A closely wound coil has a radius of 6.00 cm and carries a current of `2.50 A`. How many turns must it have if, at a point on the coil axis `6.00 cm` from the centre of the coil, the magnetic field is `6.39 xx 10^-4T` ?

Explanation

Apply `B=(mu_0NiR^2)/(2(R^2+x^2)^(3/2))`