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Difficulty: MediumElectromagnetism and Induction

When the direct current flowing through an energized inductor coil is suddenly interrupted by opening a switch, a high-voltage spark often arcs across the switch contacts. Which electromagnetic effect causes this sudden voltage surge?

  1. A
    Electrostatic discharge released from energy stored in the surrounding electric field
  2. Counter-electromotive force (CEMF) induced by the rapidly collapsing magnetic fieldAnswer
  3. C
    A continuous direct current forcing its way across the newly formed air gap
  4. D
    A sudden surge of forward current resulting from zero resistance across the contacts

Answer

Counter-electromotive force (CEMF) induced by the rapidly collapsing magnetic field causes the high-voltage spike.
According to Faraday's law of induction and Lenz's law, when current through an inductor is suddenly cut, the magnetic field around the coil rapidly collapses. Because the rate of change of magnetic flux is extremely high, the coil generates a massive counter-electromotive force (CEMF) or inductive kickback voltage that arcs across the opening switch contacts.

Step-by-Step Solution

1
Analyze the state of the circuit before and after the switch opens
Steady current established a magnetic field around the inductor coil; opening the switch abruptly drops current to zero.
Inductors store energy in magnetic fields proportional to the current flowing through them.
2
Apply Faraday's and Lenz's laws of electromagnetic induction
The rapid collapse of magnetic flux induces a large counter-electromotive force (V=−LΔIΔtV = -L \frac{\Delta I}{\Delta t}) across the coil terminals.
A faster change in flux generates a higher induced voltage spike that attempts to keep current flowing.
3
Identify the cause of the arc across the contacts
The induced voltage spike exceeds the dielectric breakdown strength of the small air gap between the opening switch contacts.
This inductive kickback produces a momentary high-voltage spark.

Key Concept

Self-Induction and Counter-Electromotive Force (CEMF)
Estimated Time:1m 0s
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Topics

Electromagnetism and Induction

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