A coil of wire of radius $r$ has 600 turns and a self inductance of $108 \mathrm{mH}$. The self inductance…

A coil of wire of radius $r$ has 600 turns and a self inductance of $108 \mathrm{mH}$. The self inductance of a coil with same radius and 500 turns is
  1. $80 \mathrm{mH}$
  2. $75 \mathrm{mH}$
  3. $108 \mathrm{mH}$
  4. $90 \mathrm{mH}$

Solution

The given, $L_1=108 \mathrm{mH}, N_1=600$ turns, $N_2=500$ turns and $L_2=$ ? By self-inductance of a plane coil $ \begin{aligned} & L_1=\frac{\mu_0 \pi N_1^2 a_1}{2} \\ & L_2=\frac{\mu_0 \pi N_2^2 a_2}{2} \end{aligned} $ From the Eqs. (i) and (ii), we get $ \begin{aligned} \frac{L_1}{L_2} & =\left(\frac{N_1}{N_2}\right)^2 \quad\left(\because a_1=a_2\right) \\ \frac{108}{L_2} & =\left(\frac{600}{500}\right)^2 \\ L_2 & =\frac{108 \times 25}{36} \\ L_2 & =3 \times 25=75 \mathrm{mH} \end{aligned} $

Asked in: AP EAMCET 2014

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