Magnetic field due to current: Which statement correctly describes the magnetic field polarity behavior around a conductor for DC versus AC current?

Introduction / Context:
Magnetic fields arise from electric currents. Their polarity and time variation depend on whether the current is direct (DC) or alternating (AC). This concept is essential in understanding inductors, transformers, and electromagnetic interference (EMI).

Given Data / Assumptions:

• Long straight conductor reference.
• Right-hand rule defines field direction for conventional current.
• Current may be constant (DC) or time-varying (AC).

Concept / Approach:
For DC, the current direction is constant, so the magnetic field has a fixed polarity and constant magnitude (neglecting edge effects). For AC, current reverses direction periodically, so the field polarity reverses as well and its magnitude varies sinusoidally (or with the waveform shape). Therefore only DC yields a fixed polarity field.

Step-by-Step Solution:

Verification / Alternative check:
Observation with a compass near a DC-carrying wire shows a steady deflection; with AC, the compass needle vibrates or averages to near zero due to rapid reversals.

Why Other Options Are Wrong:

• AC causes fixed polarity: False; polarity alternates.
• DC causes alternating field: False; polarity is fixed for unidirectional current.
• Combined statement: Reverses the true behaviors for AC and DC.

Common Pitfalls:
Equating “fixed polarity” with “constant magnitude.” In DC, both are constant; in AC, both polarity and magnitude vary with time.

Final Answer:
A DC current causes a fixed polarity magnetic field.