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 show some resistance, except for superconductors, which have a resistance of zero.

The resistance (R) of an object is defined as the ratio of voltage across it (*V*) to current through it (*I*), while the conductance (G) is the inverse:

For a wide variety of materials and conditions, *V* and *I* are directly proportional to each other, and therefore *R* and *G* are constant (although they can depend on other factors like temperature or strain). This proportionality is called Ohm's law, and materials that satisfy it are called *ohmic* materials.

In other cases, such as a transformer, diode or battery, *V* and *I* are *not* directly proportional. The ratio V/I is sometimes still useful, and is referred to as a "chordal resistance" or "static resistance",^{}^{} since it corresponds to the inverse slope of a chord between the origin and an *I–V* curve. In other situations, the derivative may be most useful; this is called the "differential resistance".

In the hydraulic analogy, current flowing through a wire (or resistor) is like water flowing through a pipe, and the voltage drop across the wire is like the pressure drop that pushes water through the pipe. Conductance is proportional to how much flow occurs for a given pressure, and resistance is proportional to how much pressure is required to achieve a given flow. (Conductance and resistance are reciprocals.)

This page was last edited on 5 March 2018, at 00:56.

Reference: https://en.wikipedia.org/wiki/Electrical_resistance under CC BY-SA license.

Reference: https://en.wikipedia.org/wiki/Electrical_resistance under CC BY-SA license.

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