I want to ask if magnetic field lines are always circular and If YES , Then WHY ??? Or on What basis we can say that magnetic field lines are always circular ?? I am confused with the topic . It is my humble request to help me out if you can . I am waiting for your answer . I shall be highly obliged to you if you clear this doubt .
Magnetic field lines always follow a closed path. That is, they are always loops. If you see a diagram showing 'ends' of field lines, it is only showing part of the lines--in that case we can assume that one 'end' of a line shown in the diagram actually loops back to the other 'end' of that same line. The loops certainly don't have to follow an exact circle, though. They can be elliptic or any oddball shape you can imagine. Their shape depends on the various structures that create them.
Magnetic field lines aren't always circular it's just that the most simple magnetic fields (those around wires or solenoids) are circular or elliptical. Really though, magnetic field shapes are just dependent on the movement and distribution of charge. You can see here how field lines in the middle of a loop with current would be straight.
Many magnetic fields are circular. Think about how currents in electro-magnets and generators flow. They flow in circular patterns so the magnetic field sometimes copies patterns of electron movement.