In a mathematical description of a circuit, I stands for current, which is simply a measure of electron flow. The amount of current flowing through a passive circuit can be calculated, according to Ohm's law, as a function of the circuit's voltage (V) and resistance (R), as follows:.

I = V / R

By anology to hydraulic systems, the current in a circuit is somewhat like the amount of water that flows through a hose as a result of water pressure pushing against the resistance offered by the cross-sectional diameter and length of the hose.