Physics quiz question: electron beam current

Physics 205B Quiz 4, Spring Semester 2010
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problems 18.2, 18.5

The FLASH accelerator at the Deutsches Elektronen-Synchrotron* in Hamburg, Germany, generated a peak electron beam current of 9 mA in 2009. How many electrons passed through a cross section of this beam in one second?
(A) 6e+16 electrons.
(B) 7e+20 electrons.
(C) 1e+28 electrons.
(D) 1e+32 electrons.

Correct answer: (A)

Current I is given by:

I = delta(q)/delta(t).

Since the charge of one electron is 1.602e-19 C, then this problem involves unit cancellation:

I = (9e-3 C/s)*(1 electron/1.602e-19 C) = 6e16 electrons/s.

Student responses
Section 31988
(A) : 7 students
(B) : 1 student
(C) : 2 students
(D) : 1 student

Success level: 63%
Discrimination index (Aubrecht & Aubrecht, 1983): 1.00

Reference:
* "Successful FLASH Tests for European XFEL and ILC," http://www.desy.de/e409/e4948/e11064/e58382/index_eng.html, September 24, 2009.