According to Lenz’s law and basic induction, when a magnetic field linked to a coil collapses, what is the polarity relationship of the induced voltage relative to the original cause (the force creating the field)?

Difficulty: Easy

Correct Answer: opposite to the force creating the field

Explanation:


Introduction / Context:
Lenz’s law provides the “minus sign” in Faraday’s induction law, indicating that induced effects oppose the change that produced them. This rule explains why inductors generate a flyback voltage when current is interrupted and why energy stored in magnetic fields tries to sustain current flow.



Given Data / Assumptions:

  • A coil with collapsing magnetic flux linkage (dΦ/dt < 0).
  • We examine the polarity of the induced EMF.
  • Linear behavior and standard sign conventions.


Concept / Approach:
Faraday’s law gives magnitude proportional to |dΦ/dt|. Lenz’s law sets polarity so that the induced current’s magnetic field opposes the change in flux. During collapse, the induced EMF polarizes itself to keep current flowing in the original direction, thereby opposing the reduction in flux—hence it is opposite to the original driving “force.”



Step-by-Step Solution:
Field decreases → induced EMF appears.Induced EMF polarity causes current that attempts to maintain original flux.Therefore, induced voltage is opposite to the cause of the original field.This manifests as a reverse-polarity “kick” across an inductor when switched off.


Verification / Alternative check:
Observe relay coils without snubbers: switch-off generates a high opposite-polarity spike. Adding a flyback diode clamps this spike by providing a path for the energy to dissipate safely.



Why Other Options Are Wrong:
Independence or randomness contradicts the deterministic nature of Lenz’s law.

“Identical” polarity would reinforce change and violate energy conservation.

Stationary force is irrelevant; change in flux is the trigger for induction.



Common Pitfalls:
Confusing “opposes the change” with “opposes the steady value.” The induced effect resists change, not necessarily the original field itself if it were steady.



Final Answer:
opposite to the force creating the field

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