physics
posted by Allee .
Four identical charges (+4.0 uC each) are brought from infinity and fixed to a straight line. The charges are located 0.33m apart. Determine the electric potential energy of this group.

Add up the work required to bring the charges into their places, one at a time.
Let the charge separation for adjacent charges be a = 0.33 m
The Coulomb constant is k, and each charge is Q = 4*10^6 C
First charge: work = 0
Second charge: work = kQ/a
Third charge: work = kQ/a + kQ/(2a)
Fourth charge: work = kQ/a + kQ/(2a) + kQ/(3a)
Add them for the total potential energy
PE = (13/3) kQ/a 
PE=(13/3)k(Q^2)/a

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