Energy and polarity in an inductor during a decreasing current When the current through an ideal inductor is forced to decrease, what happens to the inductor’s magnetic field and to the instantaneous voltage polarity across the inductor terminals?

Introduction / Context:
Inductors store energy in magnetic fields. The way they respond to changes in current is central to switching power supplies, flyback circuits, and transient suppression. Understanding field behavior and voltage polarity during current decay prevents design errors and device stress.

Given Data / Assumptions:

• Ideal inductor with inductance L.
• Current is decreasing (di/dt < 0).
• We adopt the passive sign convention: the indicated terminal voltage is positive for current entering the labeled positive terminal.

Concept / Approach:
The inductor relation is v = L * di/dt. If di/dt is negative (current decreasing), v becomes negative with respect to the passive sign convention, meaning the inductor’s polarity flips to oppose the decrease. Simultaneously, the magnetic field energy W = 0.5 * L * I^2 must diminish as current falls, so the field collapses, returning energy to the circuit. Lenz’s law captures both effects: the induced voltage polarity always opposes the change in current.

Step-by-Step Solution:

Verification / Alternative check:
Open an inductive circuit: a high-voltage spike of opposite polarity appears, often clamped by a flyback diode. The spike’s polarity is consistent with maintaining current flow briefly despite the interruption.

Why Other Options Are Wrong:

Common Pitfalls:
Forgetting sign conventions; assuming the inductor “blocks” changes entirely—instead, it resists them by generating an opposing voltage.

Final Answer:
collapses, reverses