AP Physics C e and M

Question

Two point charges of 9μC and 4μC are placed a distance of 4 m apart. 

Two point charges of 9μC and 4μC are placed a distance of 4 m apart. 

Find the point, measured in meters from the 9μC charge, between the two charges where the force on the 5μC charge is 0? 

Answer 1

Hey Gabriel,

What you want to do here is say that x = distance from the 9μC charge, which makes 4-x = distance from the 4μC charge. Now you have two equations for the force on this third charge (one caused by the 9μC charge, the other by the 4μC charge), which you can set equal to each other.

Be careful when you are squaring the distances in the denominator, (4-x)² should yield a pretty ugly denominator on that side of the equation, and then you should be looking at a quadratic equation -- when you get 2 solutions, only one should be between the 2 charges (ie: x < 4m).

Good luck.