A wire in the form of a circular loop of one turn carrying a current produces a magnetic field $B$ at the centre. If the same wire is looped into a coil of two turns and carries the same current, the new value of magnetic induction at the centre is

A coil carrying a heavy current and having large number of turns mounted in a $N-S$ vertical plane and $a$ current flows in clockwise direction. A small magnetic needle at its cente will have its north pole in

A point charge $Q\left(=3 \times 10^{-12} C \right)$ rotates uniformly in a vertical circle of radius $R(=1 \,mm )$. The axis of the circle is aligned along the magnetic axis of the earth. At what value of the angular speed $\omega$, the eff ective magnetic field at the centre of the circle .............. $rad / s$ will be reduced to zero? (Horizontal component of earth's magnetic field is $30 \,\mu T )$

Surface charge density on a ring of radius $a$ and width $d$ is $\sigma$ as shown in the figure. It rotates with frequency $f$ about its own axis. Assume that the charge is only on outer surface. The magnetic field induction at centre is(Assume that $d \ll a$ )

When equal current is passed through two coils, equal magnetic field is produced at their centres. If the ratio of number of turns in the coils is $8: 15$, then the ratio of their radii will be

Three infinite wires are arranged in space in three dimensions.(along $x, y$ and $z$ axis) as shown. Each wire carries current $i$ . Find magnetic field at $A$