# Where would the net electric field be zero if one of the charges were negative?

Two particles having charges of 0.580 and 20.9 are separated by a distance of 1.10. At what point along the line connecting the two charges is the net electric field due to the two charges equal to zero?

I've solved this part: the electric field is zero at a point .157 m away from .580 nC charge.

It's the 2nd part I'm stuck on:
Where would the net electric field be zero if one of the charges were negative? Enter your answer as a distance from the charge initially equal to .580 nC. I know that the answer should be somewhere to the right of the 20.9 nC charge, since the positive and negative electron's electric fields will be in the same direction.

Here's my work so far:
keq1/r^2 = keq2/(1.1-r)^2 (the ke's cancel out)
-.580 x 10^-9/r^2 = 20.9 x 10^-9/(1.1+r)^2
(20.9 x 10^-9)r^2 = (-.580 x 10^-9)(1.1+r)^2
(-36.034)r^2 = (r+1.1)^2
-7.003r = 1.1
r = .157
I added that to the 1.1 to get the total distance of 1.257 m, but that answer is wrong. Could someone please help me figure out what I'm doing wrong? I'm pretty sure the formula is right, but other than that I have no idea. Step-by-step what you did would be amazingly helpful!
asked Jan 25, 2013 in Physics

+1 vote