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The magnetic field $d\overrightarrow B $ due to a small current element $d\overrightarrow {l\,} $ at a distance $\overrightarrow {r\,} $ and element carrying current $i$ is
  • A$d\overrightarrow B  = \frac{{{\mu _0}}}{{4\pi }}i\,\left( {\frac{{d\overrightarrow {l\,}  \times \overrightarrow {r\,} }}{r}} \right)$
  • B$d\overrightarrow B  = \frac{{{\mu _0}}}{{4\pi }}{i^2}\,\left( {\frac{{d\overrightarrow {l\,}  \times \overrightarrow {r\,} }}{r}} \right)$
  • C$d\overrightarrow B  = \frac{{{\mu _0}}}{{4\pi }}{i^2}\,\left( {\frac{{d\overrightarrow {l\,}  \times \overrightarrow {r\,} }}{{{r^2}}}} \right)$
  • D$d\overrightarrow B  = \frac{{{\mu _0}}}{{4\pi }}i\,\left( {\frac{{d\overrightarrow {l\,}  \times \overrightarrow {r\,} }}{{{r^3}}}} \right)$
AIPMT 1996, Easy
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