An arc of a circle of radius $R$ subtends an angle $\frac{\pi}{2}$ at the centre. It carries a current $I$. The magnetic field at the centre will be ($\mu_0 =$ permeability of free space).

Two concentric coils $X$ and $Y$ of radii $16 \, cm$ and $10 \, cm$ lie in the same vertical plane containing $N-S$ direction. Coil $X$ has $20$ turns and carries $16 \, A$. Coil $Y$ has $25$ turns and carries $18 \, A$. Coil $X$ has current in anticlockwise direction and coil $Y$ has current in clockwise direction for an observer looking at the coils facing the west. The magnitude of the net magnetic field at their common centre is:

The magnetic field due to a square loop of side $a$ carrying a current $I$ at its centre is:

Shown in the figure is a conductor carrying a current $I$. The magnetic field intensity at the point $O$ (common centre of all the three arcs) is

Charge $q$ is uniformly spread on a thin ring of radius $R.$ The ring rotates about its axis with a uniform frequency $f \ Hz.$ The magnitude of magnetic induction at the center of the ring is

The figure shows the cross-section of a long cylindrical conductor through which an axial hole of radius $r$ is drilled with its centre at point $A$. $O$ is the centre of the conductor. If an identical hole were to be drilled centred at point $B$ while maintaining the same current density,the magnitude of the magnetic field at $O$: