## 20180416

Physics 205B, spring semester 2018
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 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.
"A single-pass generator cannot indefinitely continue to slide the rod along the rails to generate a constant motional emf and current."

"More common generators have a coil that rotates between the north pole and south pole of an external magnet. This also generates a motional emf that can be measured with a voltmeter, or made to generate current, but the values of the motional emf (and current) will fluctuate over each cycle of rotation, or even change direction."

"A Faraday disk is an example of a continuous generator, while motional emf are examples of single-pass generators."

"Motional emf is generated when a metal rod is moved through a magnetic field. The way to calculate motional emf is via multiplying the length of the rod by the speed of the rod by the magnetic field through which it is going through."

"Motional emf arises because a magnetic force acts on the charges in a conductor that is moving through a magnetic field."

"Single-pass generators can only be used once before having to be reset. Hoop-drag and rail generators are two types of single-pass generators."

"There are different kinds of generators. Some need to be reset (single-pass) before they can be used again, while some generators are continuous and can keep providing current and emf."

"There are many different types of generators that work in different ways. We are still using the right-hand rules for these generators."

"Single-pass generators can only be used once before having to reset them. In a uniform magnetic B field, electrons in a metal rod experience a downwards pointing force, and as long as the rod is made to move through the field, its bottom will be negative and the top positive. Therefore, moving this rod in a B field makes it a battery. Continuous generators do not need to be reset to provide emf and current. An example of this is a Faraday disk, where the north and south pole lie between the external magnet. All you do is crank it to generate emf. Rotating coils are also commonly used."

"How to use the right-hand rule when applying it to generators and all kinds of magnetic fields."

"Learned more on how to use my hands to solve problems."

"I understand that I don't know how generators work."

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.

"How to determine the direction of magnetic force for a rod that is being rotated from one end."

"Still not really intuitively understanding how generators work and what the 'resetting' is about. I get the idea of having to 'move the snowboard back up the rail,' but I don't really understand what is meant by resetting in a generator, especially for the rotating rod (Faraday disk)."

"More explanation on how to use the right-hand rules for the generators would be helpful."

"It is hard for me to use the right-hand rule to determine where the forces are going with a rod moving through magnetic fields for single-pass generator diagrams."

"The motional emf animation, and how the variables relate as well as the units."

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 ↑. *************** [15] down ↓. ******** [8] left ←. *** [3] right →. * [1] into the page ⊗. [0] out of the page ⊙. *** [3] (No direction, as this quantity is zero.) [0] (Unsure/guessing/lost/help!) ** [2]

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. ***** [5] out away from the center of rotation ******************* [19] into the page ⊗. *** [3] out of the page ⊙. ** [2] (No direction, as this quantity is zero.) [0] (Unsure/guessing/lost/help!) *** [3]

Explain what a generator is supposed to "generate."
"Generators generate a motional emf."

"Emf and current."

"Generates a current and motional emf, eventually creating power."

"Electric potential energy."

"Electricity?"

Explain the meaning of "motional" in the term "motional emf."
"That the emf is created through motion."

"Energy is created via movement of a metal rod through a magnetic field."

"It means something is constantly moving to produce the emf."

"The motion of the bar is what creates the emf, thus 'motional emf.'"

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"This is all becoming pretty foreign to me, any worksheets and practice in class will definitely benefit me!"

"what are the benefits of making a single-pass generator over a continuous generator?" (Single-pass generators are not very practical (as you must stop producing electricity in order to "reset" the system), but are much easier to understand conceptually. More practical, continuous generators (that don't have to, or automatically "reset" by returning to their starting point) are a bit more difficult to understand.)

"Aren't you still technically putting in some energy for the system to reset a continuous generator, just as in the single-pass generator?" (You actually put energy into all generators (they just convert your kinetic energy into electric potential energy), but you wouldn't have an abrupt break trying to reset a continuous generator (and can generate electricity uninterrupted), compared to a single-pass generator like a slide-rail generator, where you would have to pick up the rod at the end of a (finite) set of rails, pick it up to put it back to the beginning (or you would need to stop the rod, and start sliding it backwards along the rails.)

"I'm excited that we are learning useful stuff for the zombie apocalypse." (I'm excited, too.)