## 20160705

### Physics final exam question: increasing negative electric potential energy value

Physics 205B Final Exam, spring semester 2016
Cuesta College, San Luis Obispo, CA

Two point charges lie along the  x-axis. A –2.0 nC charge is fixed at the origin, and a +1.0 nC charge is at x = +1.0 m. In order to make the electric potential energy of this system a smaller negative number, determine whether the +1.0 nC charge should be moved to the left, or to the right, or if this is not possible. Explain your reasoning by using the properties of charges, electric forces, electric potential, and electric potential energy.

• p:
Correct. Understands that the electric potential energy of these charges of opposite sign is negative, and to make it a smaller negative number, the distance between them must increase, such that the +1.0 nC charge must be moved to the right. May also argue from doing work on the +1.0 nC to increase the electric potential energy of this system.
• r:
As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes.
• 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, and electric potential energy.
• x:
Implementation/application of ideas, but credit given for effort rather than merit. No clear attempt at applying properties of charges, electric forces, electric potential, and electric potential energy.
• y:
Irrelevant discussion/effectively blank.
• z:
Blank.
Sections 30882, 30883
Exam code: finalm4u1
p: 28 students
r: 3 students
t: 2 students
v: 1 student
x: 3 students
y: 2 students
z: 0 students

A sample "p" response (from student 3214):

Another sample "p" response (from student 3158):

Yet another sample "p" response (from student 5396):