Cuesta College, San Luis Obispo, CA
Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problems 17.1, 17.4
A +5.0 nC charge is held at the origin, and a –2.5 nC charge that can be moved horizontally along the x-axis is initially placed at x = +2.0 cm. In order to increase the electric potential energy of the –2.5 nC charge, should it be moved to the left, to the right, or would this not be possible? Explain your reasoning by using the properties of charges and electric potential energy.Solution and grading rubric:
- p:
Correct. The two charges attract each other, such that the the –2.5 nC charge will experience an electric force to the left. In order to increase its potential energy, to do work on this charge would require pulling it to the right. Makes a qualitiative argument, or demonstrates this quantitatively by plugging in numbers or argues that increasing r will make EPE = k·(+5.0×10–9 C)·(–2.5×10–9 C)/r a smaller negative number, thus increasing EPE. - r:
As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. - t:
Nearly correct, but argument has conceptual errors, or is incomplete. - v:
Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. - x:
Implementation/application of ideas, but credit given for effort rather than merit. - y:
Irrelevant discussion/effectively blank. - z:
Blank.
Grading distribution:
Section 30882
Exam code: midterm01g74S
p: 7 students
r: 1 student
t: 0 students
v: 0 students
x: 0 students
y: 0 students
z: 0 students
A sample "p" response (from student 9904) with qualitative argument based on doing work on charges to increase electric potential energy:
A sample "p" response (from student 7974), explicitly calculating the initial and final electric potential energies (without nC to 10–9 C conversions, but okay for determining the relative change in electric potential energy):
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