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,

Notice, from the geometry of Q₁ and q₂, that F₁₂ has only an x-component. Its y-component is zero. F_12x = 0.45 N and F_12y = 0.

Now, focus attention on Q₁ and q₃.

Notice, from the geometry of Q₁ and q₃, that F₁₃ 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 F₁₂ and F₁₃ 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 F₁ and F₂ are added as vectors!

F_1net = 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 F_1net?

Coulomb's Law			Electric Field
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(c) Doug Davis, 2002; all rights reserved