Online reading assignment: generators

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 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.
"There two types of generators that produce current and motional emf. One type of generators ('single-pass') needs to be reset after it has been through the passageway, and 'continuous' generators do not need to be reset as it turns around and around as it creates current and motional emf."

"You can make a battery by encouraging electrons to collect at one end of a device and leaving the other end electronless. This can be done by passing the device over a magnetic field, rotating the disk-shaped device around a magnet or rotating the coil-shaped device around a magnet."

"A ingle pass generator creates a 'motional' emf by moving a rod through a magnetic field. If this rod is touching two rails, it can take advantage of this emf. Of course, this would at some point need to be reset. Continuous generators also create a "motional" emf...keep cranking."

"Liking this section! Very much enjoyed the 'Faraday disk' and 'rotating coil' examples because, for once, I can kind of see conceptually how a generator (at a very basic level, I'm sure) works."

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.
"What was harder for me to understand was a continuous generator. Why doesn't it have to restart in order to provide current?"

"I have a very basic understanding of generators. It seems the different approaches are different set ups for the same concept, which makes sense, but some clarification would be nice."

I'm kind of stuck on left and right hand rules."

"Not sure how to visualize a hoop drag generator."

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 ↑.  ************ [12]
Down ↓.  ****************** [18]
Left ←.  *** [3]
Right →.  ** [2]
Into the page ⊗.  * [1]
Out of the page ⊙.  * [1]
(No direction, as the magnetic field is zero.)  [0]
(Unsure/guessing/lost/help!)  [0]

Explain what a generator is supposed to "generate."
"Electricity from mechanical energy."


"An emf?"

"An emf without the use of a battery or outside source of electricity."

"A motional emf that produces a current."

Explain the meaning of "motional" in the term "motional emf."
"Motional means moving or mobile system that creates current."

"A motional emf is produced by moving (this is the motional part) an object through a magnetic field."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Is it possible to connect one generator up to another in a reversed fashion so that the magnetic force of one will drive the turbine of the other and vice versa?" (Yes.)

"If you need power to rotate the devices or pass the device through the magnetic field in order to make the battery, doesn't that kind of defeat the purpose? There must be a lot more power we're getting than what we're spending...or maybe this is just a 'cool' way of generating power, but it's not really practical?" (If you have access to motional energy that is already there--such as the wind, or water--then you can use that to run a generator to convert this motional energy into electricity.)

"For the bicycle generator, would it be an ac generator with a split-ring commutator and brushes to make the emf go in one direction like a dc generator?" (Since an incandescent light bulb would work with either ac or dc current, it's hard to tell.)

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