Physics midterm question: changes in potential

Physics 205B Midterm 2, Fall Semester 2010
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Conceptual Questions 17.1, 17.4, 17.6

[10 points.] A positive source charge is held at the origin, and a negative test charge is at x = +4.0 cm. As the negative test charge is moved from x = +4.0 cm to x = +5.0 cm, determine whether it moves to a region of lower or higher potential. Explain your reasoning using the properties of electric potential.

Solution and grading rubric:
  • p = 10/10:
    Correct. The source charge is the sole relevant potential, increasing r in V = k(+Q)/r will decrease the positive numerical value of the potential.
  • r = 8/10:
    As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes.
  • t = 6/10:
    Nearly correct, but argument has conceptual errors, or is incomplete. Confounds potential of source charge with potential energy of two charges, but at least discussion of how potential energy changes is correct.
  • v = 4/10:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner.
  • x = 2/10:
    Implementation/application of ideas, but credit given for effort rather than merit.
  • y = 1/10:
    Irrelevant discussion/effectively blank.
  • z = 0/10:

Grading distribution:
Section 70856
p: 3 students
r: 0 students
t: 4 students
v: 4 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 7111):

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