Physics 205B Quiz 3, spring semester 2014
Cuesta College, San Luis Obispo, CA
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 point charge at the origin is:
(A) –1.3×10–8 C.
(B) –8.8×10–9 C.
(C) –5.3×10–14 C.
(D) +5.3×10–14 C.
(E) +8.8×10–9 C.
(F) +1.3×10–8 C.
Correct answer (highlight to unhide): (A)
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:
|F1 on 2| = k·|q1|·|q2|/(r2),
where q1 is the unknown charge at the origin, and q2 is the charge at x = +1.5 m. Then the absolute value of the unknown charge can then be solved for:
|q1| = |F1 on 2|·(r2)/(k·|q2|),
|q1| = |2.1×10–4 N|·(1.5 m)2/((8.99×109 N·m2/C2)·|–4.0×10–6 C|),
|q1| = 1.3139599555...×10–8 C.
As discussed above, this charge is expected to be negative, such that to two significant figures the charge at the origin is –1.3×10–8 C.
(Response (B) is |F1 on 2|·r/(k·|q2|); response (C) is |F1 on 2|·(r2)/k; responses (D)-(F) all have the wrong sign.)
Sections 30882, 30883
Exam code: quiz03cDvD
(A) : 22 students
(B) : 4 students
(C) : 4 students
(D) : 4 students
(E) : 0 students
(F) : 6 students
Success level: 55%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.66
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