Capacitors and inductors oppose current flow too, but they do it in a way that resistors never can, storing energy instead of burning it off, and shifting current out of step with voltage. Here is exactly what reactance is, with real phasor diagrams and a live reactance vs frequency calculator you can try right now.
What is Reactance?
Reactance is the opposition to current flow caused by a capacitor's or inductor's ability to store energy, and unlike resistance, it depends directly on the frequency of the applied signal.
Where resistance permanently dissipates energy as heat, reactance temporarily stores it in an electric or magnetic field and hands it back to the circuit a moment later. That distinction is exactly why reactance behaves so differently, and it is the missing half of impedance, the complete AC opposition to current.
Understanding what is reactance means grasping the three surprising facts covered in this guide, from why it never burns energy to why capacitors and inductors respond to frequency in completely opposite directions.
Capacitive Reactance vs Inductive Reactance
Before comparing the details, it helps to see each type's basic identity side by side.
Capacitive Reactance (Xc)
Opposition from a capacitor's electric field, which decreases as frequency rises. Current leads voltage by 90 degrees in a purely capacitive circuit.
Formula: Xc = 1 / (2πfC)
Inductive Reactance (XL)
Opposition from an inductor's magnetic field, which increases as frequency rises. Current lags voltage by 90 degrees in a purely inductive circuit.
Formula: XL = 2πfL
3 Surprising Facts About Reactance
Answering what is reactance fully means understanding these three facts, which explain why it behaves so differently from ordinary resistance.
Reactance Stores Energy, It Does Not Dissipate It
An ideal capacitor or inductor returns every bit of energy it temporarily stores back to the circuit. Unlike resistance, reactance produces no permanent heat loss at all in the ideal case.
Capacitive and Inductive Reactance Move in Opposite Directions
As frequency rises, capacitive reactance falls while inductive reactance climbs. This opposite behavior is exactly why the two can cancel each other out at a specific frequency, called resonance.
Reactance Shifts Current Out of Phase With Voltage
Resistance keeps voltage and current perfectly in step. Reactance introduces a 90 degree phase shift instead, current leading in a capacitor, lagging in an inductor, which is exactly what separates real power from reactive power.
Capacitive Circuit Phasor Diagram
Notice how the current phasor sits 90 degrees ahead of the voltage phasor in an ideal capacitive circuit.
What is happening: A capacitor's current is largest at the exact moment its voltage is changing fastest, which turns out to happen a quarter cycle before the voltage waveform itself reaches its peak.
A real example: Feed a 60Hz sine wave into a pure capacitor, and its current waveform will visibly lead the voltage waveform by exactly a quarter of a full cycle, regardless of the specific capacitance value.
Why it works: A capacitor's current depends on how fast its voltage is changing, not on the voltage's instantaneous value, which mathematically produces exactly this 90 degree leading phase relationship.
Reactance vs Frequency
This opposite relationship with frequency is the single most important practical fact about reactance.
This opposite relationship is really the entire answer to what is reactance in practical filter design. A capacitor placed in a signal path naturally attenuates low frequencies more than high ones, while an inductor does the exact reverse, and combining the two in the right arrangement produces the low-pass, high-pass, and band-pass filters found throughout analog electronics.
Reactance Formulas
Both types of reactance reduce to a compact formula involving frequency and the component value.
Capacitive reactance: XC = 1 / (2πfC)
Inductive reactance: XL = 2πfL
Worked example: f = 60Hz, C = 10µF
Xc = 1 / (2π × 60 × 0.00001) = 265.3Ω
Try It: Reactance vs Frequency Calculator
Pick a component type and value, then enter a frequency to see the resulting reactance.
Reactance Compared to Resistance and Impedance
| Feature | Reactance | Resistance |
|---|---|---|
| Energy Behavior | Stores and releases energy | Dissipates energy as heat |
| Frequency Dependence | Changes directly with frequency | None, ideally constant |
| Phase Effect | Causes a 90° phase shift | Voltage and current stay in phase |
| DC Behavior | Infinite for capacitors, zero for inductors | Unchanged regardless of frequency |
What is Reactance Used For? Real Applications
Filter Design
Capacitors and inductors shape frequency response in low-pass, high-pass, and band-pass filters.
Resonant Tuning Circuits
Radio tuners use the frequency where capacitive and inductive reactance cancel to select a station.
Power Factor Correction
Capacitor banks offset inductive reactance from motors, improving power factor on the grid.
Switching Power Supplies
Inductors and capacitors store and release energy each switching cycle in DC-DC converters.
Impedance Matching
Reactive components cancel unwanted reactance to maximize power transfer in RF systems.
Audio Crossover Networks
Reactance routes different frequency bands to the correct speaker driver in an audio system.
Every one of these six applications relies on the same basic property described throughout this guide: reactance changes predictably with frequency, and that predictability is precisely what makes it useful as a design tool rather than just an unavoidable circuit quirk.
What is Reactance Good For: Advantages and Limitations
Why Reactance Is So Useful
Limitations to Keep in Mind
Download Reactance References
These two academic references go deeper into what is reactance and its role within impedance.
Reactance and Impedance: AC Voltage and Current
San Jose State University course supplement on reactance and phasor diagrams
AC Reactance and Impedance Tech Note
Michigan State University tech note covering reactance calculations
Watch: Resistance and Reactance Explained
This video explains the core concepts of resistance and reactance in AC circuits with visuals.
FAQs on What is Reactance
These questions about what is reactance come up constantly in AC circuit fundamentals coursework.
External References
- San Jose State University, Reactance and Impedance, AC Voltage and Current
- Michigan State University, Tech Note 221b, AC Reactance and Impedance
- Electrical4U, Electrical Reactance: What is it?
- YouTube, What is Resistance and Reactance Explained
What we learn today
- Reactance is the opposition to current from a capacitor's or inductor's ability to store energy, unlike resistance which dissipates it.
- Capacitive reactance falls as frequency rises, while inductive reactance rises with frequency, an exactly opposite relationship.
- Reactance shifts current 90 degrees out of phase with voltage, leading in capacitors and lagging in inductors.
- At resonance, capacitive and inductive reactance cancel exactly, leaving only resistance to oppose current.
- Reactance forms the imaginary, frequency-dependent half of impedance, the more complete AC opposition to current.