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

A technician connects a 12 V12\text{ V} direct current (DC) battery directly across the primary winding of a 1:101:10 step-up transformer expecting a steady 120 V120\text{ V} output. Instead, the secondary winding produces only a brief momentary pulse at connection, followed by 0 V0\text{ V}. Which statement best explains this outcome?

  1. A
    The transformer converts direct current into alternating current at a frequency too high for standard measurement.
  2. A steady direct current creates a static magnetic field, which fails to produce the changing magnetic flux required for continuous induction.Answer
  3. C
    Direct current causes the iron core to act as a reverse-biased diode, blocking all electrical transfer.
  4. D
    The unidirectional flow of electrons in direct current cancels out the electromotive force across the secondary terminals.

Answer

A steady direct current creates a static magnetic field, which fails to produce the changing magnetic flux required for continuous induction.
Transformers operate on mutual electromagnetic induction, which requires a continuously changing magnetic field. Alternating current (AC) naturally provides this because its magnitude and polarity oscillate continuously. When steady direct current (DC) is applied, a brief pulse occurs only while current ramps up during connection; once steady state is reached, the magnetic field is static, resulting in zero rate of change of magnetic flux and zero induced secondary voltage.

Step-by-Step Solution

1
Analyze the nature of the power source (DC).
Direct current maintains a constant magnitude and unidirectional flow after reaching steady state.
Understanding the source characteristic is essential to determine its magnetic effect in the primary winding.
2
Evaluate the magnetic field produced by steady direct current in the transformer core.
A constant direct current establishes a stationary, unchanging magnetic field (dΦdt=0\frac{d\Phi}{dt} = 0).
A changing current is necessary to produce an expanding or collapsing magnetic field.
3
Apply Faraday's Law of Electromagnetic Induction to the secondary winding.
Induced secondary voltage is proportional to the rate of change of magnetic flux (Vsec=−NdΦdt=0 VV_{\text{sec}} = -N \frac{d\Phi}{dt} = 0\text{ V}).
Because flux is static once DC is fully established, no continuous voltage can be induced across the secondary coil.

Key Concept

Direct Current (DC) vs. Alternating Current (AC) in Transformer Operation
Estimated Time:45s
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Topics

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

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