Which of the following graphs represents the magnetic field $(B)$ versus distance $(r)$ from the centre of a long straight conducting wire of uniform cross-sectional area carrying a steady current $I$ and having radius $a$?

$A$ 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?

Two long parallel wires are at a distance $R$ apart. They carry steady equal currents in the same directions as shown in the figure. The ratio of magnetic fields at $A, B$ and $C$ respectively,is

Current $I$ is flowing along the path $ABCDA$ consisting of four edges of a cube (figure $-a$),producing a magnetic field $B_0$ at the centre of the cube. Find the magnetic field $B$ produced at the center of the cube by a current $I$ flowing along the path of the six edges $ABCGHEA$ (figure $-b$).

Two parallel wires carry currents $i_1$ and $i_2$ $(i_1 > i_2)$. When the currents are in the same direction,the magnetic field at a point midway between the wires is $10 \, \mu T$. When the direction of $i_2$ is reversed,the magnetic field at that point becomes $30 \, \mu T$. What is the ratio $\frac{i_1}{i_2}$?

$A$ long straight wire,carrying current $I$,is bent at its mid-point to form an angle of $45^{\circ}$. The magnetic field induction (in tesla) at point $P$,at a distance $R$ from the point of bending,is equal to: