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#3
Feb10-05, 11:21 PM
HW Helper
P: 2,566
The potential energy in the system is the work it would take to assemble the system. The first charge can be placed for free, but to place the second, you need to overcome the force from the first. Likewise, to place the third, you need to overcome the force from the first and second, and so on. The formula you mentioned is close, but it's missing some terms (eg, U(BC)).