Electrostatics drivng me nuts

[Brianjw]

I'm stuck on a Electrostatic problem which I just don't seem to get.

The left-hand sphere has a positive charge Q and the right-hand sphere has a negative charge -Q. Charge is distibuted uniformly over each of two spherical volumes with radius . One sphere of charge is centered at the origin and the other at x = 2R

Find the magnitude of the net electric field at the point on the x-axis.

I did the first part already where it wanted the field at the orgin which was simple:

Q/(4*pi*epislon_0*(2R)^2)

but if I try the same method for this one it doesn't seem to work. I wanted to add them together I tried:

Q/(4*pi*epislon_0*(.5R)^2) + Q/(4*pi*epislon_0*(3R/2)^2)

but doesn't like it.

Thanks

[Doc Al]

You're going to have to be clearer as to what the problem is. Where are you trying to find the field?

[Brianjw]

Err sorry. It wants me to find the E-field at the Point X=R/2.

Let me put up a picture to help:

http://mp.pearsoncmg.com/probhtml/yf.Figure.22.64.jpg

So I found that the E-field at Point X=0 is:

E = Q/(4*pi*epsilon_0*(2R) ^2 sicne the point is at the center of the first sphere its field doesn't matter, you just use the 2nd sphere which is 2R away to solve it.

I've used this method for other parts of the problem as well. Where I just the two E-fields together to get the right answer. I think there must be something difference since the E-field lies inside one of the spheres.

[Doc Al]

OK, it looks like you are trying to find the field at x = R/2.

You know how to find the field due to a uniform sphere of charge for all points outside the sphere (r > R). But what is the field for r < R? Hint: The field at a distance x from the center depends only on the charge for r < x. The field at x due to charges at r > x cancels out.

[Brianjw]

Nm, I got it, thanks for the tip.