Faraday–Lenz principle: does the induced voltage polarity in a coil oppose the change in current that created it?

Introduction / Context:
Induction underlies inductors, transformers, and many sensors. Faraday’s law quantifies induced electromotive force (emf), and Lenz’s law specifies its polarity. Together they explain why an inductor “resists” changes in current by developing a voltage that counteracts the change.

Given Data / Assumptions:

• Single coil carrying a time-varying current.
• Magnetic coupling is localized to the coil’s own field (self-inductance).
• Linear operation without core saturation.

Concept / Approach:
Faraday: induced emf magnitude is proportional to the time rate of change of magnetic flux linkage. For an inductor, v_L = L * di/dt. Lenz: the induced voltage polarity is such that the resulting current opposes the change in flux that produced it. Hence, if the current attempts to increase, the inductor produces a voltage that drops opposite to the source; if the current attempts to decrease, the inductor generates voltage that tries to keep current flowing in the same direction.

Step-by-Step Solution:

Verification / Alternative check:
Scope a switching inductor: during turn-off, the node voltage spikes of opposite polarity appear, consistent with v_L = L * di/dt and Lenz’s law.

Why Other Options Are Wrong:
Incorrect: contradicts fundamental induction behavior.
“True only for DC” or “only with iron core”: the law holds for AC and DC transients and does not require ferromagnetic cores.

Common Pitfalls:
Confusing “opposes current” with “blocks DC” (an inductor passes DC once steady state is reached); misreading polarity during switching.

Final Answer:
Correct