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Homework Help: Electric field with three charges and a point

  1. Apr 17, 2017 #1
    • Member advised to use the homework template for posts in the homework sections of PF.
    refer only to the image above, not the question

    4.) Two point charges, Q1 and Q2, are placed 0.30 m apart on the x–axis (shown in the figure above) Charge Q1 has a value of –2.50 x 10–6C. The net electric field at point P is zero.

    a. What is the sign of charge Q2? ____ Positive ____ Negative

    Justify your answer theoretically (without math)

    b. Calculate the magnitude of charge Q2.

    c. Determine the x–coordinate of the point on the line between the two charges at which the electric potential is zero. In other words, determine where you could place a third charge into the system in equilibrium. Calculate this x-coordinate distance in relation to the Q1 charge.

    d. A third charge (Q3 = 5.7 x 10–6C) is now placed into the system at the 0.60 m mark. Calculate the net electric force on the Q2 charge. Include both the magnitude and direction.

    I think that Q2 is positive because in order for the field on point P to be zero, the charges have to be opposite.
    I need help with B, C, and D. I think that B may be 4X10-5C but im not sure if im using the right equation. Im using (-KQ1/R^2)+(KQ2/R^2)=0 but that may not be correct.
    im clueless on how to do C and D. Please help in anyway that you can. Thanks!!
  2. jcsd
  3. Apr 17, 2017 #2


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    For (b) you are using the right equation. Can you show in some detail how you got the answer you posted?
    For (c) you need to review electric potential then apply the correct equation.
    For (d) if you can find the additinal contribution to the electric field by superposition.
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