Two thick wires and two thin wires,all of the same material and same length,form a square in three different ways $P, Q,$ and $R$ as shown in the figure. With the correct connections shown,the magnetic field due to the current flow at the centre of the loop will be zero in the case of:

$PQRS$ is a square loop made of uniform conducting wire. If the current enters the loop at $P$ and leaves at $S$, then the magnetic field will be

$A$ current flows through a circular coil of radius $R$. Find the distance from the center on the axis where the magnetic field is $\frac{1}{8}$th of the magnetic field at the center.

Give similarity between Biot-Savart law and electrostatic law of Coulomb.

$A$ telephone cable at a place has four long straight horizontal wires carrying a current of $1.0 \; A$ in the same direction from east to west. The earth's magnetic field at the place is $0.39 \; G$ and the angle of dip is $35^{\circ}$. The magnetic declination is nearly zero. What are the resultant magnetic fields at points $4.0 \; cm$ below and above the cable?

Give the characteristics of magnetic field lines.