7.2 Reactance
As we have already learned, the amount of electrical "friction" or opposition to the flow of a constant direct current is called resistance and is measured in ohms. Alternating currents also encounter opposition to their flow in circuits. In addition to conventional resistance, ac currents encounter a special type of electrical "friction" called reactance. Unlike conventional resistance that has a value which is independent of the frequency of the current, reactance varies with frequency. Furthermore, with conventional resistance, the voltage and current are always exactly in phase with each other. (This means that when the voltage is at its peak value, the current will also be at its peak value.) This is not the case with reactances, and the voltage and current flowing will not necessarily always be in phase with each other.
The first type of reactance is called capacitive reactance, or capacitance. The second type of reactance is called inductive reactance, or inductance. Capacitance is an effect associated with electrostatic fields, and inductance is an effect associated with electromagnetic fields.
Capacitive reactance offers an infinite opposition to the flow of dc. For other frequencies, capacitive reactance decreases inversely with frequency, offering very little opposition to very high frequencies. Capacitive reactance is symbolized by two parallel lines that represent the parallel plates of a capacitor.
Inductive reactance offers no opposition to the flow of dc. For other frequencies, inductive reactance increases linearly with frequency, offering much opposition to very high frequencies. Inductive reactance is symbolized by a coiled line.
Capacitance and inductance are frequency-dependent effects that explain the bandwidth limitations of different types of wire transmission media. Specified amounts of capacitance and inductance can be used to create filters. This is accomplished by using devices called capacitors and inductors that are constructed to have specific amounts of capacitance and inductance.
Figure 7.2 Reactance
Reactance and Phase
The current through a resistance is in phase with the applied voltage. This is not the case for reactance.
For capacitive reactance,
the current leads the applied voltage by 90 degrees. For inductive reactance, the applied voltage leads the current by 90 degrees. A good way to remember this relationship is "ELI the ICE man". "ELI" stands for Voltage (E) leads Current for inductors (inductors are typically represented with the letter 'L'. "ICE" stands for Current (I) leads Voltage (E) in capacitors (capacitors are typically represented with the letter 'C'). In order to talk about voltages and currents in a circuit with reactance, it will be necessary to represent their values with more than just numbers--their phase relationship must be shown as well. There are a number of mathematical methods for handling this: Vectors, and Complex numbers are normally used. Since phase angles are involved, trigonometry is needed to solve problems dealing with reactance in electric circuits.
Figure 7.3 Reactance and Phase