20130528

Physics final exam question: squeezing a paper-dielectric capacitor

Physics 205B final exam, spring semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problems 17.55, 17.57

A parallel-plate capacitor is constructed by placing two foil sheets between different pages in a phone book. The capacitor is then connected to a 9 V battery. As the phone book is pressed down upon to squeeze the pages between the foil sheets, discuss why the capacitor will store more charge. Explain your reasoning using the properties of capacitors.

Solution and grading rubric:
  • p:
    Correct. Discusses:
    1. how capacitance is inversely proportional to the separation distance between the plates, so squeezing the plates closer together will increase the capacitance;
    2. increasing the capacitance while keeping the electric potential constant will result in increasing the charge stored on the capacitor plates.
  • r:
    As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. One of the two arguments in (p) is only nearly complete, or has minor inconsistencies.
  • t:
    Nearly correct, but argument has conceptual errors, or is incomplete. Only one of the two arguments in (p) is complete, the other is missing or only implied.
  • 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:
Sections 30882
Exam code: finalpL3x
p: 17 students
r: 5 students
t: 8 students
v: 0 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 4444), with a happy-face unicorn:

A sample "t" response (from student 8235), not explicitly discussing how an increase in "catpacitance" (with electric potential held constant) would then result an increase in charge:

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