1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: Three point charges are arranged on a line.

  1. Aug 27, 2012 #1
    1. The problem statement, all variables and given/known data
    Three point charges are arranged on a line. Charge q3 = +5.00nC and is at the origin. Charge q2 = -4.00nC and is at x = 4.50cm . Charge q1 is at x = 2.00cm.

    What is q1 (magnitude and sign) if the net force on q3 is zero?

    2. Relevant equations

    3. The attempt at a solution

    So first thing I did was draw a diagram where q3 was located at (0,0) q1 at (2.00cm,0) and q2 was located at (4.50cm,0)

    Then after looking at the problem for a while I know I must use coulomb's law however since the net for q3 is 0 that threw me off a bit.

    So what I decided to do was make two separate coulomb's law equations and equal them to each other.


    Ultimately solving for q1 gives.

    q1= √[(q2)^2*(r1^2)/r2^2]

    Not sure if I'm on the right track just brainstorming, much help is appreciated

  2. jcsd
  3. Aug 27, 2012 #2
    Why do you use the same r for (q1, q3) and (q2, q3)? What it should it really be in these two cases?
  4. Aug 27, 2012 #3
    Wouldn't the distance for q3 be 0 since it is at (0,0)?

    Only thing else I can think of is maybe getting the distance of Q1 and Q2 and possibly use Pythagoras theorem to get the total distance?
  5. Aug 27, 2012 #4


    User Avatar
    Gold Member

    You need to find the forces on charge [itex] Q_3 [/itex] due to charges [itex]Q_1 [/itex] and [itex] Q_2 [/itex]. What does [itex] r [/itex] in Coulombs Law actually represent?
  6. Aug 27, 2012 #5
    You need the distance between q1 and q3 and between q2 and q3. They are all on the same line. You are given their distances from the beginning of the line. Moreover, q3 is at the beginning of the line. If you still can't figure it out, take a piece of paper and pencil and sketch it.
  7. Aug 27, 2012 #6


    User Avatar
    Gold Member

    I would suggest drawing a diagram, showing the directions of the forces on [itex]Q_3[/itex] due to [itex]Q_1[/itex] and [itex]Q_2.[/itex] Because you know the net force on [itex] Q_3 [/itex] is 0, it should be quite obvious what direction the force on [itex]Q_3 [/itex] due to [itex] Q_1 [/itex] is and from this you can infer the polarity of [itex] Q_1. [/itex]
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook