Examkin

Question

Difficulty: MediumDirect Current (DC) vs. Alternating Current (AC)

A technician connects the primary winding of an iron-core transformer to a 12 V12\text{ V} direct current (DC) battery and then to a 12 V12\text{ V} alternating current (AC) power supply. What voltage behavior will be observed at the secondary output terminals for each power source?

  1. A
    A continuous output voltage is produced by both sources because transformer windings respond identically to steady and changing currents.
  2. B
    A continuous output voltage is produced only with the DC battery because a static magnetic field transfers energy without inductive losses.
  3. A continuous output voltage is produced only with the AC power supply because transformers require a continuously changing magnetic field to induce a voltage.Answer
  4. D
    A continuous output voltage is produced by the DC battery, whereas the AC power supply causes the primary winding to short-circuit due to alternating polarity.

Answer

A continuous output voltage is produced only with the AC power supply because transformers require a continuously changing magnetic field to induce a voltage.
The correct answer identifies that only the alternating current (AC) supply produces a sustained secondary output voltage. A transformer functions via mutual induction, which fundamentally requires a dynamic, changing magnetic field. Because AC continuously alternates in direction and amplitude, it creates a constantly fluctuating magnetic flux in the iron core that induces an electromotive force (voltage) across the secondary winding. Steady direct current (DC) establishes an unvarying magnetic field, which cannot produce a sustained output voltage.

Step-by-Step Solution

1
Analyze the electromagnetic requirements of a transformer
Transformers operate on Faraday's law of induction, which states that a voltage is induced in a secondary winding only when there is a changing magnetic flux (ΔΦ/Δt\Delta \Phi / \Delta t).
Induction requires relative movement between magnetic lines of force and the secondary conductor.
2
Evaluate the behavior under a Direct Current (DC) input
A steady DC battery creates a constant, unvarying magnetic field in the core after initial energization, yielding zero sustained induced voltage across the secondary winding.
With steady DC, the rate of change of magnetic flux is zero after the initial switch closure.
3
Evaluate the behavior under an Alternating Current (AC) input
An AC supply continuously changes magnitude and reverses direction periodically, producing a dynamic magnetic field that continuously induces an alternating voltage in the secondary winding.
The continuous cyclical change in AC satisfies the condition for ongoing mutual inductance.

Key Concept

Transformers require a continuously changing current (AC) to produce the dynamic magnetic field necessary for mutual induction; steady DC produces a static magnetic field that cannot sustain transformer operation.
Estimated Time:45s
Rate this question

Topics

Direct Current (DC) vs. Alternating Current (AC)

More questions from this topic