resistance

In physics, that property of a conductor that restricts the flow of electricity through it, associated with the conversion of electrical energy to heat; also the magnitude of this property.

Materials that are good conductors of electricity have electrons held loosely in the outer shells of their atoms. Current can flow easily and these materials have low resistance. In poor conductors of electricity, the electrons in the outer shells of their atoms are more strongly attracted by the positively-charged nucleus. Such materials restrict the flow of electrons and have a high resistance. Resistance (R) is related to current (I) and voltage (V) by the formula: R = V/I. The statement that current is proportional to voltage (resistance is constant) at constant temperature is known as Ohm's law. It is approximately true for many materials that are accordingly described as ‘ohmic’.

Resistance depends on many factors, such as the nature of the material; its temperature; its length and cross-sectional area; the nature and state of illumination of the surface; and the frequency and magnitude of the current.

The SI unit of resistance is the ohm. A rheostat is a variable resistor.