1) A long straight wire of radius R carries a current I. The magnetic field inside the wire at distance r from its centre is expressed as A) $(\frac{\mu_{0}i}{\pi R^{2}}).r$ B) $(\frac{2 \mu_{0}i}{\pi R^{2}}).r$ C) $(\frac{\mu_{0}i}{2 \pi R^{2}}).r$ D) $(\frac{\mu_{0}i}{2 \pi R^{}}).r$ Answer: Option CExplanation:Using Ampere's law, we have $\oint \overrightarrow{B}.\overrightarrow{dl}=\mu_{0}i_{in}$ or $B \times 2\pi r =\mu_{0}\frac{i}{\pi R^{2}}\pi r^{2}$ $\therefore$ $B= \frac{\mu_{0}}{2\pi}.\frac{ir}{R^{2}}$
