Understanding Reactance — Opposition to Change in AC Circuits

When we first learn about circuits, resistance feels like the only opposition to current. But as soon as we move into AC circuits, where voltage and current are constantly changing, a new concept comes into play — reactance.

In this blog, we’ll explore what reactance is, why it exists, and how it shapes the behavior of capacitors and inductors in AC systems.

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⚡ What is Reactance?

When we first learn about circuits, we meet resistance — the opposition that limits how much current flows. But the moment we move from DC (Direct Current) to AC (Alternating Current), the story becomes much more interesting.

Here, the voltage and current are continuously changing, and this change introduces a new kind of opposition called Reactance.

Let’s explore what it really is, why it matters, and how it connects to a concept called Impedance — the total opposition in AC circuits.

In simple terms, reactance is the opposition that a capacitor or inductor offers to the flow of alternating current (AC).

It’s similar to resistance but with a key difference:

• Resistance opposes all current, whether AC or DC.

• Reactance only opposes changing current — the kind that goes up and down, like in AC.

When voltage and current are constantly varying, capacitors and inductors react to that change.

• Capacitors store energy in the electric field.

• Inductors store energy in the magnetic field.

Unlike resistors, these components don’t waste energy as heat. They temporarily store energy and return it to the circuit. This storage and return create a delay in current or voltage, and that delay is what we call reactance.

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🔧 Why is Reactance Needed?

In an AC system, voltage and current are not steady — they continuously rise, fall, and even reverse direction.

This constant change interacts with capacitors and inductors, which try to “resist” or “delay” the change due to how they store energy.

• The inductor resists a change in current because it builds a magnetic field around itself.

• The capacitor resists a change in voltage because it builds up an electric field between its plates.

This time-dependent opposition is exactly what allows us to design filters, amplifiers, and tuned circuits — all of which depend on reactance.

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⚖️ Reactance vs. Resistance

So, we don’t use the same word “resistance” for both because:

• Resistance dissipates energy as heat.

• Reactance stores and gives back energy.

Their effects on current and voltage are fundamentally different.

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🌀 Inductive Reactance — Opposition from a Coil

An inductor is simply a coil of wire that stores energy in a magnetic field when current passes through it.

But when you try to change that current, the inductor resists (thanks to Lenz’s Law).

The fundamental relation for an ideal inductor is:

V = L (dI/dt)

Where:

• V = voltage across the inductor

• L = inductance (Henry)

• dI/dt = rate of change of current

This means the voltage depends on how quickly the current changes.

If the current changes fast, the voltage across the inductor becomes large.

Here’s what happens when AC is applied:

• The inductor initially resists the change — it takes time to build a magnetic field.

• The applied voltage goes into creating that magnetic field instead of immediately increasing current.

• Once the current flows, it continues even as voltage starts to fall, because the inductor releases its stored energy.

This delay means current lags behind voltage by 90° in a pure inductor.

Inductive reactance is given by:

Xₗ = 2πfL

Where:

• f = frequency (Hz)

• L = inductance (H)

So, as frequency increases, inductive reactance also increases.

That’s why:

• For DC (f = 0), Xₗ = 0 → acts like a short circuit.

• For high-frequency AC, Xₗ becomes very large → acts like an open circuit.

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🔵 Capacitive Reactance — Opposition from a Capacitor

For a capacitor, the voltage and current relationship is:

i(t) = C (dv/dt)

Where:

• i(t) = current through the capacitor

• v(t) = voltage across the capacitor

• C = capacitance (Farad)

This equation shows that the current depends on how fast the voltage changes — not its actual value.

What really happens in AC:

• As voltage begins to rise, charges start moving instantly — current begins flowing immediately.

• It takes some time before enough charge accumulates for voltage to build up across the plates.

• Hence, current reaches its peak first — when voltage is changing fastest.

• Voltage reaches its maximum later — about a quarter cycle (90°) later.

Thus, current leads voltage by 90° in a pure capacitor.

Capacitive reactance is given by:

X꜀ = 1 / (2πfC)

So, as frequency increases, capacitive reactance decreases.

That’s why:

• Capacitors pass high-frequency signals easily.

• But they block DC (since f = 0 ⇒ X꜀ = ∞).

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🔄 The Two Types of Reactance

If you observe closely, they’re complete opposites in nature:

• In an inductor, voltage leads current.

• In a capacitor, current leads voltage.

• Inductive reactance increases with frequency, while capacitive reactance decreases.

When both are present in a circuit, their effects can even cancel each other out — a concept known as resonance.

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🔚 Conclusion

Reactance is a fundamental concept that explains how inductors and capacitors behave in AC circuits. Unlike resistance, which simply dissipates energy, reactance arises because energy is temporarily stored and returned through magnetic and electric fields.

Inductive reactance opposes changes in current, while capacitive reactance opposes changes in voltage. Their frequency-dependent nature allows engineers to design filters, oscillators, tuned circuits, and frequency-selective systems.

Understanding reactance is a crucial step toward mastering AC circuit analysis and leads naturally into deeper topics like impedance, resonance, and AC power systems.