Two long parallel wires are at a distance 2d apart. They carry steady equal current flowing out of the plane of the paper as shown. The variation of the magnetic field along the line XX ' is given by
(A)(see structure)✓ Correct
(B)(see structure)
(C)(see structure)
(D)(see structure)
Answer: (A)
Solution
Both wires carry current out of the page, so by the right-hand rule the field of each circles anticlockwise around it. At a point between the wires the two contributions oppose, and at the midpoint they are equal and cancel, giving zero field there. Moving from the midpoint towards either wire, the nearer wire dominates and the field grows, but it points in opposite senses on the two sides of the centre, so the field reverses as we cross the midpoint. The graph passing through zero at the centre with opposite signs on the two sides is the correct one.
JEE Main · 2010 · 11 Apr 2010 · Offline · Single correct option
