Q. A long straight wire of a circular cross-section with radius $a$ carries a steady current $I$. The current I is uniformly distributed across this cross-section. The plot of magnitude of magnetic field B with distance $r$ from the centre of the wire is given by
A) [image:2025J24S2_Q31_opt_a.png]
B) [image:2025J24S2_Q31_opt_b.png]
C) [image:2025J24S2_Q31_opt_c.png]
D) [image:2025J24S2_Q31_opt_d.png]
Explanation
[Solution by Dr. Abubacker] Inside the wire ($r<a$), $B=\frac{\mu_0Ir}{2\pi a^2}\propto r$ (linear rise); outside ($r>a$), $B=\frac{\mu_0I}{2\pi r}\propto\frac1r$ (falls). The maximum is at $r=a$. This is option (4). Answer (4).
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.