Difficulty: Easy
Correct Answer: Correct
Explanation:
Introduction / Context:Electromagnetism links electric currents and magnetic fields. Recognizing that even a simple straight wire carrying current generates a magnetic field is foundational for understanding inductors, transformers, motors, and EMC behavior on circuit boards and cables.
Given Data / Assumptions:
Concept / Approach:The magnetic field magnitude around a long straight conductor is given by B(r) = μ * I / (2 * π * r), encircling the wire per the right-hand rule. This holds for DC and AC instantaneous currents alike; AC simply varies the field with time. No ferromagnetic material is required to create the field; such material only concentrates or guides it.
Step-by-Step Solution:
1) State the qualitative rule: a current produces a circumferential magnetic field.2) For DC, the field is steady; for AC, the field is time-varying at the same frequency.3) Apply the right-hand rule to determine field direction around the wire.4) Conclude that any nonzero current results in a real magnetic field in space.Verification / Alternative check:Place a compass near a current-carrying wire: deflection confirms the field. Clamp-on ammeters exploit this principle to measure current non-invasively.
Why Other Options Are Wrong:Incorrect: contradicts the basic current-field relationship. “True only for AC” or “only with ferromagnetics”: the phenomenon exists for DC and without special cores; materials only modify field intensity and distribution.
Common Pitfalls:Assuming magnetism requires magnets or iron; overlooking that PCB traces and cables radiate/receive due to these very fields.
Final Answer:Correct
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