Infinite number of straight wires each carrying current $I$ are equally placed as shown in the figure. Adjacent wires have current in opposite direction. Net magnetic field at point $P$ is

A current $I$ flows a closed path in the horizontal plane of the circle as shown in the figure. The path consists of eight cars with alternating radii $r$ and $2r$. Each segment of arc subtends equal angle at the common centre $P$. The magnetic field produced by current path at point $P$ is

A mass spectrometer is a device which select particle of equal mass. An iron with electric charges $q>0$ starts at rest from a source $S$ and is accelerated through a potential difference $V$. It passes through a hole into a region of constant magnetic field $\overrightarrow{B}$ perpendicular to the plane of the paper as shown in the figure. The particle is deflected by the magnetic field and emerges through the bottom hole at a distance $d$ form the top hole. The mass of the particle is