The magnetic field at a distance $r$ from a long wire carrying current $i$ is $0.4\, Tesla$. The magnetic field at a distance $2r$ is......$Tesla$
AIPMT 1992, Medium
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(a) $B = \frac{{{\mu _0}i}}{{2\pi r}}\,i.e.\,\,B \propto \frac{1}{r}i.e.$when $r$ is doubled, $B$ is halved.
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