Physics quiz question: charge moving through magnetic field

Physics 205B Quiz 6, Spring Semester 2010
Cuesta College, San Luis Obispo, CA

[Version 1]

Consider an electron moving to the right at a through a uniform magnetic field. Increasing the speed of the electron (along the same direction) would __________ the magnitude of the magnetic force exerted on it.
(A) decrease.
(B) have no effect on.
(C) increase.
(D) (Not enough information is given.)

Correct answer: (C)

The magnetic force exerted on a moving charge is given by:

F_B = q*v*B*sin(theta),

where the angle between the charge velocity, and the magnetic field is 90 degrees, such that sin(theta) = 1, thus:

F_B = q*v*B.

Increasing the speed of the electron would increase the magnitude of the magnetic force on the electron.

Student responses
Section 31988
(A) : 3 students
(B) : 0 students
(C) : 5 students
(D) : 0 students

[Version 2]

Decreasing the speed of the electron (along the same direction) would __________ the magnitude of the magnetic force exerted on it.
(A) decrease.
(B) have no effect on.
(C) increase.
(D) (Not enough information is given.)

Correct answer: (A)

Student responses
Section 31988
(A) : 1 student
(B) : 1 student
(C) : 3 students
(D) : 0 students

Success level: 46%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.75