Impedance describes opposition to which current?

• A. Direct current
• B. Alternating current
• C. Pulsed current
• D. No current

Answer: (B)

Impedance describes opposition to current that changes with time. It’s not just resistance (which is the opposition to current in a pure, constant flow) but also reactance, which comes from energy-storage elements like inductors and capacitors and depends on how fast the voltage or current is changing. Because reactance depends on frequency, impedance is defined and most useful for alternating current, where the current continuously varies. In direct current, the current is constant, so there’s no reactance—the opposition is simply resistance. Pulsed current isn’t a steady AC waveform, so impedance isn’t used in the same way, though you can analyze instantaneous or dynamic behavior in other terms. So impedance is the opposition to alternating current.