The resistance of an electrical conductor is the opposition to the passage of an electric current through that conductor. Resistance shares some conceptual parallels with the mechanical notion of friction. The SI unit of electrical resistance is the ohm (Ω).
An object of uniform cross section has a resistance proportional to its resistivity and length and inversely proportional to its cross-sectional area. All materials, aside from super conductors, show some resistance. The resistance of an object can be defined as
R = V/I
G = I/V
R is the resistance
V is the voltage across
I is the current through
G is the conductance
For most materials and conditions, V and I are directly proportional to each other, and therefore R and G are constant. This proportionality is called Ohm’s law.
There are some cases where V and I are not directly proportional, such as a diode or battery. The I-V curve is not a straight line through the origin and Ohm’s law does not hold. In this instances, resistance and conductance are less useful concepts and more difficult to define. The ration V/I is however sometimes still useful. It refers to a chordal resistance or static resistance as it corresponds to the inverse slope of a chord between the origin and an I-V curve. Sometimes the derivative dV/dI is the most useful. This derivative is called the differential resistance.