Which one of the following graphs accurately represents the variation of magnetic field (B) with distance (r) from the exact center of a solid, current-carrying cylindrical wire of radius R? MCQ with Answer and Explanation

Which one of the following graphs accurately represents the variation of magnetic field (B) with distance (r) from the exact center of a solid, current-carrying cylindrical wire of radius R?
A. B is directly proportional to r inside, and inversely proportional to r outside.
B. B is inversely proportional to r everywhere.
C. B is zero everywhere inside, and inversely proportional to r outside.
D. B is constant inside, and directly proportional to r outside.
Answer: Option A
Solution (By JKExamLibrary)
By Ampere's Circuital Law, for a solid wire carrying uniform current, the magnetic field inside (r < R) is B = (mu_0 * I * r) / (2piR^2), meaning B ∝ r (straight line). Outside the wire (r > R), B = (mu_0 * I) / (2pi*r), meaning B ∝ 1/r (rectangular hyperbola). Thus, it increases linearly then decays.

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