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A solenoid of $N$ turn, $'l'$ length and $'r'$ radius of cross - section. If current $i$ flow in solenoid then magnetic field at axial mid point will be (where $l\, \simeq \,\,r$ ) 
  • A$\frac{{N{\mu _0}i}}{{\sqrt {{l^2}\, + \,{r^2}} }}$
  • B$\frac{{N{\mu _0}i}}{{{{(4{l^2}\, + \,{r^2})}^{3/2}}}}$
  • C$\frac{{N{\mu _0}i}}{{\sqrt {4{r^2}\, + \,{l^2}} }}$
  • D$\frac{{N{\mu _0}i}}{{{{(4{r^2}\, + \,{l^2})}^{3/2}}}}$
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