Understanding Coulomb's Law: Calculating Force on a Line of Charges

[mr_coffee]

Hello everyone, I've been having the hardest time getting the hang of coulobs law. I got a physics book, physics for complete morons. The question is simple enough:

Consider a line of charges, q1 = 8.0 micro C at the orginin, q2 = -12 micro C at 2.0 cm, and q3 = 10 micro C at 4.0cm. What is the force on q3 due to the ohter two charges?

So you find F31 and u find the F32 and then add up the two forces to find your answer. But to find F31 they used a distance of .04m which confuses me. If the charges are as follows:

(q1)---(q2)-------(q3)
.02m .04m

Why wouldn't the distance from q1 to q3 be .06m?

Thanks.

 

[whozum]

Yeah it is 0.06m, maybe you are misunderstanding what they are trying to do. What did they use for F32?

 

[Doc Al]

I suspect that those are the positions of the charges (their coordinates along the axis), not the distances between them. Calling it the x-axis, the charges are at:
x1 = 0m
x2 = 0.02m
x3 = 0.04m

Make sense?

And welcome to PF, by the way!

 

[mr_coffee]

ahhh, how simple. Thanks Doc you were right!