Automotive ignition coils: governing electromagnetic principle Traditional spark-ignition systems in automobiles step up 12 V battery voltage to tens of kilovolts to produce a spark. Which principle enables this voltage transformation in the ignition coil?

Introduction / Context:
Automobile ignition systems must convert low DC battery voltage into a very high voltage to ionize the spark-plug gap. The ignition coil is essentially a transformer adapted for pulsed operation.

Given Data / Assumptions:

• Primary winding connected to a switched 12 V source (breaker points or solid-state driver).
• Secondary winding with many more turns than the primary.
• Magnetic core provides coupling between windings.

Concept / Approach:
When current in the primary changes rapidly (opened points or transistor turn-off), the time-varying magnetic flux links both windings. By Faraday’s law, induced emf in the secondary is proportional to the turns N2 and rate of change of flux. The step-up ratio is approximately N2 / N1, demonstrating transformer, i.e., mutual inductance, action.

Step-by-Step Solution:
Primary current builds magnetic flux in the iron core.Sudden interruption causes large dI/dt in primary and hence large dΦ/dt in the core.Secondary experiences induced emf: V2 ≈ − N2 * dΦ/dt; with N2 ≫ N1, V2 reaches tens of kilovolts.Therefore, the operative principle is mutual inductance (transformer action).

Verification / Alternative check:
Modern coil-on-plug systems still rely on coupled windings; the presence of a primary and secondary confirms transformer behavior rather than capacitance-based multiplication.

Why Other Options Are Wrong:
Self-inductance contributes to primary voltage spike but does not alone provide the large step-up ratio. Capacitance and resistance are not voltage-transforming mechanisms. Electrostatic induction without magnetic coupling cannot deliver required power.

Common Pitfalls:
Confusing the condenser (capacitor) across breaker points—which reduces arcing—with the transformer principle that generates the high secondary voltage.

Final Answer:
Mutual inductance (transformer action)