An arc of a circle of radius $R$ subtends an angle $\frac{\pi }{2}$ at the centre. It carries a current $i$. The magnetic field at the centre will be
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(b) $B = \frac{{{\mu _0}}}{{4\pi }}\frac{{\theta i}}{r} = \frac{{{\mu _0}}}{{4\pi }} \times \frac{\pi }{2} \times \frac{i}{R} = \frac{{{\mu _0}i}}{{8R}}$
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