What is the behavior of an inductor in a DC circuit after a long time?

• A. It behaves as a current source.
• B. It behaves as an open circuit.
• C. It behaves as a short circuit, allowing DC current with minimal opposition (neglecting resistance).
• D. It blocks DC current completely.

Answer: (C)

In DC steady state, the current through an inductor is constant, so the rate of change of current di/dt is zero. Since the voltage across an inductor is V = L di/dt, that voltage becomes zero. With essentially zero voltage across it and, in the ideal case, negligible resistance, the inductor offers almost no opposition to the DC current. In other words, it behaves like a short circuit after a long time. Real inductors do have winding resistance, so there may be a small voltage drop, but the fundamental behavior is that DC passes through as if it were a short.