## 20170417

Physics 205B, spring semester 2017
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.
"That there is more than one way to turn motion through a magnetic into electric energy."

"A magnetic field can be used to generate current. Single-pass generators generate current as a metal object moves through a magnetic field in one direction. The object must then be move back to the starting point, which generates current with opposite polarity."

"Why rail generators are 'single-use.' You cannot have the rod run along the rails infinitely, as the rails themselves cannot go on infinitely. Because they go on finitely, they must be 'reset' before able to be used again."

"I understand that there can be two different generators. One can only be used once before having to be reset. The other is continuous."

"Generators and motors are basically the same thing because they work pretty much the same way. Continuous generators do not have to be reset."

"I understand that single-pass generators can only be used once before resetting and a continuous generator does not explicitly need to be reset in order to provide motional emf and current. I also know that in a single-pass generator, as long as the rod is made to move through the magnetic field, the bottom end of the rod becomes negatively charged, while the top end of the rod becomes positively charged."

"Nothing."

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 a rotating rod in a magnetic field generates current."

"Continuous generators, specifically the rotating coil."

"More of the right-hand rule stuff and unsure about motional emf and what that means."

"How would bringing a wire loop back out of a magnetic field create an emf with opposite polarity than if it was brought in?"

"I do not understand exactly how the motion through an magnetic field creates electricity that can be stored. Where would the + and – wires go? Do they go to a battery or a capacitor? Does it matter?"

"The formula makes sense when looking at it, but in class discussion and examples would help with a better understanding of how it all works and how to use the RHR correctly! I also do not have a firm grasp on continuous generators and how those would work."

"A lot of things."

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 ↑. ********* [9] down ↓. ****** [6] left ←. * [1] right →. **** [4] into the page ⊗. * [1] out of the page ⊙. [0] (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. *** [3] out away from the center of rotation ************ [12] into the page ⊗. * [1] out of the page ⊙. ***** [5] (No direction, as this quantity is zero.) [0] (Unsure/guessing/lost/help!) ** [2]

Explain what a generator is supposed to "generate."
"Generators create electricity."

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

"Voltage."

"It generates current and motional emf."

"A generator is supposed to generate electrical energy by converting mechanical energy into that type of energy."

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

"Energy created by movement."

"Electromotive force that is generated by moving a conductive object through a magnetic field."

"Velocity."

"I'm unsure of what it means."

"Unsure/lost/guessing/help!"

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"An alternator in a car is a small generator. Alternators have a solenoid that spins mechanically and rubs against brush magnet (coils) creating electricity to supply power back to a car's battery. The primary reason for a failing alternator is worn-out brush magnets. These can be replaced with some mechanical skill for a couple bucks compared to a \$100+ alternator." (Just be aware that replacing brush magnets (wire coils that produce magnetic fields from a current) with permanent magnets will actually convert your alternator to a magneto, which behaves differently than an alternator. An alternator needs a small amount of current put into it so the brush magnet coils inside will create a magnetic field, and the rotating solenoid inside moves through these magnetic fields to produce a greater amount of output current. A magneto does not need a small amount of current put into it to create a magnetic field for its rotating solenoid, but the output current characteristics may have specifications different enough that you shouldn't put it back in your car. However, you could instead connect it to a wind or water turbine to produce electricity, as you wouldn't need to worry about the input current to make it work.)

"Would using the current generated in a single-pass generator affect it?" (No, but you would just need to continuously put the same amount energy in to move the generator component through the magnetic field that is converted to electrical energy.)

"How would you reset a single pass generator?" (You would need to pick up the coil or rod, and move it back to its original position. Then start moving it again.)

"When would you use a single-pass generator?" (Practically speaking, only to explain the basics of generating electricity. Later we'll go over more useful and complicated generators that produce alternating current (AC) electricity.)

"Could you go over the examples for the generators that are on the blog?"

"Why are the motional emf values so inconsistent for a rotating coil and why do they change direction?" (The orientation of a rotating coil continuously changes with respect to the stationary magnets field, so the emf values will correspondingly vary as well.)

"I'm still having trouble wrapping my brain around how the magnetic field, and the subsequent force, acts on a metal coil, like in the last example."

"My hand hurts after so many contortions for the RHR1 and RHR2." (Prepare yourself for RHR3 later on.)