What is the formula for inductive reactance?

• A. XL = 2π f / L
• B. XL = 2π f L
• C. XL = fL
• D. XL = L/(2π f)

Answer: (B)

Inductive reactance shows how strongly an inductor resists changes in current in an AC circuit, and it grows with both the frequency of the signal and the inductance value. The standard relation is XL = ωL, where ω is the angular frequency. Since ω = 2πf, this becomes XL = 2π f L. So doubling the frequency or doubling the inductance doubles the reactance, meaning the inductor resists the current change more at higher frequencies or with a larger coil. Forms that divide by L, omit the 2π factor, or place L in the numerator in the wrong way do not match this behavior or the units, so they don’t describe inductive reactance correctly.