Find the magnetic field at $P$ due to the arrangement as shown.

In the given figure, net magnetic field at $O$ will be,

Rank the value of $\oint \overrightarrow{\mathrm{B}}·d\overrightarrow{\mathrm{I}}$ for the closed paths shown in figure from the smallest to the largest.

A rectangular region of dimensions $w\times I\left(w<<I\right)$ has a constant magnetic field into the plane of the paper as shown. On one side the region is bounded by a screen. On the other side positive ions of mass $m$ and charge $q$ are accelerated from rest and towards the screen by a parallel plate capacitor at constant potential difference $V<0$, and come out through a small hole in the upper plate. Which one of the following statements is correct regarding the charge on the ions that hit the screen?