9.4 Inductive Reactance

This is the fourth week of Module 9, "Relationships of Current, Counter EMF, and Voltage in LR Circuits." These modules are part of the Navy Basic Electricity and Electronics series from the 1970s. The fourth section of this module is titled, "Inductive Reactance."

9.1 Rise and Decay of Current and Voltage
9.2 LR Time Constant
9.3 Universal Time Constant Chart

1. The book thus far has only considered counter-EMF with a direct current (DC) source and a coil. With such a source, opposition to a change in current stopped once five time constants passed and the current reached a maximum or zeroed out.

But when the current has an AC (alternating current) source, the situation is different. A counter-EMF will constantly be in play from a coil and there will always be an opposition to the current, known as reactance. Reactance is symbolized by the letter X and with an inductive cause, X_L.

Because reactance is opposition to current flow, it is measured in ohms (Ω) just like resistance is. But while resistance is a permanent feature to a circuit based on its physical features, reactance changes based on the frequency of AC or the inductance of the circuit.

2. Reactance in a circuit caused by inductance is called, "inductive reactance." Its value is X_L = 2πfL, where f is the frequency in Hertz (cycles per second) and L is the inductance in henrys (volts per 1 amp change of current per second) .

Increased frequency means a greater rate of change and therefore a greater reactance.

Similarly, if the inductance is increased by increasing the number of turns in the coil, the size of the cross-section, or the permeability of the core, then the reactance to changing current increases as well.

So when inductance goes up, the reactance goes up.