A2L Item 181
- Description: Identify the direction of the net force on a current carrying semicircular wire in a uniform B-field.
- Goal: Recognizing forces on current elements
- Source: 283 Force on a half-loop
- Keywords: Electromagnetism, Forces, Magnetic Field, Right-Hand Rule
The question for students:
A
semicircular wire lies in a plane as shown. The positive z-direction is
out of the plane. The wire has current, I, in the counterclockwise
sense, and it is in a uniform external magnetic field, B, directed along
the +y axis. What is the direction of the net force, if any, acting on
the wire?
- +x
- -x
- +y
- -y
- +z
- -z
- None of the above.
Commentary for teachers:
Answer
(6) Since the current carrying semicircle lies in the x-y plane, as does the magnetic field, the net force, if any, must point perpendicular to the plane, or in the z direction. For the semicircular wire, all force contributions add. There is no contribution to the net force from current elements near the x-axis.
The force on the missing half of the loop would be out of the page. Together both forces on a full loop would create a torque tending to align the field of the current loop with the external field. If appropriate relate this situation to the torque on a magnetic dipole.