A cylindrical wire of radius R is carrying uniformly distributed current i over its cross-section. If a circular loop of radius r is taken as amperian loop, then the variation value of  ∮B→·dℓ→ over this loop with radius ‘r” of loop will be best represented by

If a wire carries current along positive x-axis, then the magnetic field due to it along the line y = x in first quadrant is upwards. If a wire carries current along positive y-axis, then the magnetic field due to it along the line y = x in first quadrant is downwards. Along the line y =xr, above two values of magnetic field are equal in magnitude but opposite in direction and thus, their resultant is zero.