## 20150424

### Online reading assignment: flux laws & devices

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 flux laws and devices.

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.
"We look at flux laws and devices here; proving them using different laws. The first law is Faraday's law which begins with the magnetic flux ΦB. The law states that the electric potential is equal to the number of coils N multiplied by the change in magnetic flux ∆ΦB over the change in time ∆t.

"I honestly do not understand any of this stuff! I know it relates to what we learned Monday and Monday's stuff made sense for the most part but I feel like this is another language! Help!"

"I understand the laws used in the presentation. Lenz's law states that direction of this induced current must 'oppose' the changes in magnetic ΦB."

"The basics of how transformers work: they take oodles of electricity and tone it down so it can actually be used."

"When there is a changing magnetic flux there is a emf. That is Faraday's law. The more coils the more emf."

"The magnetic flux ΦB is the product of the magnetic field magnitude B and the area A. The symbol '⊥' represents the maximum value for magnetic flux ΦB."

"Let's be honest. What I understand is that this stuff is confusing. I could not repeat anything back in confidence just from reading the presentation."

"When the magnetic flux is constant or unchanging the there is no induced emf in the wire loop. I understand that the amount of induced emf can be compounded by the number of coil turns N in the wire loop."

"Magnetic flux relates an area to a magnetic field. The most magnetic flux is achieved when the area is exactly perpendicular to the magnetic field lines."

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 got somewhat confused on how the primary and secondary coils gets mixed up when strength is increasing and decreasing. also, I got a little confused from how primary coil voltage can be so high and secondary too low."

"I am still kind of confused as to what a magnetic flux is exactly and how it relates to generators."

"I'm having trouble understanding when a transformer steps down voltage it actually steps up the current."

"Pretty much everything to be honest, I read the blogs and I like to tell myself that it makes it easier to see it again in class because I don't understand anything while reading."

"I need to see some simple laid-out explanations of what is expected of us in terms of formula usage and some of the basic concepts."

"So magnetic flux is sideways current? Or sideways power? I'm still trying to fully understand what a magnet is and does. Flux, coil, inducing...something? None of it makes logical sense."

"I don't really understand what magnetic flux is? Is this just magnetic force?"

"Faraday's law doesn't seem to click conceptually."

State/describe the symbol used for magnetic flux, and give its SI units.
B represents magnetic flux."

"Funny alien looking symbol."

"Wb, T·m2."

For each situation involving magnetic flux and a wire loop, determine whether or not there would be an induced current in the loop.
(Only correct responses shown.)
Constant zero magnetic flux: no induced current in loop [77%]
Constant non-zero magnetic flux: no induced current in loop. [46%]
Magnetic flux increasing in strength: induced current in loop. [82%]
Magnetic flux decreasing in strength: induced current in loop. [59%]

For an ideal transformer that "steps-down" voltage from its primary coils at 120 V to its secondary coils at 2.1 V, determine what happens to the current and to the power from its primary coils to its secondary coils.
(Only correct responses shown.)
Current: stepped-up (increases). [41%]
Power: no change. [28%]

For an ideal transformer that "steps-up" voltage from its primary coils at 1.5 V to its secondary coils at 220 V, determine what happens to the current and to the power from its primary coils to its secondary coils.
(Only correct responses shown.)
Current: stepped-down (decreases). [38%]
Power: no change. [23%]

Explain why a transformer that has the same number of primary coils and number of secondary coils would not be useful.
"The voltage stays constant."

"Because it would not transform anything, by looking at the equations we can see nothing would change. A transformer not transforming is like a heater not heating."

"There magnetic fields that are generated would be the same. So there would be no step up or down."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"The word 'flux' reminds me of the flux capacitor from the Back to the Future movies."

"Hellllppppppp."

"My mind is all fluxed up."

"Why is this class getting harder?"