Physics midterm question: two opposite charges brought closer

Physics 205B Midterm 2, spring semester 2015
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Conceptual Question 17.22, Problems 17.55, 17.56

As the distance between two charges is steadily decreased, their electric potential energy becomes a larger negative number. Discuss why these charges must have opposite signs for this to happen. Explain your reasoning using the properties of charges, electric forces, electric potential energies, and electric potentials.

Solution and grading rubric:

• p:
  Correct. With two charges of opposite signs, EPE = k∙q₁∙q₂/r must be negative, and thus decreasing r will make EPE a larger negative quantity.
• 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.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some garbled attempt at applying properties of charges, electric forces, electric potential energies, and electric potentials.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit. Approach other than that of applying properties of charges, electric forces, electric potential energies, and electric potentials.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.
  

Grading distribution:
Sections 30882, 30883
Exam code: midterm02m3tR
p: 43 students
r: 1 student
t: 3 students
v: 0 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 1288):