Four infinitely long parallel wires pass through the four corners of a square $PQRS$ of side $a$ that lies in a plane perpendicular to the wires (see figure).

They all carry equal steady currents $I$. Currents in the wires passing through $P$ and $Q$ point out of the page, and the currents in the wires passing through $R$ and $S$ point into the page as shown in the figure. The magnitude of the magnetic field at the centre $O$ of the square is 

The graph showing the variation of the magnetic field strength (B) with distance (r) from a long current carrying conductor is

A long straight wire of circular cross-section (radius $a$) is carrying steady current $\mathrm{I}.$ The current $\mathrm{I}$ is uniformly distributed across this cross-section. The magnetic field is

Two long straight wires vertically pierced the plane of the paper at vertices of an equilateral triangle as shown in figure. They each carry $2 \mathrm{A},$ out of the paper. The magnetic field at the third vertex $P$ has magnitude.

Two long straight parallel wires, carrying (adjustable) currents $I_1$ and $I_2$ , are kept at a distance $d$ apart. If the force $F$ between the two wires is taken as 'positive' when the wires repel each other and 'negative' when the wires attract each other, the graph showing the dependence of $F$, on the product $I_1{I}_2$ , would be:

As shown in the figure, two infinitely long, identical wires are bent by ${90}^o$ and placed in such a way that the segments $LP$ and $QM$ are along the $x$ - axis, while segments $PS$ and $QN$ are parallel to the $y$ - axis. If $OP=OQ=4cm,$ and the magnitude of the magnetic field at $O$ is ${10}^{-4} T,$ and the two wires carry equal currents (see figure), the magnitude of the current in each wire and the direction of the magnetic field at $O$ will be $\left({\mu }_0=4\pi \times {10}^{-7}N{A}^{-2}\right):$

A wire carrying current $\text{I}$ has the shape as shown in adjoining figure. Linear parts of the wire are very long and parallel to $\text{X}$ -axis while semicircular portion of radius $\text{R}$ is lying in $\text{Y}$ -$\text{Z}$ plane. Magnetic field at point $\text{O}$ is :

Two very long, straight, and insulated wires are kept at $90°$ angle from each other in $\mathrm{x}\mathrm{y}$ plane as shown in the figure.

These wires carry currents of equal magnitude $I$ , whose direction are shown in the figure. The net magnetic field at point $P$ will be: