Question

A coil of 200mm is to produce a field of 0.4g in its center with a current of 0.25A.How many turns must there be in the coil?

Answer 1

$0.4\ G=0.4\cdot10^{-4}\ T$

$B=\mu_0NI/l\to N=\frac{Bl}{\mu_0I}=\frac{0.4\cdot10^{-4}\cdot0.2}{4\cdot3.14\cdot10^{-7}\cdot0.25}\approx25$ . Answer