Solving for Electric Potential Difference in Square diagram

[Matthew B.]

Homework Statement

I've attached a url with a picture of the problem setup and diagram
https://instagram.com/p/7nYNXTscre/

Homework Equations

I used the equation V = KΣ_iq_i/r_i to solve for potential at A and B

The Attempt at a Solution

For V_a I used K((Q/d)+ (2Q/(d^2+d^2)^.5))
I just plugged in
d = .02
Q = 5E-9
2Q = 10E-9
and arrived to Va = 5.43E3 Volts.

For V_a I used K((Q/(d^2+d^2)^.5) + (2Q/d))
I just plugged in
d = .02
Q = 5E-9
2Q = 10E-9
and arrived to Vb =6.08E3 Volts.

I am just stuck on part c.. Which asks "What is the electric potential difference between B and A" I'm not sure exactly which equation so use to solve for electric potential difference.. I think I am misunderstanding the concept of an "electric potential difference".. is it just the difference between the electric potentials at the two points a & b? If so, I'm not sure which point (a or b) would be considered the "initial potential" which I would use to take V_final - V_Initial. If this approach is correct, picking the wrong potential as the initial or final would result in a sign error in the final answer...

Thank you guys for your help

 

[Matthew B.]

As soon as I figure out how to add a photo

 

[BvU]

Homework Statement

I've attached a url with a picture of the problem setup and diagram
https://instagram.com/p/7nYNXTscre/

Ah, there's some more guidelines on the PF forum: the threads remain on PF forever (well...) whereas photos on instagram and sites like that disappear in a short while. So we urge folks to type their problem statement (good exercise: collecting only the necessary stuff and checking for completeness; listing variables, dimensions, knowns , etc... !) and upload pictures using the button.
After all, the helpers do take the trouble to type and typeset their assistance, so it's fair to ask that the requesters do likewise...

(I wish I had a shortcut button for the guidelines and this here little admonition -- you are definitely not the only one ).

Asking for potential difference between B and A is indeed awkward. You could answer V_B - V_A = ... on paper, but in a computer program you indeed risk a sign error.

Your chances are better than 50%, because often the problem writer doesn't care about the sign, so a positive number of Volts might be the safest bet.

https://www.boundless.com/physics/textbooks/boundless-physics-textbook/electric-potential-and-electric-field-18/overview-139/electric-potential-energy-and-potential-difference-498-8078/(at t=5:07) says the potential difference between A and B is the work/charge to bring a charge from A to B, so he calculates V_B - V_A

(How did we do that in mechanics with potential energy from gravity ?)