Answer to Question #42299 in Electricity and Magnetism for Lin Di

Question

can somebody help explain how they arrived at the answer? I'm at lost at where the values -12, -6, and 8 came from and why...

3 charges are arranged at the vertices of a right-angle triangle with Q1 = 4uc = 2Q2 and Q3 = 3uc.

(1) Determine the total potential enegry of the 3charged system. (2) Determine the work done by an external force to move Q2 from its present position to infinity.

Answered:

(1) Ui = (9*10^9)*[((-12*10^-12)/(3*10^-2)) - ((6*10^-12)/(5*10^-2)) + ((8*10^-12)/4*10^-2))] = -2.88J

(2) Uf = (9*10^9) * ((-12*10^-12)/(3*10^-2)) = -3.60J