Coulomb's Law: A Numerical ExampleWhat is the force on charge Q

_{1}because of the charges q_{2}and q_{3}?

That's all there is to it!

Now let's try a real example.

What is the net force on charge Q_{1}?

First, concentrate on just Q_{1}and q_{2},

Notice, from the geometry of Q_{1}and q_{2}, that F_{12}has only an x-component. Its y-component is zero. F_{12x}= 0.45 N and F_{12y}= 0.

Now, focus attention on Q_{1}and q_{3}.

Notice, from the geometry of Q_{1}and q_{3}, that F_{13}has only a y-component. Its x-component is zero. F_{13x}= 0 and F_{13y}= - 2.7 N. The negative sign on F_{13y}= - 2.7 N means that it points down, of course.

Now we have the components of the two forces F_{12}and F_{13}so we can add them together.

Notice that the answer is NOT F_{1net}= (0.45 N) + (- 2.7 N) = - 2.25 N. It is very important that these forces F_{1}and F_{2}are added as vectors!

F_{1net}= 2.74 N is the magnitude of the force acting on charge Q_{1}. Now we need to ask what the direction of this force is. What is the angle theta between the x-axis and the force F_{1net}?

(c) Doug Davis, 2002; all rights reserved

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