Physics quiz question: forces exerted between two fixed charges

Physics 205B Quiz 3, Fall Semester 2010
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 16.13

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 1.8e-6 N. The electric force of the –2.5 nC charge on the +3.0 nC charge is directed to the:
(A) left.
(B) right.
(C) (There is no direction, as the electric force is zero.)
(D) (Not enough information is given.)

Correct answer: (A)

Opposite charges attract, so the -2.5 nC charge will experience a force to the right, while the +3.0 nC charge will experience a force to the left.

Student responses
Section 70856
(A) : 5 students
(B) : 6 students
(C) : 0 students
(D) : 0 students

Success level: 45%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.58