## 20160229

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

Physics 205B, spring semester 2016
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 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.
"The relationship between slit width and destructive interference angles. I sort of skimmed past the electrostatic charge stuff, so I think I understand what's going on, but I'll have to go back and reread it."

"The basic concepts of diffraction, and charges. But I am overwhelmed and confused by the material that has built up to this point."

"In cases with a single slit there are two relevant parameters: the wavelength and the slit opening width. These parameters affect how the waves diffract and spread out (shown by a half-angle θ). In all cases with single slit diffraction there is a faint dark region that is a destructive region that separates the two sides, this is the first minima angle."

"Smaller W results in more diffraction and a larger W results in less diffraction."
"In an insulator, valence electrons are able to move to different positions relative to the atom, but they remain relatively fixed; there is no movement of electrons from atom to atom (For the most part). In a conductor, valence electrons are able to move freely throughout the entirety of the material, from atom to atom."

"In an insulator the outer electrons are fixed to their atomic locations but can move around these locations. In a conductor the outer electrons are more free to move around the whole material."

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.
"Still confused between minima and maxima."

"For once I think I'm pretty okay with the concepts. It's probably thanks to my chemistry background, but nothing really confused me."

"The net attraction of insulators and conductors. I can't really visualize the net forces, and I'm really confused on the negative and positive attractions."

"Nothing at all."

"Cows."

Match the single slit parameter with its symbol. (Only correct responses shown.)
Width of a single slit: W [76%]
Any positive or negative non-zero whole number: m [65%]
Distance from the slit to a projection screen: L [57%]
Wavelength of light passing through the slit: λ [78%]
Direction, as measured from the centerline: θ [51%]
Position along screen, as measured from the centerline: y [46%]

A gas pump fire is inadvertently caused by the sudden discharge of an electrical spark. Describe when and how the woman became electrically charged.
"The woman became electrically charged when she got back in her car and the fabric she is wearing rubbed against the seat. She either lost, or gained electrons."

"The jacket that the woman has on is charged after she sits down and gets back up, therefore once she came in contact with the neutrally charged gas nozzle the electrons were transferred between the nozzle and woman, resulting in igniting the gasoline."

"I don't know."

"Need help!"

Electrically neutral polystyrene "packing peanuts" would be attracted to a cat that has __________ charge.
 a positive. ***** [5] a negative. ****** [6] either a positive or a negative. ******************* [19] zero (neutral). **** [4] (Unsure/lost/guessing/help!) *** [3]

An electrically neutral aluminum soda can would be attracted to a balloon that has __________ charge.
 a positive. ******** [8] a negative. **** [4] either a positive or a negative. ********************* [21] zero (neutral). * [1] (Unsure/lost/guessing/help!) *** [3]

Electrically neutral polar molecules in a water stream would be attracted to a comb that has __________ charge.
 a positive. *** [3] a negative. *** [3] either a positive or a negative. ************************* [25] zero (neutral). [0] (Unsure/lost/guessing/help!) ****** [6]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Why there is destructive interference at first minima for diffraction?" (Ideally a double slit is just two point-like (in phase) sources of light. However, a single slit is just a continuous line of many point-like sources of light that all start in phase, but they cannot all be lined up for constructive interference if θ ≠ 0°, due to all their different path lengths. So only the central θ = 0° maxima will be bright, but eventually the collective constructive interference of this continuous line of point-like charges dies off when you get to the first minima angle off to the side.)

"That video where the girl goes back to her car and it starts going up in flames--I want to know the reason behind this. I feel it could have been her either using her cell phone or sitting back down. Which one is it though? I have the tendency of going back into my car and using my cell phone, so now I'm a little paranoid." (Just remember not to sit down inside and then get back out of your car while filling up. But it's okay to use your cell phone--just remember to take a selfie if you do accidentally ignite the pump.)

"Leap year!"