Physics quiz question: electric force by known charge on unknown charge

Physics 205B Quiz 3, spring semester 2014
Cuesta College, San Luis Obispo, CA

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

Two point charges are held at fixed positions. The –4.0 µC point charge at x = +1.5 m experiences a force directed to the right with a magnitude of 2.1×10^–4 N. The magnitude of the force experienced by the point charge at the origin is directed to the __________, and its magnitude is _________ 2.1×10^–4 N.
(A) left; less than.
(B) left; equal to.
(C) left, greater than.
(D) right; less than.
(E) right; equal to.
(F) right, greater than.

Correct answer (highlight to unhide): (B)

The force on the –4.0 µC charge at x = +1.5 m points to the right, such that it is being repelled by the unknown charge at the origin. This means that the charge at the origin must also be a negative charge, as like charges repel.

The magnitude of the force on the –4.0 nC charge at x = +1.5 m is given by:

|F| = k·|q₁|·|q₂|/(r²),

where |q₁| is the amount (in coulombs) of the unknown charge at the origin, and q₂ is the charge at x = +1.5 m. This would also be the magnitude of the force on the unknown charge at the origin, which would have the same expression with the same quantities in the calculation. This could also be understood as a consequence of Newton's third law, where the force on the –4.0 µC charge at x = +1.5 m would be equal in magnitude and opposite in direction to the force on the unknown charge at the origin.

Sections 30882, 30883
Exam code: quiz03cDvD
(A) : 12 students
(B) : 18 students
(C) : 3 students
(D) : 4 students
(E) : 3 students
(F) : 0 students

Success level: 45%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.60