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? 

For the circuit shown in the figure, the direction and magnitude of the force on $PQR$ is