How To Recharge Loop Mobile?

Interaction of a Current Loop with a Magnetic Field?

• http://session.masteringphysics.com/problemAsset/1011690/27/17718_a.jpg http://session.masteringphysics.com/problemAsset/1011690/27/17718_b.jpg Consider a current I that flows in a plane rectangular current loop with height a = 4.00 cm and horizontal sides b = 2.00 cm. (Intro 1 figure) The loop is placed into a uniform magnetic field B in such a way that the sides of length a are perpendicular to B, and there is an angle theta between the sides of length b and B, as shown in the figures. (Intro 2 figure) Will the interaction of the current through the loop with the magnetic field cause the loop to rotate? A) Yes, the net torque acting on the loop is negative and tends to rotate the loop in the direction of decreasing angle (clockwise). B) Yes, the net torque acting on the loop is positive and tends to rotate the loop in the direction of increasing angle (counterclockwise). C) No, the net torque acting on the loop is zero and the loop is in equilibrium D) No, the net force acting on the loop is zero and the loop is in equilibrium

• Answer:

  The magnetic moment of a loop mu is perpendicular to the loop and points in such direction that looking along it the current flow appears clockwise. This moment interacts with a magnetic field B to produce a torque on the loop such that mu tends to point in the same direction as B. Thus the answer seems to be B. Looking at your picture 1, you must look in the direction upper-left to lower right to see I going clockwise. Then mu points to the lower right and must rotate <90 deg CCW to be aligned with B. The lower diagram in the ref. confirms the above. A <90 deg CW rotation is needed to align mu (which points along "normal to loop") and B. From the ref., the formula for loop torque is T = BIAcosθ, where θ (α in the ref.) = angle between B and the plane of the loop.