Two point charges and electric potential difference

[NihalRi]

Homework Statement

In an experiment modeling point charges I had to find equipotential lines. I did so using a voltmeter. The results looked like this:

Not a good quality picture, but the equipotential line on the right says 1.3 mV and the one on the left says 2.3mV. From this I had to calculate the value of the point charges. The idea is to compare theoretical calculations of the electric potential difference with the one I found to be -1mV. I chose two points along the line connecting the two point charges and named them A and B.

Homework Equations

Since point charges have a variable electric field the only option I had was to divide the separation between A and B into segments and use V=-Edx to find the potential difference in each segment.

The Attempt at a Solution

So I got confused because there are two point charges one that is positive and one that is negative. This means that I had to do calculations for both charges and then add them. I'm finding choosing the correct signs confusing so I reasoned that since the electric field created y both charges is in the same direction the signs for the electric field potential is the same. In other words both are negative. Is this reasoning correct. My final value for q was very small about 4.9*1^-15 C. The DC supply was 6V. Does this look right?

 

[andrevdh]

The electric potential at a point in the electric field is just the algebraic sum of the potentials created by the two point charges.

 

[NihalRi]

Thank you