From Wikipedia here: Ohm's law applies to electrical circuits; it states that the current passing through a conductor between two points is directly proportional to the potential difference (i.e. voltage drop or voltage) across the two points, and inversely proportional to the resistance between them. The mathematical equation that describes this relationship is: where I is the current in amperes, V is the potential difference between two points of interest in volts, and R is a circuit parameter, measured in ohmsresistance. (which is equivalent to volts per ampere), and is called the resistance. For all the various relationships, one can attempt to memorize the circle chart: http://www.ict4us.com/r.kuijt/en_ohm.htm But I think it's easier to remember just two mnemonic expressions: PIE in your EIR AND [IERP=AVOW or PIER=WAVO], then solve algebraically to reach the same results.