## 20140303

### Online reading assignment: diffraction, charges and materials

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

Students have a 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 diffraction and charges and materials.

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.
"For waves passing through a single-slit, the smaller the gap a is, the larger the spread angle will be and vive versa. When two different insulators with different affinities for electrons are rubbed together, electrons will move between the two."

"As the Spitzer Space Telescope has a much smaller mirror diameter, light from each part of the galaxy will diffract more and spread out more, resulting in a much less resolved image than from the Herschel Space Telescope."

"The formula for single-slit diffraction and how it was reached. Charges are not created, they are transferred. Like charges repel, opposite attract. Different materials have different degrees of conductivity for electricity."

"For an insulator, the outermost electrons are more or less fixed to their atomic locations, but at least are able to move somewhat around these locations. As in the gas station spark movie, electrons can be exchanged when two different types of insulators that have different affinities for electrons are rubbed against each other."

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 had a lot of time with electrons and some electrochemistry in second-semester chemistry so that this section seems pretty easy. And single-slit diffraction seems pretty close to double-slit interference."

"I do not understand why an insulator will still attract neutral conductors no matter what charge it acquires. It seems that it the charge would affect what it attracts."

"I'm slightly confused about the single-slit diffraction minima equation (a·sinθ = m·λ) that resembles the double-slit maxima equation (d·sinθ = m·λ)."

"I'm confused about the telescope image in the presentation. I just need a recap in class. "

"Young.gif"
Mfpiz
http://commons.wikimedia.org/wiki/File:Young.gif

The __________ slit(s) interfere in Thomas Young's experimental setup shown above.
 single. *** [3] double. ********************* [21] (Both of the above choices.) ****** [6] (Neither of the above choices.) ** [2] (Unsure/lost/guessing/help!) **** [4]

There is diffraction from the __________ slit(s) in Thomas Young's experimental setup shown above.
 single. ************* [13] double. ******** [8] (Both of the above choices.) ************ [12] (Neither of the above choices.) [0] (Unsure/lost/guessing/help!) *** [3]

Discuss the difference(s) between d and a for the d·sinθ = m·λ equation used for double slit interference, versus the a·sinθ = m·λ equation used for single-slit diffraction.
"'d' is the distance between the two slits, and 'a' is the width of the single slit."

If you started out electrically neutral, and became positively charged, excess electrons would have been:
 removed from you. ******************************** [32] added to you. *** [3] (Unsure/lost/guessing/help!) * [1]

"Gas Station Fire, Static Electricity Starts a Flash Fire"

In this video above, a gas pump fire is inadvertently caused by the sudden discharge of an electrical spark. Describe when and how the woman got electrically charged.
"The woman gets electrically charged when she goes back to her car and sits down on the fabric seat. She became charged because she slid across a different kind of insulator."

"Cell phone?"

"I feel the maybe she had turned her car on which may have led to the electrical discharge."

"Cooper and Styrofoam"
Sean McGrath
http://www.flickr.com/photos/mcgraths/235112299/

In the picture above, electrically neutral insulating polystyrene "packing peanuts" are attracted to a cat that:
 is positively charged. *** [3] is negatively charged. *** [2] can be either positively or negatively charged. ********************** [20] is electrically neutral. * [1] (Unsure/lost/guessing/help!) ** [2]

"Aluminium can static roll"
QuestaconNSTC

In the animation above, an electrically neutral aluminum soda can is attracted to a balloon that:
 is positively charged. *** [3] is negatively charged. ****** [6] can be either positively or negatively charged. ******************* [19] is electrically neutral. [0] (Unsure/lost/guessing/help!) * [1]

"Water Bending"
Malarky95

In the animation above, electrically neutral polar molecules in a water stream are attracted to a comb that:
 is positively charged. ******* [7] is negatively charged. *** [3] can be either positively or negatively charged. ***************** [8] is electrically neutral. * [1] (Unsure/lost/guessing/help!) * [1]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Shooting lasers at hair. Pachew pachew!"

"I'll have to see what I'm expected to do with the single-slit information before I can be confused. The electricity stuff doesn't seem that bad yet. I'm glad I took chemistry right before this."

"What should you do if you start a fire at the gas pump?" (Press the red emergency flow stop button. That's the first thing you look for when you pull into a gas station, right?)

"Will you state whether the small-angle approximation is valid on a quiz/exam? Would the gratings just be an extension of double-slit interference or would it be a combo of double and single?" (Yes, and mathematically, a grating can be considered a set of many double-slits with the same spacing.)

"Why would the first midterm date be changed?"(Some of you asked about changing the first midterm date. The question is, just how many of you are requesting this, versus those of you who don't want (or mind) having the date changed?)