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!

How to find the magnitude of a point charge given 2 others?

  1. Feb 8, 2015 #1
    1. The problem statement, all variables and given/known data
    Three charges are placed as shown in the figure below. The magnitude of q1 is 2.00 µC, but its sign is not known. The charge and sign of q2 is not known. We do know that q3 is +4.00 µC and the net force on q3 is entirely in the negative x-direction.

    physicproblem.png

    a.) Deduce the signs of charges q1 and q2.
    b.) Figure out the magnitude of q2

    2. Relevant equations

    Coulomb's law:

    F = (K)*(q1)(q2)/d^2

    3. The attempt at a solution
    I know how to determine the charges of q1 and q2. Since the y component of the net force vector of q3 equals 0, then q1 must be negative and q2 must be positive.

    Besides that, I have no idea on how to start figuring out the magnitude of q2
     
  2. jcsd
  3. Feb 8, 2015 #2

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Well, it could have been the other way around, but then the net force would have been in the +ve x direction.
    Your observation about net zero in the y direction tells you more than you've used. What is the y direction force from q1 on q3?
     
  4. Feb 8, 2015 #3
    The y direction force from q1 on q3 is 23.15 N. So The y direction force from q2 from q3 must be -23.15 N ? The problem is I don't know how to use this information to solve for the magnitude of q2.

    Also, what does this mean, "+ve x direction" ? I didn't understand your comment on how it could've been the other way around (sorry) :(
     
  5. Feb 8, 2015 #4

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    I don't know k off the top of my head so I can't quickly check that. For the purposes of the question, the value of k is immaterial, so we can pretend it's 1 whatever, consistent with expressing distances in cm and charges in µC.
    Yes, that would follow. You know the magnitude of q3, and the position of q2 in relation to it, so you can write an equation for that force as a function of q2's magnitude.
    Sorry, +ve means positive, -ve means negative. I thought these were very standard.
    You could set q1 positive and q2 negative and still arrange that there's not net Y field at q3. So you need to use the fact that the X field at q3 is to the left to determine which is positive and which is negative.
     
  6. Feb 8, 2015 #5
    You said this... "Yes, that would follow. You know the magnitude of q3, and the position of q2 in relation to it, so you can write an equation for that force as a function of q2's magnitude."

    I don't understand this part. I am lost on how to do this.
    And k btw is about 9 *10 ^9, atleast that is what I have been using.
     
  7. Feb 8, 2015 #6

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Yes, that's exactly what I'm saying. In your original post you wrote
    as though the zero y component was enough to determine that q1 must be negative and q2 must be positive. That observation only tells you they are of opposite sign. To deduce that it's q1 that's negative you also need to use the direction in which the arrow points.
    You probably worked all that through when you made the post, but misrepresented your own logic.

    It really is a very small point, and I'd rather get on with finding q2.
     
  8. Feb 8, 2015 #7
    I know, I am sorry. I didn't see the last part of your reply because of the computer screen I am using.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted