Particle 1 carrying 40 C of charge is fixed at the origin of

Particle 1 carrying -4.0 C of charge is fixed at the origin of an xy coordinate system, particle 2 carrying +7.0 C of charge is located on the x axis at x = 2.0 m , and particle 3, identical to particle 2, is located on the x axis at x = -2.0 m .

Solution

Potential energy of the system of particles

is the sum of PE of all pairs

There re 3 pairs

1.   -4uc, +7uc   seperated by 2m ( 1 and 2 )

2. -4uc , +7uc seperated by 2m (1 and 3)

3. +7uc , +7uc seperated by 4 m (2 and 3)

PE of the system = k(-4*7)e-12 /2 + k(-4*7)e-12/2   + k(7+7)/4 J

                             = -141.75 e-3 J   (k = 9.0e+9)