Physics quiz archive: interference, electrostatics

Physics 205B Quiz 3, spring semester 2020
Cuesta College, San Luis Obispo, CA
Sections 30882, 30883, version 1
Exam code: quiz03Cv1d



Sections 30882, 30883 results

0- 6 :
7-12 :
13-18 :	**** [low = 15]
19-24 :	***************
25-30 :	*************** [mean = 24.2 +/- 4.2] [high = 30]

Online reading assignment: interference

Physics 205B, spring semester 2020
Cuesta College, San Luis Obispo, CA

Students have a bi-weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters and previewing presentations on interference.


Selected/edited responses are given below.

Describe what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.

"I understand that if two sound speakers are wired the same, then they will be in phase which results in constructive interference and the sound will be louder. If they are not wired the same, then they will be out of phase which results in destructive interference and a more quiet sound."

"If two waves are in phase and have a whole length difference, they will produce constructive interference. If the waves are out of phase or have a half wavelength difference, destructive interference is produced."

"The principle of linear superposition states that the resultant disturbance is the sum of the disturbances from the individual waves. I also understand that interference alters the brightness of light and loudness of sound."

"This paragraph from the presentation helped me understand the basics: 'Here we have two speakers, which are our sources of two sound waves. Since they are plugged into the same frequency source, they will generate sound waves of the same frequency f (which is depends only on the source), same speed v (which depends only on the medium), and thus the same wavelength λ (which depends on both f and v). If the speakers are wired the same way--red and black wires to red and black plugs--then they will oscillate in phase, with both speaker cones moving forward and backwards in unison.'"

"I understood the in-phase and out-of-phase part, and how to tell the difference. I also understood what is constructive and destructive."

"The way two wave sources are emitted determines whether the resulting wave will be constructive or deconstructive. In-phase waves have the same velocity, wavelength, and frequency, also in-phase waves are traveling in the same direction and can form a constructive of bigger resulting wave. Out-Phase waves do not align crest to trough and they do not have the same velocity, frequency or wavelength and the resulting wave is smaller or no wave."

"My primary take-away from this set of slides is the difference between constructive interference (music playing from the neighbor's stereo), and destructive interference (silence, or quieted volume). When the crests and troughs of the two wavelengths line up, constructive interference occurs; however, if those wavelengths are not aligned, destructive interference takes place."

"There was not much that I understood from this presentation."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.

"I need a little more detail about when the interfering wave examples have a whole wavelength path length difference or half-wavelength path length difference."

"Completely grasping the constructive/destructive conditions flowchart."

"I found the flowchart of constructive/destructive conditions to be overwhelming especially because I don't fully understand the solid lines vs. the dashed lines. I could also use a bit of clarification on what path difference is."

"I would like a little more understand on phase differences. As well as how path length difference works."

"At first, the path length difference was confusing to understand because I did not comprehend the equations. However, after seeing some visual examples, I now understand and can observe whether a wave will be constructive or destructive."

"The difference between constructive and destructive seemed confusing. Its relationship with waves was easier to understand with the diagrams."

"I found a lot confusing with these concepts. I don't understand the difference between constructive and destructive waves. I also don't how changing the distance makes the two so different. I am also confused what they are considered when the waves are in unison, but the distance is changed."

"How does this work for radio waves? Please give examples of constructive, destructive and non-coherent sources. The book only uses visible light as an example."

"This isn't very confusing to me."

Classify the various interfering wave examples. (Only correct responses shown.)

1: in phase sources [91%]; whole wavelength path difference [74%]; constructive [97%]
2: in phase sources [14%]; half wavelength path difference [86%]; destructive [89%]
3: in phase sources [86%]; whole wavelength path difference [77%]; constructive [89%]
4: out of phase sources [86%]; whole wavelength path difference [40%]; destructive [94%]
5: out of phase sources [23%]; half wavelength path difference [49%]; constructive [86%]
6: out of phase sources [89%]; whole wavelength path difference [54%]; destructive [97%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"Finally lenses are over. I don't wear glasses so it was a difficult set of chapters." (I'm not so sure that this topic was any easier for student who do wear corrective optics.)

"When you change the volume on a stereo how is more sound produced? Are more sound waves generated?" (Well, "more" in the sense that the speaker cone moves in and out with a larger range of motion (larger amplitude), so more air is pushed in and out, and a greater range of pressure fluctuations result.)
"What does m stand for in the constructive/destructive conditions equation?" (Any whole positive/negative number (0, ±1, ±2, etc.).)

"Why in the world could I not find anything about path length differences?"

"May we review situation #5 specifically?"

"I just need a brief discussion about these ideas and I think I will be good on this topic."

"The wave interference problems don't seem too difficult, just need some practice."

"In that case given the two sources had the same total length and each crest and trough matched, I assumed there would be no length difference; yet, that was not an option?" (A case where there is no path length difference is the covered under the case of having a path length difference of a whole (number) of wavelengths, in this case the whole number would be zero.)

"If two sound waves are not exactly a half-wavelength off from each other, would there still be destructive interference?" (Yes, but they would not completely cancel each other out. The resulting loudness would be quieter than a single wave, but not completely quiet.)

"Will we be using noise-canceling headphones?" (That is a great application of destructive interference, but no. You can always purchase a set for yourself, though.)

"I was confused about coherent and non-coherent light sources of light. What makes a laser coherent compared to light bulbs which are incoherent?" (Basically, the light from a laser has a constant amplitude, single wavelength that starts off with a definite phase, while light from an incandescent bulb is a range of different wavelengths that can vary in amplitude and phase. It's a bit more complicated than that, but for our purposes we'll start from this working definition.)

Physics midterm problem: total electric field direction to the side of two charges

Physics 205B Midterm 1, spring semester 2019
Cuesta College, San Luis Obispo, CA

A –3.0 nC point charge is held at the origin, and +2.0 nC point charge is held at x = +1 cm. Discuss why the electric field at x = +2 cm is directed to the right. Explain your reasoning using properties of electric forces, fields, and vector superposition.

Solution and grading rubric:

• p:
  Correct. Discusses/demonstrates that the (total) electric field at x = +2 cm would point to the right by:
  1. calculating or comparing the individual electric field magnitudes for E₂ (larger magnitude, 1.8×10⁵ N/C) and E₁ (smaller magnitude, 6.8×10⁴ N/C) created by the source charges Q₁ and Q₂ at x = +2 cm; and
  2. determining the individual electric field directions for E₁ (to the right, out away from the positive Q₁ source charge) and E₂ (to the left, in towards the negative Q₂ source charge) at x = +2 cm; and
  3. explaining that the vector superposition of E₁ and E₂ at x = +2 cm will result in a total electric field that points to the right, as it (a) will have the direction of the larger of two opposite electric field vectors there, or (b) would be the result of the subtraction of a smaller E₁ magnitude (to the left) from the larger E₂ magnitude (to the right).
• 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 approach has conceptual errors, and/or major/compounded math errors. At least some attempt at finding electric field magnitudes and directions created by each source charge at x = +2 cm, and discusses result of the addition of these electric field vectors.
• v:
  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. Typically discusses the force that these charges exert on each other.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit. No clear attempt at applying electric forces, fields, and vector superposition.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.
  

Grading distribution:
Sections 30882, 30883
Exam code: midterm01Ft6G
p: 24 students
r: 3 students
t: 6 students
v: 7 students
x: 0 students
y: 0 students
z: 2 students

A sample "p" response (from student 1810):
Another sample "p" response (from student 7843), being more explicit about the different directions of the individual electric fields:
And another sample "p" response (from student 1982), ignoring the common factor of k, and nC and cm unit conversions for purposes of comparison:

Physics quiz archive: interference, electrostatics

Physics 205B Quiz 3, spring semester 2019
Cuesta College, San Luis Obispo, CA
Sections 30882, 30883, version 1
Exam code: quiz03b3Am



Sections 30882, 30883 results

0- 6 :	** [low = 3]
7-12 :	**
13-18 :	*********
19-24 :	***************** [mean = 21.5 +/- 6.0]
25-30 :	************ [high = 30]

Online reading assignment: interference

Physics 205B, spring semester 2019
Cuesta College, San Luis Obispo, CA

Students have a bi-weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters and previewing presentations on interference.


Selected/edited responses are given below.

Describe what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.

"The principle of linear superposition, states that the resultant disturbance is the sum of the disturbances from the individual waves, which can be constructive or destructive interference."

"Two waves combine when they arrive at the same point. They reinforce each other during constructive interference or cancel each other through destructive interference."

"If two waves constructively interfere, their crests and troughs line up creating a bigger wave. In contrast, if two waves destructively interfere, the crests and troughs are contrary and they cancel out each other. Also whether two waves are constructive or destructive not only depends on the source phases of the waves, but also on the path length difference."

"I like how visual this lecture was. It seemed much easier to comprehend seeing the waves and the differences in their peaks and troughs which then made it easier to understand that when they align they are constructive and when they do not they are destructive."

"In phase sources result in constructive interference and out of phase sources result in destructive interference. For two in phase sources, if one wave travels a whole wavelength longer than the other, then constructive interference occurs; if instead one wave is a half wavelength longer than the other, then destructive interference occurs. Out of phase sources have an opposite result."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.

"The mathematical equations regarding the conditions; I would like those to be explained in-depth."

"The equations are confusing. Applying things to the diagrams needs more elaboration."

"This section is pretty straightforward."

"No questions today but I think I am going to like this section."
"I think I understand this section but I feel like I'm missing something."

Classify the various interfering wave examples. (Only correct responses shown.)

1: in phase sources [85%]; whole wavelength path difference [62%]; constructive [91%]
2: in phase sources [35%]; half wavelength path difference [74%]; destructive [85%]
3: in phase sources [76%]; whole wavelength path difference [71%]; constructive [85%]
4: out of phase sources [79%]; whole wavelength path difference [38%]; destructive [85%]
5: out of phase sources [35%]; half wavelength path difference [71%]; constructive [79%]
6: out of phase sources [82%]; whole wavelength path difference [62%]; destructive [88%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"I need to better understand the source phase and path length differences. I will go over the presentation preview to better understand when a wave is considered to be constructive or destructive when the sources are out of phase."

"Okay, it was going smoothly up until the in phase and out of phase questions."

"I am not so sure about in phase vs out of phase sources so some clarification on that would be helpful."

"I'm a little confused on constructive/destructive differences. How can waves be in phase when one travels one-half of a wavelength longer than the other? I think I've only half got this concept down." ("In phase" or "out of phase" refers to how the two waves start off in relation to each other; either they both start off making crests (or troughs) together, so these two sources are said to be "in phase" with each other (regardless of how their waves travel after that); or one source makes a crest while the other source makes a trough (or vice versa), so these two sources are said to be "out of phase" with each other (regardless of how their waves travel after that).)

"I love the double-slit experiment! To me it's where what we think we know about the universe really starts to lose footing!" (We'll get there next week. #toosoon.)

Physics midterm question: charge to "cancel out" electric field

Physics 205B Midterm 1, spring semester 2018
Cuesta College, San Luis Obispo, CA

A –3 nC point charge is held at x = 0, such that there is an electric field at x = +4 cm. In order to have no electric field magnitude at x = +4 cm, discuss whether the amount of positive charge to be placed at x = +1 cm should be less than, greater than, or equal to 3 nC. Explain your reasoning using properties of electric forces, fields, and vector superposition.

Solution and grading rubric:

• p:
  Correct. The amount of the (positive) source charge Q₂ must be less than 3 nC in order for the (total) electric field magnitude at x = +4 cm to be zero by discussing/demonstrating:
  1. that the electric field at x = +4 cm of the Q₁ = +3.0 nC source charge pointing to the right (away from this positive source charge) must be "canceled" by the electric field at x = +4 cm of the negative Q₂ source charge pointing to the left (in towards this negative source charge), such that the magnitudes of their respective electric fields must be set equal to each other;
  2. solves for the (negative) amount of Q₂ source charge, which is less than 3.0 nC. (May also place a trial amount of Q₂ source charge to demonstrate that it must be less than (−)3.0 nC in order for its electric field to equal the magnitude of the Q₁ source charge.)
• r:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. May not explicitly discuss/demonstrate how the numerical values of the two source charges does not matter.
• 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 electric forces, fields, and vector superposition.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit. No clear attempt at applying properties of electric forces, fields, and vector superposition.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.
  

Grading distribution:
Sections 30882, 30883
Exam code: midterm01cVdP
p: 21 students
r: 5 students
t: 6 students
v: 3 students
x: 0 student
y: 0 students
z: 0 student

A sample "p" response (from student 2525):
Another sample "p" response (from student 7744), dropping all common k factors, and common nC and cm units:

Physics quiz archive: interference, electrostatics

Physics 205B Quiz 3, spring semester 2018
Cuesta College, San Luis Obispo, CA
Sections 30882, 30883, version 1
Exam code: quiz03Ch4R



Sections 30882, 30883 results

0- 6 :
7-12 :	** [low = 9]
13-18 :	****
19-24 :	************ [mean = 23.3 +/- 4.9]
25-30 :	************** [high = 30]

Online reading assignment: interference

Physics 205B, spring semester 2018
Cuesta College, San Luis Obispo, CA

Students have a bi-weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters and previewing presentations on interference.


Selected/edited responses are given below.

Describe what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.

"When two light waves arrive the same point they combine. they can reinforce each other during constructive interference, or cancel each other through destructive interference."

"With constructive interference the waves are in sync and reinforce each other and with destructive interference the waves cancel each other out. There are also different types of source phases and path length differences that can affect whether a wave is constructive or destructive."

"I understand that when sound waves troughs and crests line up (move in unison) there is constructive interference. On the other hand, when the troughs and crests don't line up (out of unison) there is destructive interference."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.

"I found distinguishing between in phase and out of phase sources slightly confusing. I'm not entirely sure whether a wave is considered to be constructive or destructive when the sources are out of phase. I just need some examples and I think I'll be able to figure it out."

"I don't understand the source phase and path length differences."

"I don't understand how to determine the path length differences."

"I have a good understanding of this material. Maybe some in-class examples of constructive and destructive interference would be helpful."

"I didn't find anything really confusing this lesson."

Classify the various interfering wave examples. (Only correct responses shown.)

1: in phase sources [92%]; whole wavelength path difference [96%]; constructive [92%]
2: in phase sources [63%]; half wavelength path difference [96%]; destructive [92%]
3: in phase sources [83%]; whole wavelength path difference [83%]; constructive [92%]
4: out of phase sources [83%]; whole wavelength path difference [67%]; destructive [88%]
5: out of phase sources [42%]; half wavelength path difference [63%]; constructive [83%]
6: out of phase sources [92%]; whole wavelength path difference [42%]; destructive [88%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"Are you enjoying the Olympics?" (Only while watching Mrs. P-dog's cousin compete in slopestyle and big air snowboarding.)

I think I got these diagrams...but sometimes I think I understand something and it later turns out that I'm entirely wrong."

"Lab due next Monday?" (No school next Monday. So, they're due next next Monday.)

Physics midterm question: possible directions of magnetic force

Physics 205B Midterm 2, spring semester 2017
Cuesta College, San Luis Obispo, CA

An unspecified amount of charge q moves to the right in a uniform magnetic field that is present everywhere, and experiences a force up along this page. Discuss how there can be two possible directions for the magnetic field, and how those directions depend on the sign of the moving charge. Explain your reasoning using the properties of magnetic fields and forces.

Solution and grading rubric:

• p:
  Correct. Clearly discusses or draws diagrams showing how from RHR1 the magnetic field must point into the page for a positive charge, and the magnetic field must point out of the page for a negative charge.
• 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. May have magnetic field directions switched for positive and negative charges, or may conflate field and forces in discussion.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some attempt at applying properties of magnetic fields and forces.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit. Approach other than that of applying properties of magnetic fields and forces.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 30882, 30883
Exam code: midterm02GruT
p: 21 students
r: 0 students
t: 4 student
v: 4 students
x: 0 students
y: 0 students
z: 0 student

A sample "p" response (from student 0428):
Another sample "p" response (from student 2525):

Physics midterm question: non-zero total electric field between two charges

Physics 205B Midterm 1, spring semester 2017
Cuesta College, San Luis Obispo, CA

Two point charges are held at fixed locations at x = –2 cm and at x = +2 cm. The charge located at x = –2 cm is negatively charged, and the charge located at x = +2 cm is positively charged. Discuss why it is not possible for the electric field at the origin to be zero, for any possible numerical values (equal or unequal) for these two charges. Explain your reasoning using properties of electric forces, fields, and vector superposition.

Solution and grading rubric:

• p:
  Correct. The (total) electric field magnitude at x = 0 cannot be zero for any possible numerical values for the two source charges, because:
  1. the electric field at x = 0 of the Q₁ source charge points to the left (in towards this negative source charge), and the electric field at x = 0 of the Q₂ source charge also points to the left (out away from this positive source charge), and;
  2. no matter what numerical values the two source charges have, since these two electric field vectors will add at x = 0 (because they point in the same direction, to the left), they cannot possibly cancel each other out.
• r:
  Nearly correct, but includes minor math errors. May not explicitly discuss/demonstrate how the numerical values of the two source charges does not matter.
• t:
  Nearly correct, but approach has conceptual errors, and/or major/compounded math errors.
• v:
  Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. Some garbled attempt at applying properties of electric forces, fields, and vector superposition. May discuss how the two charges attract each other, or has their electric fields pointing pointing in opposite directions at x = 0.
• x:
  Implementation of ideas, but credit given for effort rather than merit. No clear attempt at applying properties of electric forces, fields, and vector superposition.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.
  

Grading distribution:
Sections 30882, 30883
Exam code: midterm01AhC4
p: 9 students
r: 12 students
t: 0 students
v: 9 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 1381):

Physics quiz archive: interference, electrostatics

Physics 205B Quiz 3, spring semester 2017
Cuesta College, San Luis Obispo, CA
Sections 30882, 30883, version 1
Exam code: quiz03d3St



Sections 30882, 30883 results

0- 6 :
7-12 :	* [low = 9]
13-18 :	*****************
19-24 :	******** [mean = 18.6 +/- 4.4]
25-30 :	** [high = 27]

Physics midterm question: total magnetic field of two perpendicular wires

Physics 205B Midterm 2, spring semester 2016
Cuesta College, San Luis Obispo, CA

Two straight wires carry equal amounts of current, crossing perpendicular to each other in the plane of the page. Discuss why the direction of the total magnetic field at the location shown points out of the page. Show your work and explain your reasoning using the properties of magnetic fields, and superposition.

Solution and grading rubric:

• p:
  Correct. Clearly discusses or draws diagrams showing how from RHR2 the magnetic field from each wire points out of the xy plane in the lower right quadrant, resulting in a total magnetic field that points in that direction.
• 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 attempt at applying properties of magnetic fields, and superposition.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit. Approach other than that of applying properties of magnetic fields, and superposition.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 30882, 30883
Exam code: midterm02Mc4s
p: 26 students
r: 2 students
t: 1 student
v: 13 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 1157):

Physics midterm problem: comparing total electric field magnitudes

Physics 205B Midterm 1, spring semester 2016
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:

• p:
  Correct. Discusses/demonstrates that the (total) electric field magnitude would be greater at x = +3 cm than at x = +2 cm by:
  1. evaluating the individual electric field magnitudes E₁ and E₂ created by the source charges Q₁ and Q₂ at each location (four separate terms), and;
  2. 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;
  3. 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.
• r:
  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.
• t:
  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.
• v:
  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.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit. No clear attempt at applying electric forces, fields, and vector superposition.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.
  

Grading distribution:
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 students

A sample "p" response (from student 0001), explicitly evaluating the electric field magnitudes:
A sample "p" response (from student 0720), eliminating common factors:>

Physics quiz archive: interference, electrostatics

Physics 205B Quiz 3, spring semester 2016
Cuesta College, San Luis Obispo, CA
Sections 30882, 30883, version 1
Exam code: quiz03Ccf7



Sections 30882, 30883 results

0- 6 :	** [low = 6]
7-12 :	***********
13-18 :	******
19-24 :	*************** [mean = 18.8 +/- 7.2]
25-30 :	****** [high = 30]

Physics final exam problem: unknown charge contributing to total electric field

Physics 205B Final Exam, spring semester 2015
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 16.15

Two point charges are held at fixed locations. An unknown charge is at the origin, and a –4.0 µC charge is at x = +0.20 m. The electric field at x = +0.10 m points to the right and has a magnitude of 1.8✕10⁵ N/C. Determine the sign (±) and amount of the unknown charge (in either coulombs or µC) that is located at the origin. Show your work and explain your reasoning using properties of electric forces, fields, and vector superposition.

Solution and grading rubric:

• p:
  Correct. Determines that:
  1. the electric field E₁ at x = +0.10 m due to the Q₁ = 4.0 μC charge at x = +0.20 m points to the right, but is more than the total electric field E_tot at x = +0.10 m (that points to the right), thus the electric field E₂ at x = +0.10 m due to the Q₂ charge at the origin must point to the left, and so the charge Q₂ must be negative as well;
  2. from setting up vector superposition of the two oppositely directed electric fields E₁ and E₂ at x = +0.10 m, that the Q₂ charge at the origin must be –3.8 μC.
• r:
  Nearly correct, but includes minor math errors.
• t:
  Nearly correct, but approach has conceptual errors, and/or major/compounded math errors. At least recognizes that there is superposition of two electric fields at x = +0.10 m that result in the given E_tot there.
• v:
  Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. Misinterprets E_tot at x = +0.10 m m to be the electric force exerted by Q₁ on Q₂, and then solves for Q₁ from Coulomb's law.
• x:
  Implementation of ideas, but credit given for effort rather than merit. No clear attempt at applying electric forces, fields, and vector superposition.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.
  

Grading distribution:
Sections 30882, 30883
Exam code: finalLd0c
p: 3 students
r: 2 students
t: 8 students
v: 19 students
x: 4 student
y: 3 students
z: 4 student

A sample "p" response (from student 8282):
A sample "v" response (from student 9224):

Physics midterm question: comparing total electric field magnitudes

Physics 205B Midterm 1, spring semester 2015
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 16.15

Two point charges are held at fixed locations. A +8 nC charge is at the origin, and a second –4 nC charge is at x = +2 cm. The electric field at x = +4 cm is measured. The charge at the origin is then changed to +4 nC, and the electric field at x = +4 cm is again measured. Discuss why the second case has a greater electric field magnitude at x = +4 cm. Explain your reasoning using properties of electric forces, fields, and vector superposition.

Solution and grading rubric:

• p:
  Correct. Discusses/demonstrates that the (total) electric field at x = +4 cm would be greater for the second case (B) using the superposition of the electric field vectors from both source charges (Q₁ = +8 nC and Q₂ = –4 nC for case (A); Q₁ = +4 nC and Q₂ = –4 nC for case (B)), which in both cases point in opposite directions, such that superposition of these vectors is the difference of their magnitudes, and either:
  1. explicitly evaluates the numerical values of the total electric field at x = +4 cm to show that it is indeed greater for the second case (B);
  2. qualitatively discusses how the electric field produced by the Q₁ source charge always points in the opposite direction of, and has a magnitude less than the electric field produced by the Q₂ source charge, such that decreasing the amount of the Q₁ source charge (while keeping its (–) sign) will decrease the amount "subtracted" from the electric field produced by the Q₂ source charge.
• r:
  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.
• t:
  Nearly correct, but approach has conceptual errors, and/or major/compounded math errors. Nearly correct, but argument has conceptual errors, or is incomplete. At least some attempt at evaluating electric fields created by each source charge at x = +4 cm and vector superposition.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Focus on comparing force exerted by source charge Q₁ on source charge Q₂, instead of the comparing effect of electric fields created by source charge Q₁ and source charge Q₂ on x = +4 cm.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit. Approach other than that of applying electric forces, fields, and vector superposition.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.
  

Grading distribution:
Sections 30882, 30883
Exam code: midterm01p34K
p: 30 students
r: 1 student
t: 3 students
v: 12 students
x: 0 student
y: 0 students
z: 0 student

A sample "p" response (from student 1828):
Another sample "p" response (from student 8984):
A sample "v" response (from student 0203):
A sample "y" response (from student 1212):

Physics quiz archive: interference, electrostatics

Physics 205B Quiz 3, spring semester 2015
Cuesta College, San Luis Obispo, CA
Sections 30882, 30883, version 1
Exam code: quiz03wJnb



Sections 30882, 30883 results

0- 6 :	****** [low = 0]
7-12 :	*****
13-18 :	****************** [mean = 16.7 +/- 7.2]
19-24 :	***********
25-30 :	**** [high = 30]

Physics final exam problem: total magnetic field of two current-carrying wires

Physics 205B Final Exam, spring semester 2014
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problems 19.61, 19.71

Two long straight wires carry currents, both outwards perpendicular to the plane of this page. The left wire (located at x = 0) carries twice as much current as the right wire (located at x = +0.20 m). The magnitude of the total magnetic field at x = +0.30 m is 8.0×10^–7 T.

Determine (a) the direction of the total magnetic field at x = +0.30 m, and (b) the amount of current in the left wire (which has the greater amount of current). Show your work and explain your reasoning using the properties of magnetic fields, and superposition.

Solution and grading rubric:

• p:
  Correct. At x = +0.30 m, the magnetic fields of each wire point upwards, so the total magnetic field there must point upwards. Then adds magnitudes of the magnetic field due to each wire at x = +0.30 m, with the constraint that I₁ = 2·I₂, and finds that I₁ = 0.48 A.
• r:
  Nearly correct, but includes minor math errors.
• t:
  Nearly correct, but approach has conceptual errors, and/or major/compounded math errors. Typically sets up the difference in magnitudes of the magnetic field due to each wire at x = +0.30 m.
• v:
  Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. Typically has only one wire's contribution to the total magnetic field.
• x:
  Implementation of ideas, but credit given for effort rather than merit.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 30882, 30883
Exam code: finalEL7a
p: 9 students
r: 3 students
t: 5 students
v: 7 students
x: 5 students
y: 5 students
z: 1 student

A sample "p" response (from student 3420):
A sample "y" response (from student 0003), already looking forward to summer break: