An electron revolves around nucleus with rotational frequency $'f'$ in circular orbit, due to this magnetic induction produced at nucleus position is $'B'$ then radius of circular orbit is directly proportional to

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

One Tesla is equal to

The earth's magnetic induction at a certain point is $7 \times {10^{ - 5}}\,Wb/{m^2}.$ This is to be annulled by the magnetic induction at the centre of a circular conducting loop of radius $5 \,cm$. The required current in the loop is......$A$

Discuss special cases of Biot-Savart law.

Magnetic field at the centre $O$ of a square loop of side $'a'$ carrying current $I$ as shown in the figure is