Three point charge in a plane (electricity in physics II)

AI Thread Summary
The discussion centers on calculating the electrostatic potential energy and the work required to transport charge Q3 to infinity in a system of three charges located in the x-y plane. The user successfully calculated the total electrostatic potential energy as -0.0299 J but struggled with the work calculation, yielding an incorrect result of 1.51x10^10 J despite trying both positive and negative values. Additionally, there was confusion regarding the potential energy of the system with only Q1 and Q2, which was calculated incorrectly as -4.50x10^10 J. The user seeks clarification on the differences in potential energy between the two configurations of charges. Accurate calculations and understanding of the relationships between the charges are crucial for resolving these issues.
jrk012
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Homework Statement



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.


Homework Equations



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



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.

I have tried both this and its negative counterpart, neither yielding a correct answer. Please help!
 
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jrk012 said:

Homework Statement



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.

Homework Equations



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

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.

I have tried both this and its negative counterpart, neither yielding a correct answer. Please help!
What is the potential energy of the system which has only Q1 and Q2 ?
 
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.
 
jrk012 said:
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.
What's the difference between this answer, and the answer with all three charges present?
 

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