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Homework Help: Calculate Charge from Electric Field Diagram?

  1. Feb 18, 2014 #1
    1. The problem statement, all variables and given/known data

    The diagram shows the electric field lines surrounding three electric charges. Assume that q1 + q2 = -2.43 µC.

    A) Calculate q2.

    B) Calculate q1.

    C) Calculate q3.


    2. Relevant equations

    |Fe|= (k*|q1|*|q2|)/r^2

    k = 8.99*10^9 (N*m^2)/C^2
    Fe = Electrostatic force between a pair of charges
    r = charge separation
    q1, q2 = charge magnitudes
    E = electric field
    Q = charged object

    3. The attempt at a solution
    I am having a very hard time with this question. I'm not sure where to start. Based on the diagram I assume that q1 and q3 will be equal in charge and magnitude but am not sure where to go from here.

    Any help would be greatly appreciated!
  2. jcsd
  3. Feb 18, 2014 #2
    Have you learned Gauss's Law/Maxwell's first equation yet? This problem is essentially that. It involves the fact that electric field lines begin on positive charges, and end on negative charges. So the number of electric field lines around a certain charge distribution is proportional to the net amount of charge in that distribution. If you look at the diagram, you have twice as many field lines entering [itex]q_{2}[/itex] as you have leaving [itex]q_{1}[/itex] and [itex]q_{3}[/itex]. Using that, what can you determine about the signs and magnitudes of the charges?
  4. Feb 19, 2014 #3
    Hi again jackarms!

    No we haven't learned those concepts yet, we got this assignment about 1 week early and I was trying to be keen. I'm going to read ahead and see if I can figure this out.

    Thank you!
  5. Feb 23, 2015 #4
    Not sure if you got this yet, but the key thing is to look at the direction of the fields. If they travel away, the charge is positive, where as if they travel to, the charge is negative. The amount of field lines entering a charge are proportional to the charge when comparing two charges. So basically, -2Q1 = Q2, as Q1 is positive, but Q2 is negative. Does this make sense?
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