Physics quiz question: force on test charge exerted by electric field

Physics 205B Quiz 3, spring semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 16.25

A point charge is held at the origin. The electric field at x = +0.40 m is directed to the left, and has a magnitude of 3.5×10⁵ N/C. A –4.0×10^–6 C point charge placed at x = +0.40 m will experience a force of magnitude:
(A) 1.1×10^–11 N.
(B) 1.4 N.
(C) 8.8×10¹⁰ N.
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (B)

The magnitude of the force F experienced by a test charge q due to an electric field E is given by:

|F_{on q}| = |q·E| = |(–4.0×10^–6 C)·(3.5×10⁵ N/C)| = 1.4 N.

(Because the test charge is negative, the direction of the force exerted on it by the field (to the right) is opposite the direction of the field (to the left).)

Response (A) is |q/E|; response (C) is |E/q|.

Section 30882
Exam code: quiz03rD1o
(A) : 8 students
(B) : 16 students
(C) : 6 students
(D) : 2 students

Success level: 50%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.44