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Difficulty: MediumInductors and Transformers

An electrical technician connects an inductor and a resistor in series with a direct current (DC\text{DC}) power source and a mechanical switch. Immediately after the switch is closed, which statement accurately describes the initial behavior of the inductor?

  1. A
    It acts as an immediate short circuit with zero initial resistance, allowing maximum current to surge instantly before blocking all current once energized.
  2. It creates a counter-electromotive force (back-EMF) that opposes the sudden change in current, momentarily holding the circuit current at zero.Answer
  3. C
    It completely blocks the electrical current indefinitely because inductive components cannot conduct direct current under any conditions.
  4. D
    It instantly steps up the source voltage to double its rated value across the resistor without drawing any initial energy from the power supply.

Answer

The inductor creates a counter-electromotive force (back-EMF) that opposes the sudden change in current, momentarily holding the circuit current at zero.
Inductors store energy in a magnetic field and oppose any rapid change in electrical current. At the exact moment a DC switch closes, the rate of current change is at its maximum, producing an opposing counter-electromotive force (back-EMF) equal and opposite to the applied source voltage. Consequently, the initial current through the circuit is momentarily zero and rises exponentially over time.

Step-by-Step Solution

1
Identify the fundamental property of self-inductance upon circuit connection.
An inductor inherently opposes any instantaneous change in the electrical current flowing through its coils (VL=LdidtV_L = L \frac{di}{dt}).
According to Lenz's Law, when current attempts to rise from zero, the expanding magnetic flux induces an opposing voltage (counter-electromotive force or back-EMF).
2
Determine the initial electrical state at time t=0t = 0 (immediately upon switch closure).
The induced back-EMF across the inductor equals the full source voltage, meaning initial current I(0)=0 AI(0) = 0\text{ A} and initial resistor voltage drop VR=0 VV_R = 0\text{ V}.
Because current cannot change instantaneously through an inductor, it momentarily behaves like an open circuit before current gradually rises exponentially toward its Ohm's Law limit (I=VRI = \frac{V}{R}).

Key Concept

Inductor Transient DC Response and Back-EMF
Estimated Time:1m 0s
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Topics

Inductors and Transformers

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