$A$ solenoid has $N$ turns,length $l$,and cross-sectional radius $r$. If a current $i$ flows through the solenoid,what is the magnetic field at the axial midpoint? (Given $l \simeq r$)

  • A
    $\frac{N\mu_0 i}{\sqrt{l^2 + r^2}}$
  • B
    $\frac{N\mu_0 i}{(4l^2 + r^2)^{3/2}}$
  • C
    $\frac{N\mu_0 i}{\sqrt{4r^2 + l^2}}$
  • D
    $\frac{N\mu_0 i}{(4r^2 + l^2)^{3/2}}$

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