## 20160418

### Online reading assignment: generators

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 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 changing magnetic field, a changing area of a source, and a source moving in a magnetic field all induce an emf in the source, which induces a current in the source. When an emf is induced using a magnetic field it is called electromagnetic induction."

"One thing that I understand and found interesting is that, moving a rod through a magnetic field makes the rod a battery!"

"I understand how to do RHR1. RHR2 I'm a little confused on. I don't know when to use which."

"I understand the concept of generators but I couldn't make one if you asked me to."

"Motional emf is created from potential difference that happens when rods are passed through a magnetic field. this provides current."

"As long as the moving rod is moving through an electric field, the bottom end of the rod will be negative. In this case, the top rod will be positive."

"A continuous rotational generator acts similar to that of an object experiencing an ideal rotational inertia. As a magnet rests along side a constant rotating disc, the magnetic field that is provided as wires are connected to the axis and edges of the of the disc remains constant as expected. But the rotational movement of the disc allows for the amount of electric potential energy supplied by the generator to remain constant as well. It gives a decent idea as to how a standard generator is made."

"You can create emf by moving a metal rod or coil through magnetic fields."

"'Continuous' generators do not explicitly need to be reset in order to continuously provide motion emf and current. While 'single-pass' generators can only be used once before having to be reset."

"A generator uses magnetic field and a moving object to generate electric potentials. A rail generator needs to be stopped and reset. A rotational generator can infinitely spin."

"he rail generator has a rod made to move through a magnetic field while the ends of the rod rest on rails to make a complete circuit."

"We are going to eventually see some examples in class about RHR and how they apply to generators. There are single pass generators when a rod slides along a rail or a wire loop enters a magnetic field which need to rest after one use. There are also continuous generators which don't need resetting and theyre discs and coils that you just keep on rotating."

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 know I'm probably being slow, but I'm still a little tripped up about when to use which hand rules. For the problems on this page we need to use two each, right? One to first find the current and then another to find the magnetic field force? Are all of the hand rules and the situations that they're applicable in listed in one place somewhere? I need to just sit down and stare at them."

"I find the continuous generators to be a little tricky. I'm not quite sure how much they differ from other generators."

"I am still confused on generators. Is it generating electricity? I would think so because when the lights go out, they say go look for the generator to bring the power back."

"I could definitely use some good discussion on recent topics just to solidify concepts."

"I am confused on how to find out the direction of the magnetic field with a rotating rod. I am confused on what 'motional' means."

"So I think I understand the rail example with the snowboard however you lost me with the next example with the loops, where a wire loop encounters the edge of a uniform magnetic field. 'As long as some part of the loop is still entering the magnetic field, there will be a motional emf that will produce current in the loop, until the loop is completely inside of the magnetic field. Practically speaking, at some point you would need to stop the loop after it has completely entered the magnetic field, and bring it back out of the magnetic field--this would still generate a motional emf and current, but with opposite polarity, in order to 'reset' the system.' I don't understand this, completely lost here..."

"I found the rotating coil generator a little confusing. What is the benefit of a coil vs a ring/disk? More surface area?"

"When you use RHR1 and RHR2. How do you know when to use which one?"

"Single-pass generators are weird. I don't understand why you wouldn't just use a continuous generator."

"Not sure when to apply the different hand rules."

"What I found confusing about the reading was understanding a rail generator. I can understand how an electric charge can be provided to a generator as an object slides along a rail enough to provide a charge. However, I found it confusing as to why a generator would ever be constructed in this way, I just don't how you can make the electric potential applied to the generator constant. An explanation would be chill."

"The hoop drag generator is hard for me to understand and visualize in my head."

"Does the ε = vBL equation in the presentation preview equal the amount of motional emf produced? And if so does how much can that amount vary. Will we have to use that equation in the future? I don't understand how simply moving a rod through a magnetic field can generate current.eI did not find the material confusing, except some review of the material to see if I understand it would be nice.eI don't get how to use the RHRs for the rods. Can you go over these in class?"

"I also am still really confused about what we covered in class on Friday with the right hand rules and fields and I didn't feel like we had enough time for questions about the worksheet before class ended."

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 ↑. ************* [13] down ↓. ******* [7] left ←. ** [2] right →. ***** [5] into the page ⊗. * [1] out of the page ⊙. * [1] (No direction, as this quantity is zero.) * [1] (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. *** [3] out away from the center of rotation *************** [15] into the page ⊗. *** [3] out of the page ⊙. ** [2] (No direction, as this quantity is zero.) * [1] (Unsure/guessing/lost/help!) ********** [10]

Explain what a generator is supposed to "generate."
"Converts mechanical energy into electrical energy."

"Voltage?"

"I wasn't too sure, but don't they generate electricity? And since it generates electricity, it generates current."

"A generator is suppose to generate current and motional emf. Ultimately creating power."

Explain the meaning of "motional" in the term "motional emf."
"A motional emf is produced by moving an object through a magnetic field."

"Motion refers to the motion of charges in a conductor, moving through a magnetic field. This emf only exists as long as the conductor moves."

"'Motional' means something is moving, so its emf is generated by movement."

"I have no idea what it is supposed to mean. I don't understand this concept at all."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Mondays make me sad."

"Someone made fun of me at work when doing RHRs lol." (You're just presenting and representing. No shame.)

"Do many generators use magnetism?" (Only because magnetic fields exert forces on moving charges, so it is a very convenient way to change kinetic energy into making electric currents flow.)

"I am so behind in your class! I'm freaking out!" (I'm freaking out too! ≡:O Let's all try to get unfreaked out this week.)

"I was actually confused with what a generator is supposed to generate. I really don't know, I just assumed that it supplies a steady amount of volts but an explanation on that would be nice." (Yes, some generators produce a steady amount of volts (DC, or direct current generators), while some others produce an oscillating amount of volts (AC, or alternating current generators); but the main idea is that the volts generated will produce a current to flow in a useful circuit containing light bulbs, etc.)