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For many conductors of electricity, the electric current which will flow through them is directly proportional to the voltage applied to them. When a microscopic view of Ohm's law is taken, it is found to depend upon the fact that the drift velocity of charges through the material is proportional to the electric field in the conductor. The ratio of voltage to current is called the resistance, and if the ratio is constant over a wide range of voltages, the material is said to be an "ohmic" material. If the material can be characterized by such a resistance, then the current can be predicted from the relationship:

Electric current = Voltage/Resistance

AC Ohm's Law[]

The AC analog to Ohm's law is

(I = V/R, is for a pure resistor where Z=R, preserving the DC Ohm's law as stated above)

where Z is the impedance of the circuit and V and I are the rms or effective values of the voltage and current. Associated with the impedance Z is a phase angle, so that even though Z is also the ratio of the voltage and current peaks, the peaks of voltage and current do not occur at the same time. The phase angle is necessary to characterize the circuit and allow the calculation of the average power used by the circuit.


The illustration is for a case where the inductive reactance is dominant over the capacitive reactance as shown in the phasor diagram.

The above treatment of Ohm's law is valid for typical wires and elements used for household electricity which is at 60Hz frequency. For these low frequencies, the current density in wires can be assumed to be constant across the cross-section of a wire. This is not true for high frequencies like radio frequencies and above. For such frequencies the magnetic effects are significant and the currents are predominantly in the outer parts of the conductors. This is typically called the "skin effect", and must be taken into account in the design of high frequency circuits.