(N/A) $1$. Magnetic field due to a circular loop: When a current flows through a circular loop,the magnetic field lines are circular near the wire. As we move towards the center of the loop,the arcs of these circles become larger and appear as straight lines at the center. The direction of the magnetic field can be determined using the Right-Hand Thumb Rule.
$2$. Effect of $n$ turns: The magnetic field produced by each turn of the coil adds up in the same direction. Since the current in each turn flows in the same direction,the magnetic effects of all the turns are additive. Therefore,if a coil has $n$ turns,the total magnetic field produced at any point is $n$ times the magnetic field produced by a single turn.