# A solenoid is 2.7-m long, has a radius of 0.85 cm, and has 600 turns. It carries a current I of 2.5 A. What is the magnitude of the magnetic field B inside the solenoid and far from either end?

$\begin{array}{1 1} 0.007\;mT \\ 7\;mT \\ 0.07\;mT \\ 0.7 \;mT \end{array}$

The formula for the field inside the solenoid is $B = \mu_0 n I$, where $n = \large\frac{N}{L}$ or the turn density and $I$ is the current.
Given that number of turns, $N = 600$ and the length of the solenoid, $L = 2.7m \rightarrow n = \large\frac{600}{2.7}$$\approx 222.22 \;m^{-1}$
$\Rightarrow B = 4\pi \times 10^{-7} N/A^{-2} \times 222.22 m^{-1} \times 2.5A \approx 0.7mT$