AP Physics B : Using Coulomb's Law
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Example Question #1 : Coulomb's Law
Charges A and B are placed a distance of 
What is the ratio of 
This question is very simple if you realize that the force experienced by both charges is equal.
The definition of the two electrostatic forces are given by Coulomb's law:
In this question, we can rewrite this equation in terms of our given system.
It doesn’t matter if the charges of the two particles are different; both particles experience the same force because the charges of both particles are accounted for in the electrostatic force equation (Coulomb's law). This conclusion can also be made by considering Newton's third law: the force of the first particle on the second will be equal and opposite the force of the second particle on the first.
Since the forces are equal, their ratio will be 
Example Question #71 : Ap Physics B
An excess charge of 
Two principal realizations help with solving this problem, both derived from Gauss’ law for electricity:
1) The excess charge on an ideal conducting sphere is uniformly distributed over its surface
2) A uniform shell of charge acts, in terms of electric force, as if all the charge were contained in a point charge at the sphere’s center
With these realizations, an application of Coulomb’s law answers the question. If 
In this equation, 
Using the given values in this equation, we can calculate the generated force:
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