Question

Sol. [Key Concept: Ampère's law — B inside a solid current-carrying wire] For a long straight solid wire carrying steady current, which is uniformly distributed across its cross-section, the variation of magnetic field $(B)$ with distance $(r)$ from axis will be

$$\begin{matrix} & B = \frac{\mu_{0}Ir}{2\pi a^{2}} \Rightarrow B \propto r,\text{~}\text{for}\text{~}r < a \\ & B = \frac{\mu_{0}I}{2\pi r} \Rightarrow B \propto \frac{1}{r},\text{~}\text{for}\text{~}r > a \end{matrix}$$