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: Coulombs Law ?

  1. Apr 19, 2005 #1
    We are dealing with Coulombs law or w/e in Physics right now AND I DONT GET IT lol In my homework. it gives numbers like +6.0 (weird u symbol)C and i jus dont understand how to find the magnitude and direction of everything..

    Im a junior in Physics..so someone of higher class...like..college...that could help.. PLEASE HELP ME! lol

    Patrick :confused: :yuck: :eek:
  2. jcsd
  3. Apr 19, 2005 #2
    Well, do you have a sample problem you have trouble understanding with... If so post it.
  4. Apr 19, 2005 #3
    ok... heres a sample prob. actually its the one im on..

    1. Three point charges, q1, q2, and q3, lie along the x-axis at x=0, x=3.0 cm, and x= 5.0 cm, respectively. Calculate the magnitude and direction of the electric force on each of the three point charges when q1= +6.0 mu symbol C, q2=+1.5 muC and q3= -2.0 muC.

    This all deals with "the superposition principle" and yea..this is hard stuff considering we were just working with lens...and i hated them and wish we were back to them already lol.


  5. Apr 20, 2005 #4
    The "weird u symbol" you speak of means micro, which is 1 * 10^-6. So [tex]6 \mu C[/tex] is the same thing as [tex]6 \cdot 10^{-6} C[/tex]. Remember that the equation for force between two charges is [tex]F=\frac{kq_{1}q_{2}}{r^{2}}[/tex]
    Last edited: Apr 20, 2005
  6. Apr 20, 2005 #5


    User Avatar

    ok, calm down. This isn't really that hard, it's just in a new format. I assume that you've done gravity? [tex]F=\frac{G m_{1} m_{2}} {r^2}[/tex]? Well coulomb's law is basically the same. Instead of G you have k, and instead of masses you have charges. So for the force between two charges, you just put q1 as one charge (say q1=+6.0 muC) and q2 as the other charge (say 1.5muC) and the r is the distance between them (say 3.0cm). so you just plug those numbers into the equation and you've got the force between those two charges. If say q2=-2.0muC then the force would be negative (the negative charge q2 times the positive charge q1).

    Now superposition is really not that difficult either. Basically, if you have three charges, q1, q2, and q3, the TOTAL force on q1 is the force on q1 by q2 PLUS the force on q1 by q3. And there you go. You've got the force. Just a small hint: Don't forget the directions of the force. If one of the force is negative, take that into account. If you have three charges and your looking for the force on the one in the middle, don't forget which way each force is going. You'll understand the hint better once you do the problem.
  7. Apr 20, 2005 #6
    [tex]F = k\frac{q_1q_2}{r^2}[/tex]

    When the magnitude of the force is positive (negative-negative or positive-positive) then it is a push force, pointing away from the other charge.
    When the magnitude of the force is negative (negative-positive) then it is a pull force, pointing to the other charge.
    Also you will usually see charges in micro-Coulombs: [tex]\mu C[/tex], which is a milionth of a Coulomb. The SI unit C is usually considered too large for normal static charges, which are just a few [tex]\mu C[/tex] or [tex]nC[/tex] (nano-Coulombs, or bilionth of a Coulomb).
    Last edited: Apr 20, 2005
  8. Apr 20, 2005 #7
    Thanks 4 the Help

    I got to class and had some more explanation... i wish in class they would be as blunt and understandable though... only thing i still have a problem with is like.. trying to find the vectors part..the directions.. like

    q1 q2

    q3 q4 i dont understand after finding the Forces acting on them..like the directions...which comes out as a Degree.. its jus...confusing i guess..i dont know how to explain in on here lol. easier by mic.

    but thanks for the help.. anyone have calculator programs for physics??? i have a ti -84 plus...

    patrick :smile:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook