The magnetic field at a distance $r$ from a long straight wire carrying current $I$ is $0.4 \text{ tesla}$. The magnetic field at a distance $2r$ will be (in $\text{ tesla}$)

$A$ straight long wire is carrying current $I$. The ratio of magnetic field due to this wire at perpendicular distance $2 \text{ cm}$ and $5 \text{ cm}$ respectively from the wire is

Two concentric coils of $20$ turns each are placed in the same plane. Their radii are $30 \text{ cm}$ and $60 \text{ cm}$,and they carry $0.4 \text{ A}$ and $0.6 \text{ A}$ currents,respectively,in opposite directions. The magnetic induction at the centre in tesla is ....... .

The figure shows two semicircular loops of radii $R_1$ and $R_2$ carrying current $I$. The magnetic field at the common centre '$O$' is

An infinitely long straight wire carrying current $I$ is bent in a planar shape as shown in the diagram. The radius of the circular part is $r$. The magnetic field at the centre $O$ of the circular loop is:

The magnetic field intensity at the centre of a circular wire of radius $0.1 \,m$ carrying a current of $0.2 \,A$ is: