Electric current with respect to ground. Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equation that describes this relationship: where "I" is the current through the conductor in units of amperes, "V" is the potential difference 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. In
Electric current with formula_20 being the elementary charge and formula_21 the electron density. The carriers move in the direction of decreasing concentration, so for electrons a positive current results for a positive density gradient. If the carriers are holes, replace electron density formula_21 by the negative of the hole density formula_23. In linear anisotropic materials, "σ", "ρ" and "D" are tensors. In linear materials such as metals, and under low frequencies, the current density across the conductor surface is uniform. In such conditions, Ohm's law states that the current is directly proportional to the potential difference between two ends (across) of that