12 Modern Commercial Wiring Copyright Goodheart-Willcox Co., Inc. Inductance opposes any change in cur- rent fl ow. If the voltage across a coil increases, the current fl ow increases slowly due to the self-induced voltage (which opposes current change). Conversely, if the voltage across a coil decreases, the current value decreases gradu- ally due to the self-induced voltage. The major difference between an ac circuit with resistance alone and one with induction is response speed. The current fl ow in a circuit having only resistance will change instantly if the voltage changes. Current fl ow in an induc- tive circuit will respond slowly if a voltage change occurs. This opposition due to inductance is called inductive reactance. It is measured in ohms, as is resistance. The symbol for inductive reac- tance is X L . Inductance (L) is measured in units called Henrys (H). An inductance of one Henry is present if one volt is applied to an inductor and the current fl ow increases at a rate of one ampere per second. Ohm’s law, as applied to a circuit with pure inductance, can be rewritten as follows: I = E ___ 2πfL = E __ X L where I = Current (A) E = Voltage (V) π = Pi (3.14) f = Frequency (Hz) L = Inductance (H) X L = Reactance (Ω) In other words, resistance in a purely induc- tive circuit is due to inductive reactance, and is numerically equal to 2πfL. One key idea to remember about purely inductive circuits is that the current lags the voltage by 90°. Also, since inductive reactance increases or decreases with the frequency of a circuit, it has the same effect as resistance and impedes the current fl ow. 1.3.6 Capacitance Capacitance (C) is the ability of a material to store an electric charge. This occurs whenever there are two conducting materials, such as metal plates, separated by an insulating mate- rial (such as air or paper). A device specifi - cally constructed in such a manner is called a capacitor or a condenser, Figure 1-18. If a source of direct current is connected to the two plates of a capacitor, the current will fl ow initially, but then gradually decrease to zero. The electrical energy will be stored in the capacitor. The capacitor is charged. If the source is not disconnected and the two plates are con- nected by a wire or resistor, current will fl ow through that connection for a limited time. When the current reaches zero, the capacitor is discharged. However, a capacitor in a dc circuit cannot maintain a charged condition because the current fl ows in one direction only. In an ac circuit, the voltage is constantly changing direction. The current is initially in one direction, charging the capacitor, and then in the opposite direction, discharging the capacitor. Therefore, current continues to fl ow, alternating back and forth, the capacitor plates charging and discharging. The unit of measure for capacitance is the farad (F). Capacitive reactance (X C ) is the opposition to the fl ow of current due to capaci- tance in the circuit. It is measured in ohms, just as is inductance and resistance. In a purely capacitive ac circuit (without any inductance or resistance), the current will lead the Conductor Metal plates Conductor Air space Goodheart-Willcox Publisher Figure 1-18. A simple capacitor is composed of two layers of conducting material (metal plates) separated by a layer of insulating material (air).