Physics quiz question: relativistic electrons

Physics 205B Quiz 7, Spring Semester 2011
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 26.45

If electrons had a Lorentz factor γ = 2.00, how fast would they be traveling?
(A) 0.414c.
(B) 0.500c.
(C) 0.866c.
(D) 1.41c.

Correct answer: (C)

Inverting the Lorentz factor equation γ = 1/sqrt(1 - (v/c)^2)) to solve for v results in:

v = c*sqrt(1 - 1/(γ^2)),

such that with γ = 2.00, v = 0.886c. Response (A) is 1 - c*sqrt(2*(γ - 1)); response (B) is c*sqrt(1/(γ^2)); response (D) is c*sqrt(2*(γ - 1)), which results in an implausible result.

Section 30882
Exam code: quiz07t0LO
(A) : 0 students
(B) : 1 student
(C) : 5 students
(D) : 2 students

"Success level": 75%
Discrimination index (Aubrecht & Aubrecht, 1983): -0.50

Compare to an older version of this question from Fall 2010.