Resistors and resistance

Electrical resistance is a broad term given to the opposition of flow of current within an electrical circuit. However, when considering components such as capacitors or inductors, or when speaking about resistance to alternating current (AC) flow, certain other terminology is used. 

The total of the resistive and reactive components of opposition to electrical flow (ohms, fi). 

All three of these terms have units of ohms as they are all measures of some form of resistance to electrical flow. The reactance of an inductor is high and comes specifically from the back electromotive force (EMF p. 46) that is generated within the coil. It is, therefore, difficult for AC to pass. The reactance of a capacitor is relatively low but its resistance can be high therefore, direct current (DC) does not pass easily. Reactance does not usually exist by itself as each component in a circuit will generate some resistance to electrical flow. The choice of terms to define total resistance in a circuit is, therefore, resistance or impedance. 

The strength of an electric current varies directly with the electromotive force (voltage) and inversely with the resistance. 

The equation can be used to calculate any of the above values when the other two are known. When R is calculated, it may represent resistance or impedance depending on the type of circuit being used (AC/DC). 

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From Whitaker, J.C., Resistors and resistive materials, in The Resource Handbook of Electronics, CRC Press, Boca Raton, FL, 2001, p. 187. 

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