Question

Three charges are located at the corners of a rectangle as follows:

Charge A: lower left corner, 3.0 ?C

Charge B: upper left corner, 6.1 ?C

Charge C: upper right corner, -3.7 ?C

The distance between A and B is 0.16 m and between B and C it is 0.25 m.

How much work must an external force acting on the particles (like, your hands) do in order to move the charges infinitely far from each other? Units of work are Joules. Make sure to enter the correct sign.

Answer 1

The work done by an external force is the difference in PE of the charges;
W = PEf - PEi

At infinity the PE is zero ,so PEf=0 and;
W = - PEi

The initial PE is the algebraic sum of the PE of all possible pairs;
PEi = kq1q2/r12 + kq1q3/r13 + kq2q3/r23

where q1,q2 & q3 are the given charges (it doesn't matter which you call q1 or q2 or q3) and r12, r13, r23 are the distances between the corresponding charges.
Be sure to use the correct sign of the charge, so two of the three terms will be negative.