Cuesta College, San Luis Obispo, CA
Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 16.35
Two point charges are held at fixed locations. A +1.0 nC charge is at the origin, and a second –1.5 nC charge is at x = +2.0 cm. Discuss why the electric field to the left of both charges
(x < 0) is weaker than the electric field to the right of both charges (+2.0 cm < x). Explain your answer using the properties of charges, electric fields, and superposition. Solution and grading rubric:
- p:
Correct. Discusses:- that to the left or to the right of both charges, the electric fields from each charge point in different directions, thus the (total) electric field at either location will be the difference of the two electric field magnitudes;
- the negative charge will have a greater magnitude electric field compared to the positive charge's electric field at comparable distances;
- such that there is a greater difference between the opposing electric fields to the right of both charges than to the left of both charges, resulting in a stronger (total) electric field to the right of both charges.
- r:
As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. Chooses stronger electric field contribution to show greater magnitude of total electric field without explicit demonstration of superposition. - 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.
Sections 30882
Exam code: finalpL3x
p: 1 student
r: 5 students
t: 10 students
v: 10 students
x: 1 student
y: 1 student
z: 2 students
A sample "p" response (from student 7582):
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