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The problem I am having is:

Two positive charges (+8.0 mC and +2.0 mC) are separated by 300 m. A third charge is placed at distance r from the +8.0 mC charge in such a way that the resultant electric force on the third charge due to the other two charges is zero.

I don't really know where to start with this problem.

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1 Answer

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Write down the coordinates of the fixed charges A(0,0), B(300,0). Now the third charge can be located at some point in space, let the coordinates of this point be C(x,y). The constraints are :

1) Distance between A and C is r meters.

2) The Electric Field due to charges located at A and B is 0.

You have to draw the picture first and use the above constraints to solve the problem. If you are still stuck, let me know.

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Ok, So I have a picture now that looks something like this: A(0,0)------C(x,y)---------B(0,300) So now how would you calculate the distance, r? – Avalon-96 Sep 9 '12 at 20:38
@Avalon-96, you've got to try a little harder than that. Here's a hint: think of a balance beam with unequal weights on the end with one weight 4 times the other. How far away from the heavier weight must the fulcrum be for the beam to be balanced? – Alfred Centauri Sep 9 '12 at 23:43
Avalon, remember that C(x,y) need not be collinear with A or B. From coordinate geometry, write down the distance between points A and C. – Antillar Maximus Sep 10 '12 at 14:27

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