$A$ coil having $N$ turns is wound tightly in the form of a spiral with inner and outer radii $a$ and $b$ respectively. When a current $I$ passes through the coil,the magnetic field at the centre is

At what distance on the axis,from the centre of a circular current-carrying coil of radius $r$,does the magnetic field become $1/8$th of the magnetic field at the centre?

Two long parallel wires carrying currents $8 \,A$ and $15 \,A$ in opposite directions are placed at a distance of $7 \,cm$ from each other. $A$ point $P$ is equidistant from both the wires such that the lines joining the point to the wires are perpendicular to each other. The magnitude of the magnetic field at point $P$ is $(\sqrt{2}=1.4)$ $(\mu_0=4 \pi \times 10^{-7} \,T \cdot m/A)$.

Two infinitely long wires carry currents $4 \text{ A}$ and $3 \text{ A}$ placed along the $X$-axis and $Y$-axis respectively. The magnetic field at a point $P(0, 0, d) \text{ m}$ will be ...... $\text{T}$.

When equal current is passed through two coils,equal magnetic field is produced at their centres. If the ratio of the number of turns in the coils is $8: 15$,then the ratio of their radii will be

$Weber/m^2$ is equal to