Example: Cylindrical Capacitor
Calculate the capacitance:
Assume +Q, -Q on surface of cylinders with potential difference V.
If we assume that inner cylinder has +Q, then the potential V is positive if we take the zero of potential to be defined at r = b:
http://wug.physics.uiuc.edu/courses/phys112/fall97/acts/lect07/act07b/sld001.htm