A long solenoid is formed by winding $20$ $turns/cm$. The current necessary to produce a magnetic field of $20$ $milli\,tesla$ inside the solenoid will be approximately .....$A$ $(\frac{{{\mu _0}}}{{4\pi }} = {10^{ - 7}}\,tesla - metre/ampere)$
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(a)$B = {\mu _0}ni \Rightarrow i = \frac{B}{{{\mu _0}n}}$$ = \frac{{20 \times {{10}^{ - 3}}}}{{4\pi \times {{10}^{ - 7}} \times 20 \times 100}}$
=$7.9\,amp = 8\,amp$
=$7.9\,amp = 8\,amp$
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