20150518

Astronomy quiz question: dark matter evidence

Astronomy 210 Quiz 7, spring semester 2015
Cuesta College, San Luis Obispo, CA

Stars further out orbiting at the same speed as stars nearer the center of the Milky Way is evidence of:
(A) dark matter.
(B) the central supermassive black hole.
(C) the expansion of the universe.
(D) unequal amounts of matter and antimatter.

Correct answer (highlight to unhide): (A)

The orbital speeds of stars are nearly the same for all distances from the center of the Milky Way, instead of decreasing with increasing distance. This is due to the mass of the Milky Way not being concentrated at the very center (despite a supermassive black hole there), but being diffusely distributed above and below the disk of the Milky Way. Because this amount of mass is not visible in a manner such as luminous stars and gas, this unseen mass is termed "dark matter."

Section 30674
Exam code: quiz07Ctlu
(A) : 5 students
(B) : 11 students
(C) : 15 students
(D) : 5 students

"Success level": 18% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.50
Section 30676
Exam code: quiz07Ctlu
(A) : 9 students
(B) : 16 students
(C) : 15 students
(D) : 4 students

"Success level": 26% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.52

Astronomy quiz question: red dwarf nucleosynthesis?

Astronomy 210 Quiz 7, spring semester 2015
Cuesta College, San Luis Obispo, CA

A red dwarf produced the:
(A) hydrogen in the sun.
(B) gold and silver in Earth's crust.
(C) heat inside Earth's core.
(D) lithium in car batteries.
(E) (More than one of the above choices.)
(F) (None of the above choices.)

Correct answer (highlight to unhide): (F)

A red dwarf is a low-mass main sequence star, which produces energy from fusing hydrogen into helium during its (extremely long) main sequence lifetime, which is so long (50+ billion years) that none have yet died, as it is longer than the age of the universe (14 billion years). Thus no atoms from a red dwarf have ever been released to be incorporated into the formation of our solar system.

Section 30674
Exam code: quiz07Ctlu
(A) : 3 students
(B) : 2 students
(C) : 1 student
(D) : 2 students
(E) : 8 students
(F) : 20 students

Success level: 57% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.40

Section 30676
Exam code: quiz07Ctlu
(A) : 4 students
(B) : 0 students
(C) : 2 students
(D) : 1 student
(E) : 15 students
(F) : 21 students

Success level: 49% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.82

Education research: SPCI statistics (spring semester 2015)

Students at Cuesta College (San Luis Obispo, CA) were administered the Star Properties Concept Inventory (SPCI version 3.0, developed by Janelle Bailey, University of Nevada-Las Vegas) during the first and the last week of instruction. Astronomy 210 is a one-semester introductory general science course, with a separate optional adjunct laboratory (Astronomy 210L).

The pre- to post-test gain for this semester at Cuesta College (excluding students with negative informed consent forms (*.pdf), and missing pre- or post-tests) is:

Astronomy 210 spring semester 2015 sections 30674, 30676
N = 68 (matched-pairs)
<initial%>= 31% ± 12%
<final%>= 56 ± 19%
<g>= 0.38 ± 0.23 (matched-pairs); 0.36 (class-wise)

This semester's SPCI pre- and post-instruction scores are comparable to results from previous semesters at Cuesta College.

20150517

Physics midterm question: electric potential energy change of two opposite charges brought closer

Physics 205B Midterm 2, spring semester 2015
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Conceptual Question 17.22, Problems 17.55, 17.56

As the distance between two charges is steadily decreased, their electric potential energy becomes a larger negative number. Discuss why these charges must have opposite signs for this to happen. Explain your reasoning using the properties of charges, electric forces, electric potential energies, and electric potentials.

Solution and grading rubric:
  • p:
    Correct. With two charges of opposite signs, UE = kq1q2/r must be negative, and thus decreasing r will make UE a larger negative quantity.
  • r:
    As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. One of the two arguments in (p) is only nearly complete, or has minor inconsistencies.
  • 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 energies, and electric potentials.
  • x:
    Implementation/application of ideas, but credit given for effort rather than merit. Approach other than that of applying properties of charges, electric forces, electric potential energies, and electric potentials.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Sections 30882, 30883
Exam code: midterm02m3tR
p: 43 students
r: 1 student
t: 3 students
v: 0 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 1288):

Physics midterm question: scuffing your shoes on a rug and jumping up

Physics 205B Midterm 2, spring semester 2015
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Conceptual Question 17.22, Problems 17.55, 17.56

A voltmeter is used to measure the potential difference between a person's shoes and a rug[*]:
Scuff [your shoes] on the rug, and watch the voltage-reading on the meter [connected to the shoes and the rug] as it climbs up to several thousand volts... To make the voltage go up, leap into the air.
Assume that the shoes and the rug act as a parallel-plate capacitor, and that the amount of charge stored remains constant as the person leaps up in the air. Discuss why the voltmeter reading increases when the person leaps up into the air. Explain your reasoning by using the properties of capacitors, charge, and electric potential.

[*] William J. Beaty, "'Static Electricity' means 'High Voltage'--Measuring your Body-Voltage," amasci.com/emotor/voltmeas.html.

Solution and grading rubric:
  • p:
    Correct. Discusses/demonstrates:
    1. that an increase in distance between capacitor plates (shoe and rug) decreases their capacitance C = A/(4π∙kd); and
    2. a decrease in capacitance, while charge remains the same results in an increase in voltage difference ΔV = Q/C.
    May instead argue from work argument that the electric potential energy of two oppositely (but equal amounts) of charge will increase when they are separated from each other, resulting in an increase in ΔV from UE = (1/2)∙Q∙ΔV.
  • 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 applying properties of capacitors, charge, and electric potential.
  • x:
    Implementation/application of ideas, but credit given for effort rather than merit. Approach other than that of applying properties of capacitors, charge, and electric potential.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Sections 30882, 30883
Exam code: midterm02m3tR
p: 35 students
r: 0 students
t: 8 students
v: 3 students
x: 1 student
y: 0 students
z: 0 students

A sample "p" response (from student 8984):

Physics midterm problem: currents through each of two parallel bulbs

Physics 205B Midterm 2, spring semester 2015
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problems 18.40, 18.73, 18.75

An ideal emf source is connected to an ideal light bulb, and a certain amount of current (say, 0.40 A) flows through the bulb. A common misconception in analyzing circuits[*] is that if two of these bulbs (each with the same resistance as before) are connected in parallel to the same ideal emf, each of those bulbs would then get 0.20 A. Discuss why this is incorrect. Show your work and explain your reasoning using Kirchhoff's rules and Ohm's law.

[*] Dennis Albers, cited in Thomas O'Kuma, David P. Maloney, Curtis J. Hieggelke, Ranking Task Exercises in Physics, Prentice Hall, 2000, p. 204.

Solution and grading rubric:
  • p:
    Correct. Discusses/demonstrates:
    1. that the parallel circuit light bulbs would still each have the same amount of current (0.40 A) as the lone light bulb attached to the same ideal emf source; or
    2. specifically argues why the parallel circuit light bulbs would not have one-half of the current of the lone light bulb attached to the same ideal emf source.
  • r:
    As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. Typically solves for the equivalent resistance of the two light bulbs in parallel, and the total equivalent circuit current of 0.80 A, but only implicitly (if at all) mentions how each light bulb would divide up this current such that each does not get the incorrect 0.20 A value.
  • t:
    Nearly correct, but argument has conceptual errors, or is incomplete. At least understands equivalent resistance for light bulbs in parallel.
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some garbled attempt at discussing Kirchhoff's rules and Ohm's law.
  • x:
    Implementation of ideas, but credit given for effort rather than merit. Approach other than that of discussing Kirchhoff's rules and Ohm's law.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Sections 30882, 30883
Exam code: midterm02m3tR
p: 19 students
r: 13 students
t: 11 students
v: 3 students
x: 1 student
y: 0 students
z: 0 students

A sample "p" response (from student 1111), qualitatively appealing to Kirchhoff's loop rule:

A sample "p" response (from student 0196), quantitatively using specific numbers:

Physics midterm question: loop moving through opposite magnetic fields boundary

Physics 205B Midterm 2, spring semester 2015
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Example 20.1, Conceptual Example 20.5

A square metal loop of resistance R is dragged from a region with an external magnetic field that points into the plane of this page, to a region with an external magnetic field that points out of the plane of this page. The magnitudes of the magnetic fields in these two regions are the same, only their directions differ. Discuss why the induced current in the loop while it is passing from one region to the other will be clockwise in direction. Explain your reasoning using the properties of magnetic fields, forces, motional emf, Faraday's law and Lenz's law.

Solution and grading rubric:
  • p:
    Correct. Discusses/demonstrates that current induced in the square must be clockwise using either (or both) of the following (equivalent) arguments:
    1. right-hand rule 1 to show that the force on fictitious positive charges in the top of the square loop point to the right, while the force on the bottom points to left, resulting in a clockwise flow; or
    2. Faraday's (and Lenz's) law to the changing external magnetic flux through the square loop--as it moves downwards, the flux pointing into the page decreases (while the flux pointing out of the page increases), such that the current induced in the square loop must be clockwise in order to provide a counteracting flux that points into the page.
  • 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. At least some attempt at using magnetic forces and/or magnetic flux.
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner.
  • x:
    Implementation of ideas, but credit given for effort rather than merit. Approach other than that of applying properties of magnetic fields, forces, motional emf, Faraday's law and Lenz's law.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Sections 30882, 30883
Exam code: midterm02m3tR
p: 31 students
r: 1 student
t: 11 students
v: 4 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 0550), discussing the Lorentz force exerted on fictitious positive charges in the top and bottom segments of the wire loop:

A sample "p" response (from student 8167), using both the Lorentz force, and also applying Lenz's law to the changing flux through the wire loop:

20150516

Physics quiz archive: radioactive decay, Feynman diagrams

Physics 205B Quiz 7, spring semester 2015
Cuesta College, San Luis Obispo, CA
Sections 30882, 30883, version 1
Exam code: quiz07d4wN


Sections 30882, 30883 results
0- 6 :   **** [low = 3]
7-12 :   ******
13-18 :   *************
19-24 :   ****** [mean = 18.4 +/- 7.8]
25-30 :   *********** [high = 30]

20150515

Astronomy current events question: 55 Cancri e temperature fluctuations

Astronomy 210L, spring semester 2015
Cuesta College, San Luis Obispo, CA

Students are assigned to read online articles on current astronomy events, and take a short current events quiz during the first 10 minutes of lab. (This motivates students to show up promptly to lab, as the time cut-off for the quiz is strictly enforced!)
Sarah Collins, "Astronomers Find First Evidence of Changing Conditions on a Super Earth" (May 5, 2015)
http://www.cam.ac.uk/research/news/astronomers-find-first-evidence-of-changing-conditions-on-a-super-earth
NASA’s Spitzer Space Telescope observed variations in atmospheric temperatures from extrasolar planet 55 Cancri e, which may be evidence of:
(A) volcanic activity.
(B) a runaway greenhouse effect.
(C) a polar vortex.
(D) an asteroid impact.
(E) radioactive decay.

Correct answer: (A)

Student responses
Section 30680
(A) : 12 students
(B) : 3 students
(C) : 0 students
(D) : 1 student
(E) : 0 students

Astronomy current events question: "thick-disk" quasars

Astronomy 210L, spring semester 2015
Cuesta College, San Luis Obispo, CA

Students are assigned to read online articles on current astronomy events, and take a short current events quiz during the first 10 minutes of lab. (This motivates students to show up promptly to lab, as the time cut-off for the quiz is strictly enforced!)
Jennifer Harbaugh, "NASA's Chandra Suggests Black Holes Gorging at Excessive Rates" (April 30, 2015)
http://www.nasa.gov/mission_pages/chandra/nasas-chandra-suggests-black-holes-gorging-at-excessive-rates.html
NASA's Chandra X-ray Observatory observed ultraviolet and x-ray emissions fainter than expected around supermassive black holes, which may be caused by:
(A) large amounts of infalling material.
(B) cold, dark matter.
(C) antimatter.
(D) neutron stars.
(E) protoplanet formation.

Correct answer: (A)

Student responses
Section 30680
(A) : 9 students
(B) : 4 students
(C) : 1 student
(D) : 2 students
(E) : 0 students