Calculating the Electric Potential at the Center of the Rectangle

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The discussion focuses on calculating the electric potential at the center of a rectangle formed by three charges, resulting in a potential of V_center = 1.78×10^5 V. Participants express confusion about determining the electric energy expended when moving charge q3 to infinity while keeping q1 and q2 fixed. Key equations mentioned include V = kq/r for electric potential and W = Vq for work done. Clarifications emphasize the importance of distinguishing between electric potential and potential energy, and the need to use the correct distances between charges for calculations. The conversation highlights the necessity of understanding the total potential energy of the system and how it changes with the movement of charge q3.
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Homework Statement


q3---a-----|
| |
| b
| |
q1---------q2

The figure shows three charges q1, q2 and q3 situated at corners of a rectangle of sides a = 15.0 cm and b = 6.0 cm.
For q1 = 5.90 μC, q2 = -5.90 μC, and q3 = 1.60 μC find the electric potential at the center of the rectangle.
V_center = 1.78×105 V

Continuing with the figure above, how much of the electric energy of the system would be expended in moving q3 to infinity while q1 and q2 remain in their positions?

Im at a loss as how to do this part, I have tried to find the field at the center with q3 no longer there and everytime it works out to be 0, so I am just not sure what to do here.


Homework Equations


V = kq/r
W = Vq



The Attempt at a Solution

 
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What's the electric potential energy between a pair of charges?
 
using V=kq1/a + kq2/a = 0... or at least I think.
 
kraigandrews said:
using V=kq1/a + kq2/a = 0... or at least I think.
Don't mix up potential with potential energy.

See: Electric Potential Energy
 
ok so U = kq1q2/a... correct? Or should I use the center of the rectangle as the radius?
 
kraigandrews said:
ok so U = kq1q2/a... correct? Or should I use the center of the rectangle as the radius?
You should use the distance between the charges. What's the total PE of the system? How does that change when q3 is moved to infinity?
 
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