$A$ long straight wire of radius $a$ carries a steady current $i$. The current is uniformly distributed across its cross-section. The ratio of the magnetic field at $a/2$ and $2a$ is

The magnetic field at the centre of an equilateral triangular loop of side $2L$ and carrying a current $i$ is

Two concentric circular coils $A$ and $B$ having radii $20 \ cm$ and $10 \ cm$ respectively lie in the same plane. The current in coil $A$ is $0.5 \ A$ in anticlockwise direction. The current in coil $B$,so that net magnetic field at the common centre is zero,is

An equilateral triangle is made by uniform wires $AB, BC, CA$. A current $I$ enters at $A$ and leaves from the midpoint of $BC$. If the length of each side of the triangle is $L$, the magnetic field $B$ at the centroid $O$ of the triangle is:

$A$ current $i$ flows through a square loop of side $a$. What is the magnetic field at its center?

Two identical circular wires of radius $20\,cm$ and carrying current $\sqrt{2}\,A$ are placed in perpendicular planes as shown in the figure. The net magnetic field at the centre of the circular wires is $.............\times 10^{-8}\,T$. (Take $\pi=3.14$ )