PHYS 1220 Round 4 Retests

The criteria for satisfying the standards are explained in the table below.

Standard	Quiz	Cutoff
1	No quiz
2	Standard 2 Retest 4	3 of 3
3	No quiz
4	No quiz
5	No quiz
6	No quiz
7	Standard 7 Retest 3	2 of 2
8	Standard 8 Retest 3	4 of 4
9	Standard 9 Retest 3	6 of 7
10	Standard 10 Retest 3	6 of 7
11	Standard 11 Retest 1	2 of 2
12	Standard 12 Retest 3	1 of 1
13	Standard 13 Retest 1	7 of 8
14	Standard 14 Retest 2	4 of 4
15	Standard 15 Retest 2	3 of 3
16	Standard 16 Retest 2	14 of 16
17	Standard 17 Retest 2	3 of 3
18	Standard 18 Retest 2	3 of 3
19	Standard 19 Retest 2	3 of 3
20	No quiz
21	Standard 21 Retest 1	3 of 3
22	Standard 22 Retest 1	1 of 1
23	Standard 23 Retest 1	4 of 4
24	Standard 24 Retest 1	7 of 9
25	Standard 25 Retest 1	2 of 2
26	Standard 26 Retest 1	5 of 6
27	Standard 27 Retest 1	3 of 3
28	Standard 28 Retest 1	2 of 2
29	Standard 29 Retest 2	3 of 3
30	Standard 30 Retest 1	7 of 8
31	Standard 31 Retest 1	2 of 2

If you think anything was graded inconsistently or in error, let me know.  I want you to be credited for what you understand.  Also, if you have questions about any of the retests, please contact me and I'll be happy to review.  I don't think I'll be able to post detailed solutions to these.

Grader's notes

Time doesn't allow me to go into much detail about the quizzes here.  If you have a question that isn't answered here, contact me soon.  There isn't much time until the final exam.

Standard 12 Retest 3.  The answer key was wrong, because I neglected to convert from grams to kilograms.  I changed the point values by hand.  Let me know if I missed yours.

Standard 13 Retest 2.  I think there was one tricky question here, which was the last question.  I don't believe I spent much (if any) time in class on the Carnot cycle.  The relevance to this question is that a Carnot cycle operates at maximum theoretical efficiency e = (Th−Tc)/Th.

Standard 16 Retest 2.  A couple of features of this quiz seemed to trip students up.  The first was that the conductor (yellow) carried a net charge of −2Q.  In the end, half of this charge was on the inner “skin” of the hollow, and the other half was on the outer “skin” of the conductor.

The second point, which is a key point to understanding 80 percent of Gauss's law problems, is that the electric field everywhere inside a static (no currents) conductor is zero.  This comes first; from that, Gauss's law allows us to conclude that the flux through any closed surface entirely within a conductor is zero, which further requires that the net electric charge enclosed by that surface is also zero.

There also seemed to be sone confusion about the Gaussian surface “just inside the outer skin of the conductor.” This was meant to be entirely within the conductor, but just up to the edge.  Thus it enclosed zero charge, and any excess charge on the conductor must be outside of it (on the skin).

Standard 19 Retest 2.  The capacitor with its plates being pulled apart apparently was confusing.  It wasn't explicitly stated that the separation of the plates had to remain small compared to the plate areas, so that the electric field between the plates remained uniform and there weren't significant fringing effects.  That said, as the plates are pulled apart, the field between them remains E = σ/ε₀.  The voltage then is Ed, which is directly proportional to the plate separation distance d.  The charge doesn't change, so the charge per voltage (the capacitance) decreases as the plate separation widens.  The potential energy of the capacitor increases, because the plates attract each other as they are separated.

Standard 24 Retest 1.  There are two Kirchhoff's circuit rules: the Voltage rule, which applies to loops, and the Current rule, which applies to nodes.  If your equation has voltage terms, it should describe a loop.  If it has current terms, it should describe a node.

My preferred set of equations to describe this particular circuit is
0 = ε₁ − I₁R₁ + ε₂ − I₂R₂
0 = I₂R₂ − I₄R₄ − I₃R₃
0 = ε₂ − I₄R₄ − I₅R₅
0 = I₁ − I₇
0 = I₇ − I₂ − I₃
0 = I₂ + I₄ − I₆
0 = I₃ + I₅ − I₄

Standard 31 Retest 1.  The graph of Question 1 is a graph of magnetic flux versus time.  The emf depends on the rate of change of the flux, not on the particular value of the flux at one time.  The greatest emf occurs when the slope of the plot is steepest.