What Is Impedance? A Plain-Language Guide to Resistance, Inductive Reactance, and Capacitive Reactance

If you’re still building your foundation in basic electricity, start with this beginner-friendly overview: 🔹 “Electricity 101: The Complete Beginner’s Guide to How Power Really Works”
After reading it, the concepts in this article will make a lot more sense.

What is impedance?
If you’re just starting to learn AC circuits, studying for an exam, or trying to understand what’s really happening in your home’s 120V / 240V wiring, this article will walk you through what impedance is and how it works in plain language.

We’ll see how impedance Z is built from resistance R, inductive reactance Xₗ, and capacitive reactance X꜀, and how they affect current, voltage, and frequency in real-world AC circuits, so that by the end you can clearly explain what impedance is to someone else.

Watch first: the three pieces of impedance — R, Xₗ, and X꜀

If you just typed “what is impedance” into Google, don’t scroll away yet.
Start with this 1-minute short video. It gives you a very visual way to remember the three key players in impedance (Z):

• Resistance R
• Inductive reactance Xₗ
• Capacitive reactance X꜀

and how they work together to decide how easily current can flow in an AC circuit.

Whether you’re preparing for an electrician exam, taking a trade-school class, or simply trying to understand your home’s 120V / 240V power, you will run into impedance sooner or later.
Let the video plant the overall picture in your head first, then we’ll unpack everything with text and examples.

What is impedance? Start with a one-sentence definition

When people ask “what is impedance in AC circuits?”, this is the simplest way to answer it. Let’s pin down a clear definition first:

Impedance (Z) is the “total opposition” to AC current in a circuit. It includes resistance, inductive reactance, and capacitive reactance.

In DC circuits, we usually only talk about “resistance R,” because the voltage and current don’t keep changing direction.

But in AC (alternating current), voltage and current are always swinging up and down. Now the circuit doesn’t just “burn power” as heat — we also get:

• Resistance R: honestly turns electrical energy into heat, not very sensitive to frequency.
• Inductive reactance Xₗ: comes from inductors, which hate it when current changes too fast.
• Capacitive reactance X꜀: comes from capacitors, which hate it when voltage changes too slowly.

Add those three together and you get Z (impedance) — the quantity we use all the time when designing AC circuits.

You can think of impedance as:

“The actual total opposition that AC current feels at a given frequency.”

A lot of people see the question “What is impedance?” in a textbook or exam and assume it’s just another word for resistance. In reality, impedance hides a bigger story: frequency, phase angle, and how resistance and reactance work together.

If you’d like to see a more formal, math-heavy explanation of what impedance is, you can also check the electrical impedance overview on Wikipedia. In this guide, we’ll stay focused on practical, exam-friendly intuition.

In this article, you’ll learn:

• What resistance, inductive reactance, and capacitive reactance each represent — and their formulas.
• Why inductive and capacitive reactance depend on frequency.
• How to calculate impedance Z, and what “inductive” vs “capacitive” circuits mean.
• Where these ideas show up in real life: home wiring, audio, power filters, and communication circuits.

Resistance (R) — the part that simply “eats power” as heat

Resistance: not very interested in frequency

Resistance is the most intuitive component. When current flows through a resistor, part of the electrical energy turns into heat. It follows Ohm’s Law:

V = I × R

Where:

• V = voltage (volts)
• I = current (amps)
• R = resistance (ohms, Ω)

In most real-world situations, the value of a resistor doesn’t change much with frequency.

So in both DC and AC circuits, its job is pretty straightforward:

• limit current
• divide voltage
• and generate heat as power is dissipated

Common applications: from phone chargers to space heaters

You’ll see resistance used to:

• Limit current: keep LEDs and ICs from being destroyed by too much current.
• Divide voltage: use two resistors to “slice” a high voltage into several lower voltages.
• Generate heat on purpose: in space heaters, irons, electric kettles, and toasters, the heating element is basically a big resistor.

Inductive reactance (Xₗ) — the inductor telling AC current to “slow down”

Inductive reactance: the higher the frequency, the more it resists current changes

Think of an inductor as a coil of wire. When current flows through it, a magnetic field builds up around it.

If the current tries to speed up or slow down suddenly, the magnetic field changes. That changing field induces a voltage that pushes back against the current change.

This “I don’t want the current to change so fast” behavior is what we call inductive reactance, Xₗ.

The formula is:

Xₗ = 2π f L

Where:

• Xₗ = inductive reactance (Ω)
• f = frequency (Hz)
• L = inductance (H, henries)

From this formula we can see:

• The higher the frequency f, the larger Xₗ → high-frequency signals have a harder time getting through an inductor.
• The larger the inductance L, the larger Xₗ → a “bigger” coil fights current changes even more.

Where does inductive reactance show up?

You’ll bump into inductive reactance in many places:

• Power line filters: before AC enters a power supply, inductors help block high-frequency noise on top of the 50/60 Hz mains.
• Motors and transformers: if there’s a coil and an iron core, there’s inductive reactance.
• High-frequency/RF circuits: tiny inductors are used to tune frequency, build filters, and match impedances in radios and wireless systems.

Capacitive reactance (X꜀) — the capacitor telling AC to “move faster”

Capacitive reactance: the higher the frequency, the easier it lets current through

A capacitor is two conductive plates separated by an insulator. With AC voltage across it, the capacitor keeps charging and discharging.

In effect, current can start flowing before the voltage has fully “caught up.”

The “opposition” a capacitor gives to AC is called capacitive reactance, X꜀, and the formula is:

X꜀ = 1 / (2π f C)

Where:

• X꜀ = capacitive reactance (Ω)
• f = frequency (Hz)
• C = capacitance (F, farads)

This time, the relationship flips:

• The higher the frequency f, the smaller X꜀ → high-frequency signals pass through a capacitor more easily.
• The larger the capacitance C, the smaller X꜀ → a large capacitor looks more like a “short” to AC.

Where does capacitive reactance show up?

Typical examples:

• Coupling capacitors: block DC while letting AC signals pass (for example, between amplifier stages in audio gear).
• Power supply filters: used with inductors to smooth voltage and reduce ripple.
• Wireless and communication circuits: used in filters, frequency-shaping networks, and antenna matching.

Impedance and phase: inductive or capacitive? One formula tells you

So far we’ve mostly answered “what is impedance” in terms of its size in ohms. To really see what’s happening in AC circuits, we also need to look at how impedance affects the phase between voltage and current.

In AC analysis, we combine R, Xₗ, and X꜀ into a single complex vector called impedance Z.

For a simple series circuit, the magnitude of Z is:

Z = √(R² + (Xₗ − X꜀)²)

Conceptually, you can remember it this way:

• R lies on the real axis → it represents power that’s actually consumed (heat).
• (Xₗ − X꜀) lies on the imaginary axis → it represents the phase difference between voltage and current.

Depending on the relative sizes of Xₗ and X꜀, the circuit can behave in three main ways:

• Xₗ > X꜀ → the circuit is inductive, and current lags voltage.
• X꜀ > Xₗ → the circuit is capacitive, and current leads voltage.
• Xₗ = X꜀ → the circuit is at resonance and behaves like pure resistance. This is extremely important in oscillators, wireless circuits, and many kinds of filters.

You don’t have to memorize all the complex-number math right away.

For now, just keep this picture in mind:

Impedance Z is like a slanted arrow. The horizontal part is R (resistance), and the vertical part is the net reactance (Xₗ − X꜀), pointing up for inductive and down for capacitive.

Later, when you get into more advanced electrical, electronics, or communication courses, this mental picture will make the math much easier to digest.

Simple experiment: actually “see” how impedance shapes the waveform

Materials (for a hands-on lab)

If you have access to a basic lab, you can try this yourself:

• Resistor (around 1 kΩ)
• Inductor (around 10 mH)
• Capacitor (around 100 μF)
• Function generator
• Oscilloscope

Steps

1. Start with just the resistor
   Put the resistor between the AC signal source and the load. Change the frequency and watch the voltage and current waveforms.
   You’ll notice the resistor doesn’t really care about frequency — behavior stays about the same.
2. Swap in the inductor
   Use the same setup, but replace the resistor with the inductor. Increase the frequency.
   As frequency goes up, the current gets smaller, and you’ll likely see a clear phase shift between voltage and current.
3. Swap in the capacitor
   Now use the capacitor instead.
   As you change frequency, you’ll see the opposite behavior: higher frequencies pass more easily, while low-frequency components are blocked more.

If you don’t have hardware on hand, you can do the same thing in a free circuit simulator like Falstad Circuit Simulator or LTspice: connect R, L, and C in the same spots, sweep the frequency, and watch the waveforms change.

🔍 What you should observe:

• Resistor: current depends mainly on V and R; frequency doesn’t matter much.
• Inductor: as frequency increases, current decreases → Xₗ grows with frequency.
• Capacitor: as frequency increases, current increases → X꜀ shrinks as frequency goes up.

That’s impedance in action, in a way your eyes can literally see — a very concrete way to answer the question “what is impedance?” without staring only at formulas.

Impedance FAQ

If you made it this far and the question “what is impedance?” still feels a bit fuzzy, start with a few small R–L–C examples and use the formulas until it becomes second nature.

Summary: once you get impedance, AC circuits start to “make sense”

At this point, you can clearly answer the core question people type into Google: “what is impedance?”

What is impedance?
Impedance is the total opposition to AC current in a circuit, combining resistance, inductive reactance, and capacitive reactance. It changes with frequency and sets the phase relationship between voltage and current.

Resistance quietly burns power as heat.
Inductors fight sudden changes in current.
Capacitors make it easier for high-frequency signals to move.

Together, they decide whether a circuit behaves more inductively, more capacitively, or sits right at resonance.

No matter where you go next — electrician licensing, electronics design, industrial power systems, or simply being a homeowner who actually understands their electrical system — impedance is the common language of the AC world.

Once you clear this level, topics like transformers, motors, EMI filters, and wireless communication will feel much less mysterious.

Further reading

If you want to go deeper after learning the basics of impedance, these related guides will help you review each part of the concept step by step:

“What Is Resistance? The Key Player in Every Circuit”
Start from the basics of resistance, power, and voltage division so the “R” part of impedance feels rock-solid.

“How Do Inductors Affect a Circuit? Basic Theory and Real-World Uses”
Go deeper on inductors: power supplies, filtering, wireless charging, and more.

“What Are Capacitors Used For? How They Store and Release Energy”
Fill in the capacitor side: coupling, filtering, decoupling, timing circuits — where capacitive reactance really matters.

“How to Calculate Impedance in AC Circuits” (coming soon)
A step-by-step guide to practice series and parallel R–L–C examples using both formulas and vector diagrams.