Physics quiz question: magnetic force of current-carrying wire on moving charge

Physics 205B Quiz 6, spring semester 2012
Cuesta College, San Luis Obispo, CA

A long straight wire carries a current. A q = +6.0 nC charge experiences a force to the right. The direction of the charge's velocity is:
(A) up ↑.
(B) down ↓.
(C) left ←.
(D) right →.
(E) into the page ⊗.
(F) out of the page ⊙.
(G) (Some other (diagonal) direction not listed above.)
(H) (No direction, as the magnetic force is zero.)

Correct answer (highlight to unhide): (A)

For the field model of magnetism, the source object is the current in the wire, which generates a magnetic field around itself. This magnetic field (generated by the current in the wire) then exerts a force on the test object: the moving positive charge (although its direction of motion is unknown).

At the position of the charge, the direction of the magnetic field B points out of the plane of the page, as given by the second right-hand rule (where the thumb is the direction of current, and fingers "wrap around" the wire for the direction of the magnetic field lines surrounding it).

Using the first right-hand rule (RHR1), the direction of the positive charge's velocity v (the thumb) is unknown. However, the direction of the magnetic field B (the index finger) points out of the plane of the page, and the direction of the force F on the positive charge (the middle finger) is given as pointing to the right along the plane of the page. The direction of the positive charge's velocity v must then point up along the plane of the page.

Section 30882
Exam code: quiz06LR1r
(A) : 20 students
(B) : 2 students
(C) : 0 students
(D) : 1 student
(E) : 1 student
(F) : 3 students
(G) : 0 students
(H) : 0 students

Success level: 74%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.75