An infinite current carrying wire passes through point O and in perpendicular to the plane containing a current carrying loop ABCD as shown in the figure. Choose the correct option (s).
(A)Net force on the loop is zero.✓ Correct
(B)Net torque on the loop is zero.
(C)As seen from O, the loop rotates clockwise.✓ Correct
(D)As seen from O, the loop rotates anticlockwise
Answer: (A), (C)
Solution
The infinite wire creates a magnetic field in circles around it. At any point on the loop, the field is tangent to circles centered at O. The force on a current-carrying segment is F=Idl×B. For wire BC (closer to O), the magnetic field is stronger and the force is perpendicular to the plane, pointing outward. For wire DA (farther from O), the field is weaker and the force points inward. Since the forces on opposite sides are equal and opposite (by symmetry of the rectangular loop), the net force is zero (option A correct). However, the forces on BC and DA form a couple (unequal magnitudes due to different distances from O), creating a net torque that rotates the loop clockwise as seen from O (option C correct, B and D incorrect).
JEE Advanced · 2006 · 09 Apr 2006 · One or more correct options
