In the below figure,$A$ and $B$ represent two straight wires carrying equal currents in a direction at right angles to the plane of paper inwards. Sketch separately the magnetic field lines produced by each current. Give a reason why the magnetic field at $K$ (mid-point of the line joining $A$ and $B$) will be zero.
What will be the effect on the magnetic field at $K$ if the current in wire $B$ is reversed?

(N/A) The magnetic field lines produced by the current in wires $A$ and $B$ are concentric circles centered at the wires,with their direction determined by the Right-Hand Thumb Rule. Since both currents flow inwards,the field lines are clockwise.
Point $K$ is equidistant from wires $A$ and $B$. Since both wires carry equal currents,the magnetic fields produced at $K$ by wire $A$ and wire $B$ are equal in magnitude. According to the Right-Hand Thumb Rule,the magnetic field due to wire $A$ at $K$ points in one direction (e.g.,upwards),while the magnetic field due to wire $B$ at $K$ points in the opposite direction (e.g.,downwards). Because these two fields are equal in magnitude and opposite in direction,they cancel each other out,making the net magnetic field at point $K$ zero.
If the direction of current in wire $B$ is reversed (i.e.,it flows outwards),the direction of the magnetic field produced by wire $B$ at point $K$ also reverses. Consequently,both magnetic fields at point $K$ will now point in the same direction and add up,resulting in a non-zero net magnetic field.