The magnetic field at the center of a circular coil of radius r carrying current is B1. The field at the center of another coil of radius r2 carrying same current is B2 then ratio of B1/B2 is :-
A magnetic field is inversely proportional to the r3 of the radius of the coil
So B1/B2=(1/r3)/(1/(r/2)3)
B1/B2=8/1
B1:B2=8:1