Current flows through uniform,square frames as shown. In which case is the magnetic field at the centre of the frame not zero?

Two parallel beams of protons and electrons,carrying equal currents,are fixed at a separation $d$. The protons and electrons move in opposite directions. $P$ is a point on a line joining the beams,at a distance $x$ from the proton beam. The magnetic field at $P$ is $B$. If $B$ is plotted against $x$,which of the following best represents the resulting curve?

$A$ charge $q$ is accelerated through a potential difference $V$. It is then passed normally through a uniform magnetic field,where it moves in a circle of radius $r$. The potential difference required to move it in a circle of radius $2r$ is (in $V$)

Which of the following statements is correct?

Two circular coils $X$ and $Y$,having an equal number of turns,carry equal currents in the same sense and subtend the same solid angle at point $O$. If the smaller coil $X$ is midway between $O$ and $Y$,then if we represent the magnetic induction due to the bigger coil $Y$ at $O$ as $B_Y$ and that due to the smaller coil $X$ at $O$ as $B_X$,then:

The current flowing along the path $A B C D$ of a cube (shown in the left figure) produces a magnetic field at the centre of the cube of magnitude $B$. Dashed lines depict the non-conducting part of the cube. Consider a cubical shape shown to the right which is identical in size and shape to the left. If the same current now flows in along the path $D A E F G C D$,then the magnitude of the magnetic field at the centre will be