Inductive Reactance And Capacitive Reactance

When alternating current (AC) circuits are studied, two key concepts that emerge are inductive reactance and capacitive reactance. These are the measures of how inductors and capacitors oppose the flow of alternating current. Inductors and capacitors behave differently under the application of AC. Inductive reactance is usually related to the magnetic field produced by the current-carrying coil or wire, whereas capacitive reactance is correlated to the changing electric field between two conducting plates or surfaces that are separated by some insulating medium.

Inductive reactance is the resistance provided by the inductor in an AC circuit to the flow of AC current. It is denoted by (XL) and expressed in units of ohms (Ω). Inductive reactance is low in the case of lower frequencies and high in the case of higher frequencies. For DC current, it is negligible. An inductor is a loop of wire that accumulates energy in the form of a magnetic field when current passes through it. In AC circuits, the current is always reversing direction, so the magnetic field surrounding the inductor is also always reversing. According to Faraday's Law of Electromagnetic Induction, the reversing magnetic field creates an opposing voltage, or back emf (electromotive force), in the coil. This opposing voltage opposes changing current.

The formula for inductive reactance is:

formula

Through this formula, we know that inductive reactance increases with frequency. That means at higher frequencies, an inductor offers greater resistance to the AC current.

(where f = 0), the inductive reactance is zero, and the inductor behaves like a simple wire. In an AC circuit, the current in an inductor lags behind the voltage by 90 degrees. This lag is due to the back emf opposing the change in current.

formula

From the formula above, we can understand that capacitive reactance is lower with higher frequencies. This means that capacitors offer less opposition to high-frequency signals. At very low frequencies (in DC, where f = 0), the capacitive reactance is very high and practically blocks DC current. In an AC circuit, the capacitor current leads the voltage by 90 degrees. This is due to the fact that the capacitor begins to pass current as soon as the voltage starts to change, even before the voltage reaches its peak.

Graph Representation

formula

The graph indicates the phase relationship between current (I) and voltage (V) in capacitive and inductive AC circuits. In the inductive circuit (upper graph), voltage precedes current by 90° because the inductor resists a change in current. In the capacitive circuit (lower graph), current precedes voltage by 90° because the capacitor enables current to respond rapidly to changes in voltage. Therefore, voltage leads current in inductive circuits and current leads voltage in capacitive circuits.

Difference between Inductive Reactance and Capacitive Reactance

Characteristics	Inductive Reactance (XL)	Capacitive Reactance (XC)
Device	Inductor	Capacitor
Formula
Depends on	Frequency (f) and inductance (L)	Frequency (f) and capacitance (C)
Effect of Frequency	Increases with an increase in frequency	Decreases with an increase in frequency
Behaviour in DC (f = 0)
Phase	Current lags voltage by 90°	Current leads voltage by 90°
Energy Stored	In the form of a magnetic field	In the form of an electric field

Summing Up

Inductive and capacitive reactance are fundamental in learning why AC circuits will behave differently from DC circuits. Inductive reactance goes up with frequency and makes current lag behind voltage, whereas capacitive reactance goes down with frequency and makes current lead voltage. Both are crucial in regulating current flow.