Potential Energy of a Collection of charges

[Josh930]

Homework Statement

Three equal point charges, each with charge 1.35 \mu C, are placed at the vertices of an equilateral triangle whose sides are of length 0.350 m. What is the electric potential energy U of the system? (Take as zero the potential energy of the three charges when they are infinitely far apart.)

Homework Equations

U=k(Qq/r)

The Attempt at a Solution

should i just find the U for each pair and then just multiply it by the number of possible pairs since the distances and charges are all equal? my answer = .138J

 

[Fightfish]

Yup, seems correct to me. The magnitude of the total potential energy of a system of charges is the amount of energy required to separate each of the charges to infinity and thus to "break up" each pair.

 

[tomcue]

Yes, that is a correct approach. The electric potential energy of a system of charges is the sum of the potential energies of all possible pairs of charges. In this case, since all three charges are equal and the distances between them are also equal, you can simply calculate the potential energy for one pair and then multiply it by the number of possible pairs (in this case, 3). Therefore, your final answer of 0.138 J is correct.