Coulomb's Law: A Numerical Example
What is the force on charge Q1 because of the charges q2 and q3?
That's all there is to it!
Now let's try a real example.
What is the net force on charge Q1?
First, concentrate on just Q1 and q2,
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Notice, from the geometry of Q1 and q2, that F12 has only an x-component. Its y-component is zero. F12x = 0.45 N and F12y = 0.
Now, focus attention on Q1 and q3.
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Notice, from the geometry of Q1 and q3, that F13 has only a y-component. Its x-component is zero. F13x = 0 and F13y = - 2.7 N. The negative sign on F13y = - 2.7 N means that it points down, of course.
Now we have the components of the two forces F12 and F13 so we can add them together.
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Notice that the answer is NOT F1net = (0.45 N) + (- 2.7 N) = - 2.25 N. It is very important that these forces F1 and F2 are added as vectors!
F1net = 2.74 N is the magnitude of the force acting on charge Q1. Now we need to ask what the direction of this force is. What is the angle theta between the x-axis and the force F1net?
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(c) Doug Davis, 2002; all rights reserved
Return to Ch 23 ToC