Physics midterm question: attraction via charge induction

Physics 205B Midterm 1, fall semester 2010
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Conceptual Question 16.14

A Physics 205B student holds a charged plastic rod near a neutral conducting rod, hanging from a non-conducting string attached to its center. Discuss why the neutral conducting rod will always be attracted by the charged plastic rod, regardless of whether the charged plastic rod is positive or negative. Explain your reasoning by drawing diagrams using the properties of charges and induction.

Solution and grading rubric:
  • p:
    Correct. Electrons are mobile in the conductor (while protons in atomic nuclei sites are fixed). Bringing a positively/negatively charged plastic rod nearby will attract/repel the electrons in the conductor, which will cause the end of the conductor to have a negative/positive charge, which is a stronger attractive force (due to being closer) than the positive/negative charge on the far side of the condcutor, which will have a weaker repulsion force (due to being farther). Thus the net force on the conductor will be attractive.
  • r:
    As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. One argument missing or switched. Has protons mobile in the conductor, or did not explicitly consider the second case where the plastic rod has the opposite charge.
  • t:
    Nearly correct, but argument has conceptual errors, or is incomplete. Both omissions in (r).
  • 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:

Grading distribution:
Section 70856
Exam code: midterm01voL7
p: 7 students
r: 3 students
t: 1 student
v: 0 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 9999):

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