## 20180227

### Physics quiz question: separation distance between two point charges

Physics 205B Quiz 3, fall semester 2010
Cuesta College, San Luis Obispo, CA

Two point charges  (–2.5 nC and +3.0 nC) are held a certain distance apart.  The magnitude of the electric force of the –2.5 nC charge on the +3.0 nC charge is 1.8×10–6￼ N. The distance between these charges is:
(A) 2.0×106 m.
(B) 0.037 m.
(C) 0.050 m.
(D) 0.19 m.

Correct answer (highlight to unhide): (D)

The magnitude of the force of the q1 = –2.5 nC charge on the q2 = +3.0 nC charge is given by Coulomb's law:

|F1 on 2| = k·|q1|·|q2|/(r2),

where r is the center-to-center distance separating these two charges, and can be given by:

r = √(k·|q1|·|q2|/|F1 on 2|),

r = √(8.99×109 N·m2/C2)·|–2.5×10-9 C|·|+3.0×10–9 C|/|1.8×10–6￼ N|),

r = 0.1935415545 m,

or to two significant figures, 0.19 m.

(Response (A) is √(|q1|·|q2|/|F|); response (B) is (k·|q1|·|q2|/|F|); response (C) is √(k·(1×10–18)·(|q1| + |q2|)/|F|).)

Section 30882
Exam code: quiz03C0u7
(A) : 0 students
(B) : 3 students
(C) : 2 students
(D) : 6 students

Success level: 54%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.67