1. Feb 14, 2012

1. The problem statement, all variables and given/known data
I have three problems. I am absolutely desperate for answers. I cannot figure out what to do from the examples my book gives. It doesn't give examples in the book that relate to these problems, and I simply do not know what to do. It's very important that these get done fast. I'm basically screwed and this forum is my last resort, so please, please help.

A small charge of -2 x 10^ -8 C is at the point x = 2 m, y = 0 on the x axis/ A second small charge of -3 x 10^ -6 C is at the point x=0, y= -3.0 m on the y axis. What is the electric force that the first charge exerts on the second? What is the force that the second charge exerts on the first? Express your answers as vectors.

I know the answer is (-2.3 x 10^-5 N)i + (-3.5 x 10^-5 N)j, (2.3 * 10^-5 N)i + (3.5 * 10^-5 N)j, but I don't know how to get that.

Also:
Point charges +Q and -2Q are separated by a distance d. A point charge q is equidistant from these charges, at a distance x from their midpoint. What is the electric force on q?

Also:
A thin rod of length L is placed near a point charge q, with the nearest end a distance d from the charge and oriented radially as shown. The rod carries a uniform distribution of charge C coulombs per meter. Find the electric force that acts on the rod. (Hint: sum the force contribution dF = kqdq'r^2) due to each small charge dq' = Cdx on the rod to obtain the total force F = the antiderivative of dF.

If you could help with any or all of these, I would much appreciate it.

2. Relevant equations

F = (8.99 x 10^9 N*m^2/C^2)(qQ/r^2)

3. The attempt at a solution
See above

2. Feb 14, 2012

### tiny-tim

using the formula, i don't understand why you're not getting that result

how far have you got?

did you get the magnitude ok?

3. Feb 14, 2012