How do I solve these physics problems involving variables and equations?

[hoseA]

http://img423.imageshack.us/img423/4778/hwhelp1bs.jpg
The first one on the left is a Multiple Choice question. Just the mention of variables shuts down my brain. I'm still researching for the equation for the pbm.


The pbm in the right upper corner i think it's C and E. (by simply looking at it)

The pbm on the bottom right corner i used -E times delta r... I got -17.59.

Any help in the next 20-30 minutes will be much appreciated... as it'll double my homework score...

 

[hellraiser]

Potential is a scalar quantity. The distance of the points in all the 3 cases from the charges is same (they lie on the perpendicular bisector). So the net potential will be zero along these points (all are equal = 0).

For the right upper corner, consider the other point where the potential may be zero. There are three possibilites - right of +Q, between the charges and left of -2Q. One case is already given - right of +Q. Left of -2Q will be ruled out (Why?). So one case is left. Solve. (d/3)

For 3rd one use E = - (V/d)
or V = - E . d (E dot product d).
where d is the radius vector pointing from B to A.

 

[hellraiser]

For the second question answer is D+(2D/3) = 5D/3