# Resistance

Resistance is a measure of the… resistance of flow of electricity. Let’s try that again without breaking a cardinal rule of definitions. Resistance is how much the electrons struggle to move in comparison to the voltage across something. So, if there’s high voltage and high resistance, there will be low current, just as if there’s low voltage but low resistance. There. Hopefully that’s better.

The ability to resist the flow of electric current; it is measured in ohms.

Fundamentals of Electric Circuits, 5th Edition by Charles K. Alexander and Matthew N. O. Sadiku

The opposition to the flow of current in an electric circuit.

Grob’s Basic Electronics, 11th Edition by Mitchel E. Schultz

The **electrical resistance** of an object is a measure of its opposition to the flow of electric current. The inverse quantity is **electrical conductance**, and is the ease with which an electric current passes. Electrical resistance shares some conceptual parallels with the notion of mechanical friction. The SI unit of electrical resistance is the ohm (Ω), while electrical conductance is measured in siemens (S).

The resistance of an object depends in large part on the material it is made of—objects made of electrical insulators like rubber tend to have very high resistance and low conductivity, while objects made of electrical conductors like metals tend to have very low resistance and high conductivity. This material dependence is quantified by resistivity or conductivity. However, resistance and conductance are extensive rather than bulk properties, meaning that they also depend on the size and shape of an object. For example, a wire's resistance is higher if it is long and thin, and lower if it is short and thick. All objects 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 reciprocal:

For a wide variety of materials and conditions, *V* and *I* are directly proportional to each other, and therefore *R* and *G* are constants (although they will depend on the size and shape of the object, the material it is made of, and 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* over *I* is sometimes still useful, and is referred to as a "chordal resistance" or "static resistance",^{[1]}^{[2]} since it corresponds to the inverse slope of a chord between the origin and an *I–V* curve. In other situations, the derivative