1. The problem statement, all variables and given/known data A +15μC point charge Q1 is at a distance of 30mm from -30μC charge Q2. Show that the electric potential is zero at a point between the two charges which is 10mm from Q1 and 20mm from Q2. 3. The attempt at a solution I know that V = Ed (where V = electric potential, E = electric field strength, d = distance from the point to the charge) I have two problems though: 1) I'm not sure where electric potential is being measure from, from charge Q1 or Q2, so I don't know what value (10mm or 20mm) to take as distance, d. With the examples I'm given in the textbook, its simple - just one charge point and a test charge. 2) I know I'm not supposed to find 'E', the resultant electric field strength, because its only in the next question that I'm asked to calculate this. So I think this makes the equation I've given at the top ( V = Ed) redundant as it probably won't be used. so I'm kinda stuck on where to start.