The number of turns per unit length of a long solenoid is $10$ . If its average radius is $5 \,cm$ and it carries a current of $10 \,A$, then the ratio of flux densities obtained at the centre and at the end on the axis will be
A$1: 2$
B$2: 1$
C$1: 1$
D$1: 4$
Medium
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B$2: 1$
b (b)
$\bar{B}_{\text {centre }}=\mu_0 n i \quad n=10$
$\vec{B}_{\text {end }}=\frac{\mu_0 n i}{2}$
$\frac{\vec{B}_{\text {centec }}}{\bar{B}_{\text {end }}}=\frac{\mu_0 n i}{\left(\frac{\mu_0 n i}{2}\right)}=\frac{2}{1}$
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