\textbf{185-} The length of a coreless solenoid is $50\ \text{cm}$ and the area of each turn is $10\ \text{cm}^2$. This solenoid has $2000$ turns close together and a current of $0.5\ \text{A}$ passes through it. What is the self-inductance coefficient of the solenoid in SI?
$$\left(\mu_\circ = 12.5\times10^{-7}\ \frac{\text{T.m}}{\text{A}}\right)$$
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(1) $0.01$ \hspace{2cm} (2) $0.05$ \hspace{2cm} (3) $0.10$ \hspace{2cm} (4) $0.50$
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