Question

A long straight wire of a circular cross-section (radius $a$ ) carries a steady current $I$ and the current $I$ is uniformly distributed across this cross-section. Which of the following plots represents the variation of magnitude of magnetic field $B$ with distance $r$ from the centre of the wire?

• A.
• B.
• C.
• D.

Answer 1

Answer: (A)

Answer 2

Magnetic field inside a conductor [if $r \leq a$ ]
$B_{i n}=\frac{\mu_{0} I}{2 \pi a^{2}} . r$
or $B_{i n} \propto r$
and magnetic field outside the conductor [if $r \geq a$ ]
$B_{\text {out }}=\frac{\mu_{0} I}{2 \pi r} B \propto \frac{1}{r}$