20130314

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

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