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What is Reactance?

The imaginary part of impedance: how strongly a coil or capacitor opposes an AC current [Ω]. A coil has XL = ωL = 2πfL, which grows in proportion to frequency, and a capacitor has XC = 1/(ωC) = 1/(2πfC), which shrinks in inverse proportion to frequency. For example, a 1 µF capacitor has about 159 Ω at 1 kHz. Unlike resistance it consumes no power, and it shifts the phase between voltage and current by 90°.

Read the beginner explainer: AC vs DC Explained: Impedance, Phase and Power Factor

Japanese name: リアクタンス

Reactance appears in 9 practice questions across 3 topics, including Circuit Analysis: AC, Linear Circuits and Transients, Circuit and electromagnetism basics, Analog and electronic circuits.

Thevenin equivalent circuit with a load impedance Vth Zth a b ZL
Thevenin equivalent circuit with a load impedance · View in diagrams

Field: Circuit Analysis: AC, Linear Circuits and Transients

Try it: questions about Reactance

What is the magnitude of the impedance of a circuit with a resistor R = 30 Ω in series with a coil of reactance XL = 40 Ω? (Ignore the coil's resistance.)

  1. A70 Ω
  2. B10 Ω
  3. C50 Ω
Show answer and explanation

Answer: C. 50 Ω

The voltages across a resistor and an inductor are 90° out of phase, so they are combined as complex numbers (vectors), not added directly. Z = R + jXL = 30 + j40 Ω, with magnitude √(30² + 40²) = √2500 = 50 Ω. The current lags the voltage by φ = tan⁻¹(40/30) ≈ 53°, and the power factor is cos φ = 30/50 = 0.6.

Reactance in the explanations

Questions that mention Reactance (9)

  1. Which is closest to the reactance of a 1 µF capacitor at a frequency of 1 kHz?
  2. What is the magnitude of the impedance of a circuit with a resistor R = 30 Ω in series with a coil of reactance XL = 40 Ω? (Ignore the coil's resistance.)
  3. A sinusoidal voltage of 100 V RMS is applied to a load consisting of a resistor R = 8 Ω in series with a coil of reactance X = 6 Ω. Which of the following is the active power of the load?
  4. As in the figure, a load impedance ZL is connected to terminals a-b of an AC circuit whose Thevenin equivalent voltage is Vth = 10 V RMS and internal impedance Zth = 3 + j4 Ω. If both the resistance and the reactance of ZL can be chosen freely, which of the following is closest to the maximum power consumed by the load?
  5. In the circuit in figure (a), R = 10 Ω, the capacitor reactance at the operating angular frequency is 1/(ωC) = 10 Ω, and the input voltage is vin = 10∠0° V (RMS). Which of the following is the internal impedance Zth of the Thevenin equivalent circuit seen from the output terminals?
  6. How does the magnitude of a capacitor's impedance (capacitive reactance) change as frequency increases?
  7. Which is closest to the resonant frequency of a series resonant circuit with L = 1 mH and C = 10 nF?
  8. In the series RLC circuit in the figure, R = 10 Ω, L = 10 mH and C = 1 µF, and the frequency of a 1 V RMS sinusoidal voltage is set to the resonant frequency. Which of the following is the RMS voltage VC across the capacitor?
  9. Which expression gives the resonant frequency of a series LC circuit?

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Related terms

Impedance Resonant circuit Power factor Thevenin's theorem Reactive power Phasor Q factor (quality factor) Admittance Mesh analysis Nodal analysis

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