Ohm's law

 Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points. 

where I is the current through the conductor in units of amperes, V is the voltage measured across the conductor in units of volts, and R is the resistance of the conductor in units of ohms. More specifically, Ohm's law states that the R in this relation is constant, independent of the current. Ohm's law is an empirical relation that accurately describes the conductivity of the vast majority of electrically conductive materials over many orders of magnitude of the current. However some materials do not obey Ohm's law, these are called non-ohmic.