Electric Potential: Solve for Energy to Move -5uC Charge to Infinity

[bob19841984]

Homework Statement

A point charge of +8C is located at (-6,0)cm. A second point charge of -11C is located
at(-5,-12)cm. If a -5C charge were at the origin, how much energy is required to move it to innity?

Homework Equations

V=KQ/R
U = -q
V

The Attempt at a Solution

I have the solution. It is given as below... I just don't understand why!
http://a-s.clayton.edu/jdyer/physics/phys1112/phys1112S09/Quizzes/1112_Quiz2_solution.pdf"

If the origin now has a -5 micro C charge, why is the energy potential still calculated as if it was not there? I have been banging my head on this for a while, any help would be greatly appreciated.

 

[rl.bhat]

1. T

The Attempt at a Solution

I have the solution. It is given as below... I just don't understand why!
http://a-s.clayton.edu/jdyer/physics/phys1112/phys1112S09/Quizzes/1112_Quiz2_solution.pdf"

If the origin now has a -5 micro C charge, why is the energy potential still calculated as if it was not there? I have been banging my head on this for a while, any help would be greatly appreciated.

V₁ and V₂ are the potentials at origin due to two charges. -5 μC charge is taken from origin to infinity and the work done to do this is -q*ΔV.