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:

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 $a$?

Two similar coils each of radius $R$ are lying concentrically with their planes at right angles to each other. The currents flowing in them are $I$ and $2I$. The resultant magnetic field of induction at the centre will be ($\mu_0 =$ Permeability of vacuum).

The magnetic induction due to an infinitely long straight wire carrying a current $i$ at a distance $r$ from the wire is given by:

$PQ$ and $RS$ are long parallel conductors separated by a certain distance. $M$ is the mid-point between them. The net magnetic field at $M$ is $B$. Now,the current $2\,A$ is switched off. The field at $M$ now becomes

Two concentric circular coils $X$ and $Y$ of radii $16\; cm$ and $10\; cm$ respectively,lie in the same vertical plane containing the north to south direction. Coil $X$ has $20$ turns and carries a current of $16\; A$. Coil $Y$ has $25$ turns and carries a current of $18\; A$. The sense of the current in $X$ is anticlockwise,and clockwise in $Y$,for an observer looking at the coils facing west. Give the magnitude and direction of the net magnetic field due to the coils at their centre.