How does the strength of the magnetic field at the centre of circular coil of wire depend on [i] the radius of the coil, [ii] the number of turns of wire in the coil, and [iii] the strength of current flowing in the coil ?

Explanation

i] It is inversely proportional to the radius of the coil.
B `prop` 1/r
ii] It is directly proportional to the number of turns n of the coil. As the direction of current in each circular turn is same, the fields due to the various turns get added up.
B `prop` n
iii] It is directly proportional to the strength of current passing through the coil B `prop` l