Posted by Anonymous on .
four particles with charges +5x10^6C, +3x10^6, +3x10^6C, and +5x10^6C are placed at the corners of a 2m x 2m square. (a)determine the electric potential at the center of the square, assuming zero potential at infinity. (answer:3.83x10^4V)(b) If one of the +3x10^6C charges is replaced by a 4x10^6C charge, what is the electric potential at the square's center? (answer: 6.36x10^3 V) Any help is appreciated.

physics 
drwls,
To get the potential energy at any point P, add up the k*Qi/ri terms, where Qi is the charge of the ith particle and ri is it's distance away from point P. The sign of Q is important.
At the center, ri = sqrt2 meters for each corner particle
Do both problems the same way, but changing the value of one of the charges, qi. The other three charges stay the same. 
physics (for drwls) 
Anonymous,
there is a typo. the last charge is suppose to be 5c10^6C
Can you check my work and see what I did wrong? Thank you!
Work:
K(5x10^6C)/1 + K(3x10^6)/1 + K(3x10^6)/1+ K(5x10^6C)/1+K(5x10^6C)/sqrt(2) + K(3x10^6)/sqrt(2) + K(3x10^6)/sqrt(2)+ K(5x10^6C)/sqrt(2)