The magnetic field along the axis of a long current-carrying wire is:
Aconstant
Bproportional to 1/r
Czero on the wire's axis
Dproportional to 1/r²
Answer & Solution
Correct answer: C. zero on the wire's axis
Symmetry: on the axis of an ideal current line, the field is along the axis only for special geometries; for a straight wire there is no axial field at points on the wire's own line.
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