(b) $i = \frac{q}{t} = 100 \times e$ $B_{centre} = \frac{\mu_{o}}{4\pi}.\frac{2\pi i}{r} = \frac{\mu_{o}}{4\pi}.\frac{2\pi \times 100e}{r}$ $= \frac{\mu_{o} \times 200 \times 1.6 \times 10^{- 19}}{4 \times 0.8} = 10^{- 17}\mu_{o}$
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Ph.D. in Physics with 19+ years of teaching experience.