A long solenoid with $20$ $turns$ per $cm$ is made. To produce a magnetic field of $20$ $millitesla$ inside the solenoid, the necessary current will nearly be......$ampere$
Medium
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$B=\mu_{0} n i ;$ where $n=\frac{20}{10} \frac{t u r n}{c m}$
$=2000 \frac{t u r n}{m} \cdot$ So, $20 \times 10^{-5}$
$=4 \pi \times 2000 \times i$
$\Rightarrow i=8 A$
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