## 20150420

### Online reading assignment: generators

Physics 205B, spring semester 2015
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 generators.

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.
"That some generators do not have to be reset and keep moving. These are continuous generators."

"How a generator and a motor are essentially the same thing. It all depends on whether it is receiving a rotational input on the shaft or is receiving a current flow to cause it to rotate."

"That a metal rod can become a battery when moving at constant speed. The force on the rod pumps the charge in the rod in order to maintain a constant potential difference."

"Hmmm, every lecture we are getting asked what we 'understand.' The more lectures we delve into, the less I am putting it all together and understanding."
"Still haven't gotten to it. I don't understand anything yet."

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 'hoop-drag' generator was a little difficult to visualize."

"How you would reset a single-pass generator or what constitutes as resetting."

"How continuous generators work, and how to find the forces."

"I don't quite get the point in having ac over dc, or vice versa."

"How as long as a rod moves through a magnetic field its bottom is negative and its top is positive."

"I still don't understand how to make a bicycle-powered generator to fend off the zombie hordes..."

A metal rod moves to the right along a magnetic field that points into the page. The direction of the magnetic force on (fictitious) positive charges in the rod is:
 up ↑. ********************** [22] down ↓. ****** [6] left ←. *** [3] right →. ***** [5] into the page ⊗. [0] out of the page ⊙. ** [2] (No direction, as this quantity is zero.) [0] (Unsure/guessing/lost/help!) **** [4]

A metal rod pivoted at one end rotates counterclockwise in a magnetic field that points out of the page. The direction of the magnetic force on (fictitious) positive charges in the rod is:
 in towards the center of rotation. ****** [6] out away from the center of rotation *************** [15] into the page ⊗. ***** [5] out of the page ⊙. ***** [5] (No direction, as this quantity is zero.) *** [3] (Unsure/guessing/lost/help!) ******** [8]

Explain what a generator is supposed to "generate."
"The movement of electric charges."

"emf."

"Voltage and current."

"Electrical power."

Explain the meaning of "motional" in the term "motional emf."
"It moves."

"The resulting difference in potential when moving a rod through a magnetic field."

"That the emf is created as long as the object is in motion."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"It seems that there are starting to become more than a couple lectures where this 'flipped classroom' style is not working for me. I'm reading and looking at these lectures and not much is sinking in or clicking. I guess this is just how physics goes. When you're not in love with the subject (but still quite fascinated by it) trying to comprehend how things (electricity and magnets) work is just extra-challenging. #stayfocused #onedayatatime" (At the very least your pre-reading responses let me know exactly what you are stuck on--even if it's on the basic stuff--so I know what key points to hit in the following lecture. #notallwhowanderarelost)

"Speaking of a zombie apocalypse, I am watching The Walking Dead right now and it is pretty much epic :-) Also I got a new German Shepherd today, so I am pretty sure I don't have to worry about zombies anymore."

"Still lost on concepts from this past quiz." (Let's make sure that you get caught up on this before the second midterm. See me after lecture, during office hours (make an appointment if necessary), and/or email me.)

"Can you go over the rotating metal rod? It was confusing."

"Can we go over the equations and possibly ways to remember all of them?" (Meh. The right-hand rules are probably more important.)