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A charge $q$ $coulomb$ moves in a circle at $n$ revolutions per second and the radius of the circle is $r$ $metre$. Then magnetic field at the centre of the circle is
  • A$\frac{{2\pi q}}{{nr}} \times {10^{ - 7}}$ $N/amp/metre$
  • B$\frac{{2\pi q}}{r} \times {10^{ - 7}}$ $N/amp/metre$
  • C$\frac{{2\pi nq}}{r} \times {10^{ - 7}}$ $N/amp/metre$
  • D$\frac{{2\pi q}}{r}$ $N/amp/metre$
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