V=I*R, I=V/R, R=V/I
Ohm's Law is one of the fundamental laws of circuit theory, along with Kirchhoff's Voltage Law, and Kirchhoff's Current Law. It states that the voltage drop, V, across any resistive element is proportional to the current, I, flowing through it. Further, the constant of proportionality is the resistance, R, of the element.
With the introduction of capacitance or inductance into a circuit, one must substitute impedance, Z, for resistance:
V=I*Z
From this law, one can derive equations for voltage division, current division, and others.
In
electromagnetic fields, one may express Ohm's Law as:
J=σE
Here J is the current density, σ is the conductivity (1/resistivity), and E is the electric field intensity.