What do inductors oppose?

• A. Changes in voltage
• B. Changes in current
• C. Changes in resistance
• D. Changes in frequency

Answer: (B)

Inductors oppose changes in current. Their voltage-current relationship is v = L di/dt, so the rate at which current changes determines the voltage the inductor develops. When current tries to rise or fall, the changing magnetic flux induces an emf that acts to oppose that change, per Lenz’s law. This is why a constant DC current passes through an ideal inductor with little opposition after transients die out, while a changing current experiences a growing opposition—especially at higher frequencies, where the inductive reactance X_L = ωL becomes larger. The magnetic energy stored, W = 1/2 L I^2, grows as the current increases, illustrating why changing current requires work from the source.