The adjacent figure shows the E-field lines (with arrows) and equipotential contours for a certain charge configuration. Determine for each of the following statements whether it is true or false.
Assuming that these contours are those of a point charge at the origin, calculate the value of the charge.
Instructor says "Yes - read off the "r" values right from the diagram.
I don't think you'll need to integrate anything to solve this one. (Unless you feel like repeating the integral we already solved for a point charge.)
Also - looks like the diagram is assuming V is zero at r = infinity."
The Attempt at a Solution
okay, the first contour line of -9000V is at 1.00 meters and the third of -7000V is at 1.24 meters.
I use dV = kQ/rf - kQ/ri = kQ ( 1/rf - 1/ri)
Q = 2000 / (8.99E9 * (1/1 - 1/1.24))
then I switch it to negative since the E-field lines are pointing toward the charge
<the question is what am i doing wrong... grr...>