The magnetic field at the centre of a circular coil of radius $R$,carrying current $2 \ A$ is $B_1$. The magnetic field at the centre of another coil of radius $3R$ carrying current $4 \ A$ is $B_2$. The ratio $B_1: B_2$ is

$A$ current of $1.5 \, A$ is flowing through an equilateral triangle of side $9 \, cm$. The magnetic field at the centroid of the triangle is (Assume that the current is flowing in the clockwise direction.)

An infinitely long straight conductor is bent into the shape as shown in the figure. It carries a current $I$ ampere and the radius of the circular loop is $r$ metre. Then the magnetic induction $B$ at the centre $C$ of the circular part is:

$A$ straight wire carrying a current $10\, A$ is bent into a semicircular arc of radius $5\, cm.$ The magnitude of the magnetic field at the center is

An electric current $I$ enters and leaves a uniform circular wire of radius $r$ through diametrically opposite points. $A$ particle carrying a charge $q$ moves along the axis of the circular wire with speed $v$. What is the magnetic force experienced by the particle when it passes through the centre of the circle?

$A$ steady current flows in a long wire. It is bent into a circular loop of one turn and the magnetic field at the centre of the coil is $B$. If the same wire is bent into a circular loop of $n$ turns,the magnetic field at the centre of the coil is