# Homework Help: Three point charge in a plane (electricity in physics II)

1. Jun 7, 2012

### jrk012

1. The problem statement, all variables and given/known data

Three charges are located in the x-y plane (see plot below), with Q1 = -6.00 μC, Q2 = 5.00 μC and Q3 = -3.00 μC. Note that the charges are located at grid intersections points.

*I couldn't put the grid on here, but the points are Q1 = (-2,-4), Q2 = (-2, 2). and Q3 = (3, -4)

a) Calculate the total electrostatic potential energy.

b) Calculate the work required (by external forces) to transport Q3 from its location on the figure to infinity.

2. Relevant equations

PE = k[(Q1Q2/r12) + (Q1Q3/r1r3) + (Q2Q3/r2r3)]

3. The attempt at a solution

I got part a) to be -.0299 J, which was right. This is what I did for the second part:

k[(Q1Q3/r1r3) + (Q2Q3/r2r3)]=

=k[(-15/√61) + (18/5)]

=1.51x10^10 J.

2. Jun 7, 2012

### SammyS

Staff Emeritus
What is the potential energy of the system which has only Q1 and Q2 ?

3. Jun 7, 2012

### jrk012

I guess it would be k[Q1Q2/r1r2] = k[-5] = -4.50x10^10 J, but I tried that and its positive counterpart as well and neither are correct.

4. Jun 8, 2012

### SammyS

Staff Emeritus
What's the difference between this answer, and the answer with all three charges present?