The figure below shows three charges at the corners of a rectangle of length x = 0.35 m and height y = 0.22 m.
http://www.webassign.net/walker/20-23alt.gif (rectangle image)
(a) How much work must be done to move the +2.7 µC charge to infinity?
(b) Suppose, instead, that we move the -6.1 µC charge to infinity. Is the work required in this case greater than, less than, or the same as when we moved the +2.7 µC charge to infinity?
(c) Calculate the work needed to move the -6.1 µC charge to infinity?
The Attempt at a Solution
I got the correct answer for (a) like this:
W= (9e9)(2.7e-6)(6.1e-6)/.35 + (9e9)(2.7e-6)(3.3e-6)/.4134, with .4134 as the distance between the -3.3 charge and the 2.7 charge via the pythagorean theorem.
.61749 J, was correct; and I guessed that it would take less work to move the -6.1 µC charge for (b). But I'm not entirely sure why this is the case-- is it simply because the distances between the charges are smaller? And for some reason, when I use the same method on (c) as I did on (a), I'm wrong:
(9e9)(6.1e-6)(2.7e-6)/.35 + (9e9)(6.1e-6)(3.3e-6)/.22 yields 1.247, which is not the correct answer. Why is this?