Question

An equilateral triangular loop is carrying current i, is kept near to a current carrying long wire as shown in figure. Under the action of magnetic force alone the loop

• A. must move along positive y-axis
• B. must move in x-y plane and not along x-axis or y-axis
• C. must move along positive x-axis
• D. must move along negative x-axis

Answer 1

The correct option is D must move along negative x-axis

Let $B$ be the magnetic field due to the current carrying straight wire in the region of triangle, whose direction is vertically downwards in the plane of paper according to right hand thumb rule. The three current carrying equal sides of triangle are in this field, hence experience equal magnetic forces ($F=Bil$).

According to Fleming's left hand rule, all three sides of triangle, experience equal forces in perpendicularly outward direction of the sides, the lines of action of these forces meet at the center of triangle, the resultant of these forces will be zero as all three equal forces are making an angle of ${120}^o$ with each other i.e. the net force is zero on the triangular loop , hence it will not move in any direction.