Change in Kinetic Energy of a moving charge

[yankeekd25]

Homework Statement

A charge of +5.5 micro-Coulombs lies at x = -3.6 cm, y = 0 cm, a charge of +5.5 micro-Coulombs lies at x = +3.6 cm, y = 0 cm, a charge of +5.5 micro-Coulombs lies at x = 0 cm, y = +3.6 cm, and a charge of -5.5 micro-Coulombs lies at x = 0 cm, y = -3.6 cm. A charge of -6.5 micro-Coulombs moves from x = -3.6 cm, y = +3.6 cm to the x = -1.86 cm and y = +1.86 cm. What is the -6.5 micro-Coulomb charge's change in kinetic energy during this path? Answer in Joules.

Homework Equations

Delta KE= - delta EPE
EPE= q V
V= Kq/r

The Attempt at a Solution

I know I need to find the V that the four charges on the axis produce, but I'm not sure what to use for r, or what to do after that.

 

[berkeman]

Work in the rectangular coordinate system that you are given in the problem. The r values are just the hypoteneuses (sp?) of the x,y offsets for each charge. You are correct to calculate the PE initially and the PE finally to get the change in PE to then tell you the change in KE.

 

[yankeekd25]

Work in the rectangular coordinate system that you are given in the problem. The r values are just the hypoteneuses (sp?) of the x,y offsets for each charge. You are correct to calculate the PE initially and the PE finally to get the change in PE to then tell you the change in KE.

Ok based on what you told me, I've calculated the voltage within the rectangular system. I know that the new charge will be in that system, so how do I find the initial EPE and final EPE?