Physics final exam question: electric field of two charges

Physics 205B Final Exam, Spring Semester 2011
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 16.32

[10 points.] Two point charges are fixed at the locations shown at right. Determine the direction of the total electric field at the location x = +3.0 cm. Explain your answer using the properties of charges, electric fields, and superposition.

Solution and grading rubric:
  • p = 10/10:
    Correct. At x = +3.0 cm, electric field of -8.0 µC charge points to the left, while electric field of 2.0 µC charge also points to the left, such that the total electric field at that location points to the left.
  • 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. Chooses stronger electric field contribution to determine direction of total electric field.
  • t = 6/10:
    Nearly correct, but argument has conceptual errors, or is incomplete. Calculates magnitudes of each electric field contribution at x = +3.0 cm, but garbles directions and/or superposition.
  • 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 30882
Exam code: finalv0L7
p: 3 students
r: 2 students
t: 3 students
v: 0 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 3773):

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