5 Magnetic forces on current A l Example

5. Magnetic forces on current A 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? 1) left 2) right I 3) zero 4) into the page B 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? 1) left I 2) right 3) zero 4) into the page B 5) out of the page

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? 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 z B center of the loop. Thus, the forces of the opposing segments cancel, so the net force is zero x y

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 F 2) move down 3) rotate clockwise 4) rotate counterclockwise S N N B S 5) both rotate and move B field out of North B field into South F 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. up At the south pole, the same logic leads to a downward force. Thus the loop rotates clockwise

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

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 0 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 Example: B) = max C) 0< < max