AC Inductive and Capacitive Reactance

This tutorial will showcase how inductance, capacitance and resistance affect alternating current. A circuit may have the alternating current through it and the voltage across it rising and failing together if having pure resistance. The image below shows the sine waves for current and voltage in a purely resistive circuit having an AC source. The current and voltage are in phase but they do not have the same amplitude.

Inductive Reactance in AC

In a resistor-inductor circuit, the resistor will offer resistance to AC regardless of frequency and the inductor will offer reactance to AC current at certain frequency. The combined effect of the two components will be an opposition to current equal to the complex sum of the two numbers. The combined opposition will be a vector combination of resistance and reactance. In the case of a circuit having inductance, the opposing force of the counter EMF would be enough to keep the current from the remaining in phase with the applied voltage.

In the case of an AC source, the inertia effect of the CEMF is greater than with DC when the current flowing through an inductor continuously reverses itself. The greater the amount of inductance (L), the greater the opposition from this inertia effect. The name given to the opposing force is Inductive Reactance because it is the reaction of the inductor to the changing value of alternating current and is measured in Ohms. It cannot be called resistance since it is not the result of friction within a conductor. The sine wave below is compared to a circle since the time of one cycle of a sine wave can be marked off into 360 degrees.

The formula for inductive reactance is shown below where the induced voltage increases with the increase in inductance. This is because the induced voltage in a conductor is proportional to the rate at which magnetic lines of force cut the conductor. So the greater the rate or the higher the frequency, the greater the CEMF.

Capacitive Reactance in AC

The variation of the alternating voltage and current in a capacitive circuit for each one cycle is shown in the four parts of figure below. The voltage across the capacitor is represented by the solid line while the current is represented by the dotted line. For both the voltage and the current, the line running through the center is the reference point or the zero. The time of the cycle is marked off by the bottom line in terms of electrical degrees.

A very real opposition to current flow is offered by capacitors themselves which arises from the fact that the number of electrons going back and forth from plate to plate is limited by the storage ability (capacitance) of the capacitor. The opposition offered by a capacitor is decreased by increasing the frequency. The opposition is called Capacitive Reactance where the capacitive reactance decreases with increasing frequency or for a given frequency, the capacitive reactance decreases with increasing capacitance.

References
http://www.tpub.com/neets/book2/4b.htm
http://www.tpub.com/neets/book2/4a.htm

Image Sources
http://www.sayedsaad.com
http://www.electricianeducation.com