A long cylindrical wire of radius $R$ carries a uniform current $I$ flowing through it. The variation of magnetic field with distance $\text{'}r\text{'}$ from the axis of the wire is shown by

Two very long straight wires are set parallel to each other. Each carries a current $I$ in the same direction and the separation between them is $2r.$ The intensity of the magnetic field at point $P$ as shown in the figure is _________.

A toroidal coil has $3000$ turns. The inner and outer radii are $10 \mathrm{cm}$ and $12 \mathrm{cm}$, respectively. If current flowing is $5 \mathrm{A}$, then the magnetic field inside the toroid will be

Four identical long solenoids $\mathrm{A}, \mathrm{B}, \mathrm{C}$ and $\mathrm{D}$ are connected to each other as shown in the figure. If the magnetic field at the center of $\mathrm{A}$ is $3 \mathrm{T}$ the field at the center of $\mathrm{C}$ would be : (Assume that the magnetic field is confined with in the volume of respective solenoid).