

    User Avatar
    Homework Helper

    You need to square the total distance between the charges.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook