Electric Potential Energy of Three Point Charges
Use ε0 = 8.85×10−12 C2/(N*m2) for the permittivity of free space.
Mastering Physics says to use ε0 but we don’t have to (we can just use k directly). The formula to find the potential energy between two charges is:
U = (k*q1*q2)/r
And since there are 3 pairs of charges (1-2, 1-3, and 2-3), we can solve for the potential energy of the system if we multiply the above by 3 (k is the Coulomb constant, which equal’s 8.99*109N*m2/C2). Note that each charge is 1.3 μC, which equals 1.3*10-6C:
U = 3*(k*q1*q2)/r
U = 3*((8.99*109)*(1.3*10-6)*(1.3*10-6))/0.5
U = 9.11*10-2 J