Cuesta College, San Luis Obispo, CA
Two point charges are held at fixed locations. A +5 nC charge is at the origin, and a second –1 nC charge is at x = +4 cm. Discuss why the magnitude of the electric field at x = +2 cm is less than the magnitude of the electric field at x = +3 cm. Show your work and explain your reasoning using properties of electric forces, fields, and vector superposition.
Solution and grading rubric:
Correct. Discusses/demonstrates that the (total) electric field magnitude would be greater at x = +3 cm than at x = +2 cm by:
- evaluating the individual electric field magnitudes E1 and E2 created by the source charges Q1 and Q2 at each location (four separate terms), and;
- discussing how for each location the individual electric field magnitudes add together, as they both point in the same direction to the right (away from Q1 = +5 nC, and in towards Q2 = −1 nC), and;
- either completely evaluating the total electric field magnitude at each location, or comparing their values in terms of in common relative terms of k, nC, and cm2.
As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. May have a minor computational error, but at least conclusion is consistent with result.
Nearly correct, but approach has conceptual errors, and/or major/compounded math errors. At least some attempt at evaluating electric fields created by each source charge at both locations and vector superposition.
Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some garbled attempt at applying electric forces, fields, and vector superposition.
Implementation/application of ideas, but credit given for effort rather than merit. No clear attempt at applying electric forces, fields, and vector superposition.
Irrelevant discussion/effectively blank.
Sections 30882, 30883
Exam code: midterm01rx1C
p: 22 students
r: 7 students
t: 8 students
v: 4 students
x: 2 student
y: 0 students
z: 0 student
A sample "p" response (from student 0001), explicitly evaluating the electric field magnitudes:
A sample "p" response (from student 0720), eliminating common factors: