Physics quiz question: source charge of electric field

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

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Example 16.6

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. The point charge at the origin is:
(A) –2.0×10¹⁶ C.
(B) –1.6×10^–5 C.
(C) –6.2×10^–6 C.
(D) +6.2×10^–6 C.
(E) +1.6×10^–5 C.
(F) +2.0×10¹⁶ C.

Correct answer (highlight to unhide): (C)

The magnitude E of the electric field located at a distance r away from a source charge Q is:

E = k·|Q|/r²,

such that:

|Q| = E·r²/k,

|Q| = (3.5×10⁵ N/C)·(0.40 m)²/(8.99×10⁹ N·m²/C²).

|Q| = 6.229143493×10^–6 C,

or to two significant figures, the absolute value of the source charge Q is 6.2×10^–6. Since the direction of the electric field points in towards the source charge, then the source charge Q must be negative, such that Q = -6.2×10^–6 C.

(Responses (A) and (F) are from k·E/r², responses (B) and (E) are from E·r/k.)

Section 30882
Exam code: quiz03rD1o
(A) : 2 students
(B) : 0 students
(C) : 11 students
(D) : 5 students
(E) : 3 students
(F) : 11 students

Success level: 34%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.56