There is a long straight wire carrying electric current i. A circular region of radius r is selected perpendicular to wire in such a manner that wire lies along the axis of this circular region. How much magnetic flux is associated with this circular region?

Explanation

We know that magnetic lines of a straight wire are concentric circles with their planes perpendicular to the wire. Hence, magnetic lines in this case are passing parallel to the circular region shown. No magnetic line is passing perpendicular to this circular area and hence net magnetic flux associated with this region is zero.