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:

Astronomy quiz archive: solar system

Astronomy 210 Quiz 4, spring semester 2019
Cuesta College, San Luis Obispo, CA

Section 30674, version 1
Exam code: quiz04n3Ni


Section quiz04n3Ni

0- 8.0 :
8.5-16.0 :	* [low = 9.0]
16.5-24.0 :	*********
24.5-32.0 :	************ [mean = 26.7 +/- 7.1]
32.5-40.0 :	******** [high = 40.0]

Section 30676, version 1
Exam code: quiz04Si3n


Section 30676

0- 8.0 :
8.5-16.0 :	***** [low = 9.5]
16.5-24.0 :	************
24.5-32.0 :	************** [mean = 25.1 +/- 8.4]
32.5-40.0 :	******** [high = 40.0]

Astronomy midterm question: Cassiopeia and Orion visible in March 27 night sky?

Astronomy 210 Midterm 1, spring semester 2019
Cuesta College, San Luis Obispo, CA

Shown at right is a comic strip[*] published on March 27, 1994. Determine a plausible time for an observer in San Luis Obispo, CA to be able to see both the constellation Cassiopeia and the constellation Orion somewhere in the night sky on this date, or discuss why this comic strip is not plausible. Defend your answer by clearly explaining how you used your starwheel to do this, along with any assumptions that you may have made.

[*] Mort Walker and Chance Browne, "Hi & Lois" (March 27, 1994), King Features Syndicate.

Solution and grading rubric:

• p:
  Correct. Locates the constellations Cassiopeia and Orion on the starwheel, and rotates starwheel such that both are still visible in the sky on March 27 (from approximately 12 PM to 11 PM), and within that range selects a time that is dark enough to see stars (after approximately 7 PM), such that this would be plausible for time within the range 7 PM to 11 PM on that date.
• r:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Discusses how this is not plausible, because at 12 AM on March 27 Orion will have already set, but does not consider a slightly earlier time that night where both Cassiopeia and Orion would be visible.
• t:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Discusses some date other than March 27 to find a time when both Cassiopeia and Orion would be visible in the night sky.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. At least attempts to use starwheel in a systematic manner.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit. Discussion not clearly based on using a starwheel in a systematic manner.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Section 30676
Exam code: midterm01SFBk
p: 31 students
r: 5 students
t: 3 students
v: 4 students
x: 1 student
y: 0 students
z: 0 students

A sample "p" response (from student 1929):
Another sample "p" response (from student 4132):
Another sample "p" response (from student 7415):
A sample "r" response (from student 0815), only considering midnight on March 27 to test for (im)plausibility:
A sample "t" response (from student 7563), testing for a plausible time on another date besides March 27:

Astronomy midterm question: waning gibbous moon with sun low in opposite sky?

Astronomy 210 Midterm 1, spring semester 2019
Cuesta College, San Luis Obispo, CA

Shown at right is a photographic montage[*] of the moon, observed by a person on a sand dune, with the sun low in the sky somewhere beyond the right edge of the photograph. Discuss why this composition is not plausible. Support your answer using a diagram showing the positions of the sun, moon, Earth, and an observer on Earth.

[*] instagram.com/p/Bq0oQ9PHlK9.

Solution and grading rubric:

• p:
  Complete diagram and reasoning, using one of the following explanations:
  1. identifies the moon as a waning gibbous (due to slightly more than the left side of the moon being illuminated), at or very nearly overhead at or just before 3 AM, which is inconsistent with the sky still illuminated by the sun "low in the sky" sometime during daylight hours (after 6 AM to before 6 PM); or
  2. identifies the moon as third quarter, which is overhead at 6 AM, argues that is too early for the sun to already be rising "low in the sky," but does not recognize the inconsistency in lighting; or
  3. recognizing that the moon as shown is illuminated from a light source from the left, which is inconsistent with the sun being low in the sky "somewhere beyond the right edge of the photograph."
• r:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. May make one of the following explanations:
  4. since this is just before sunset (without discussing possibility that this may be just after sunrise), argues how a waning gibbous moon would not be visible then until rising later at 9 PM, when it is dark; or
  5. misidentifies the moon as waxing gibbous, at or very nearly overhead at 9 PM, which is inconsistent with the sky still illuminated by the sun "low in the sky" sometime during daylight hours (after 6 AM to before 6 PM).
• t:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Primarily discusses waning crescent moon being overhead.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. At least attempts to draw a moon phase diagram and apply rise/overhead/set times.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit. Discussion not clearly based on a moon phase diagram.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Section 30674
Exam code: midterm01NtwT
p: 27 students
r: 3 students
t: 1 student
v: 3 students
x: 0 students
y: 0 students
z: 1 student

Section 30676
Exam code: midterm01SFBk
p: 23 students
r: 11 students
t: 6 students
v: 3 students
x: 1 student
y: 0 students
z: 0 students

A sample "p" response (from student 2005), discussing that the sun would not be out in the sky when the waning gibbous moon is high overhead:
Another sample "p" response (from student 2517), observing that the wrong side of the moon is illuminated:
Another sample "p" response (from student 7563), making both arguments mentioned above:
A sample "r" response (from student 1377), only discussing how the waning gibbous moon would not be visible at sunset:
Another sample "r" response (from student 1106), instead identifying this as a waxing gibbous moon, which would not be high overhead during daylight hours:

Astronomy midterm question: Jupiter, not Venus low in the west at dawn

Astronomy 210 Midterm 1, spring semester 2019
Cuesta College, San Luis Obispo, CA

An astronomy question on an online discussion board was asked and answered[*]:

deanmajiet65: Just before dawn, I saw a planet in the sky in the west. Is it Jupiter or Venus?
Rich: It's not Venus. You were facing west, so that was Jupiter.

Discuss why this planet could not be Venus, and would have to be Jupiter for an observer in San Luis Obispo, CA, and how you know this. Support your answer using a diagram showing the positions of the sun, Venus, Jupiter, Earth, and an observer on Earth.

[*] answers.yahoo.com/question/index?qid=20090727104303AA5cFyR.

Solution and grading rubric:

• p:
  Complete diagram and reasoning includes the following explanations for how it is possible for an observer on Earth to be able to see Jupiter, but not Venus in the west at dawn:
  1. places observer on Earth at dawn (or some time after midnight but before sunrise), with east and west horizons clearly indicated; and
  2. is able to place Jupiter (in an outer orbit) such that it is visible above the west horizon for this observer, and also indicates how Venus (in an inner orbit) can only be seen in the east horizon if it is visible at all at dawn.
• r:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Typically diagram is correct, but supporting argument is contradictory or incomplete.
• t:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. May have an observer at sunset instead of sunrise, and/or switched east/west horizons; but still has Jupiter and Venus along outer and inner orbits around the sun, respectively.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some attempt at diagram(s), but no clear indication of observer/horizon on Earth and how placement of outer and inner planets would (or would not) be visible at dawn in the west. Typically misplaces Jupiter and Venus in their relative outer/inner orbits around the sun.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Section 30674
Exam code: midterm01NtwT
p: 17 students
r: 3 students
t: 10 students
v: 4 students
x: 0 students
y: 1 student
z: 0 students

Section 30676
Exam code: midterm01SFBk
p: 26 students
r: 2 students
t: 9 students
v: 3 students
x: 2 students
y: 1 student
z: 1 student

A sample "p" response (from student 7563):
A sample "v" response (from student 1735), with both Venus and Jupiter as inner planets, and with east and west horizons switched for an observer on Earth, just before dawn:

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]

Astronomy quiz archive: telescopes

Astronomy 210 Quiz 3, spring semester 2019
Cuesta College, San Luis Obispo, CA

Section 30674, version 1
Exam code: quiz03nrDr


Section 30674

0- 8.0 :	** [low = 5.0]
8.5-16.0 :	*****
16.5-24.0 :	********* [mean = 24.1 +/- 9.3]
24.5-32.0 :	*********
32.5-40.0 :	******** [high = 40.0]

Section 30676, version 1
Exam code: quiz03SnDr


Section 30676

0- 8.0 :
8.5-16.0 :	****** [low = 8.5]
16.5-24.0 :	**************** [mean = 23.1 +/- 7.7]
24.5-32.0 :	*********
32.5-40.0 :	****** [high = 40.0]