5. Magnetic forces on current l Example: A straight wire carrying a current is placed in a region containing a magnetic field. The current flows in the +x direction. There are no magnetic forces acting on the wire. What is the direction of the magnetic field?

Example: A horizontal wire carries a current and is in a vertical magnetic field. What is the direction of the force on the wire? B I 1) left 2) right 3) zero 4) into the page 5) out of the page Example: A vertical wire carries a current and is in a vertical magnetic field. What is the direction of the force on the wire? B I 1) left 2) right 3) zero 4) into the page 5) out of the page

z B y x 6. Current loop in magnetic field Example: A rectangular current loop is in a uniform magnetic field. What is the direction of the net force on the loop? B x z y 1) + x 2) + y 3) zero 4) - x 5) - y Using the right-hand rule, we find that each of the four wire segments will experience a force outwards from the center of the loop. Thus, the forces of the opposing segments cancel, so the net force is zero.

Wire and Loop with Electric Current in Magnetic Field Battery Axis of rotation Magnetic field Loop The loop will rotate (clockwise)

Example: If there is a current in the loop in the direction shown, the loop will: 1) move up 2) move down 3) rotate clockwise 4) rotate counterclockwise 5) both rotate and move F N S N S B F B field out of North B field into South At the North Pole, the magnetic field points to the right and the current points out of the page. The right-hand rule says that the force must point up. At the south pole, the same logic leads to a downward force. Thus the loop rotates clockwise.

a b 6a. Current loop and magnetic moment b N – number of loops in the coil M - magnetic dipole moment b

Example: A current-caring circular coil is in uniform magnetic field Example: A current-caring circular coil is in uniform magnetic field. The maximum magnitude of the torque on the coil occurs when the angle between the field and magnetic moment is 45° 90° 135° 180°

Example: A coil is oriented initially with its magnetic dipole moment antiparallel to a uniform magnetic field. In this orientation torque  is equal to: A)  = 0 B)  = max C) 0<  < max Example: