## 20170421

### Online reading assignment: flux laws & devices

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 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.
"The concept of magnetic flux. For any imaginary or actual area A (such as that enclosed by a wire loop) in the presence of a (uniform magnitude and direction) magnetic field B, the magnetic flux ΦB is the product of the magnetic field magnitude B and the area A."

"The concept of Faraday's law and the relationship between electromagnetic force going through a wire loop and the magnetic flux changing while going through the loop. The magnetic flux must always be changing, and if it is constant, then the electromagnetic force is zero."

"According to Faraday's law, in order for emf to be induced, the flux has to change."

"Change in magnetic flux is necessary to induce current. Even if a magnetic field is present, it will not induce current if it remains constant."

"What I was able to understand from tonight's reading is that the magnetic flux ΦB is the product of the magnetic field B and the area is A. The perpendicular sign '⊥' means the maximum value for the magnetic flux ΦB."

"I think I get the basic concepts relating to how a changing magnetic flux creates an induced current, and how that is applied in the coil and transformer we saw. I also get the voltage can vary in a transformer, because the number of 'windings' in the core corresponds to number of turns, i.e. N."

"How to convert grams to newtons."

"Haven't gotten to it yet."

"Nothing really..."

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.
"Faraday's Law. I didn't understand the concept of it."

"I'm not quite sure what's going on with Lenz's law. I don't see what's going on with the coils inducing a loop & creating a magnetic field."

"How to properly use all the symbols in each formula, because it seems like they are all over the place."

"How a coil resists change. I understand the comparison to throwing a brick, but a brick has mass that requires force to displace."

"The rotating coil generator was unclear. I think some clarification of Lenz's law would be very helpful. I can't seem to visualize what is going on in the explanations on the blog. You LOST me on the transformer part for sure. So, the more you could explain about that, the better."

"The physics of changing a transformer into a metal melter could be described a bit better in detail."

"I am confused on mostly every part of this. Very confused on magnetic flux."

"Nothing that I can think of."

"A lot of things man, a lot."

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

"An O with a capital I running through it, followed by a small B; Teslas times meters squared (T·m2), or webers."

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 [91%]
Constant non-zero magnetic flux: no induced current in loop. [57%]
Magnetic flux increasing in strength: induced current in loop. [83%]
Magnetic flux decreasing in strength: induced current in loop. [83%]

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). [39%]
Power: no change. [30%]

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). [39%]
Power: no change. [40%]

Explain why a transformer that has the same number of primary coils and number of secondary coils would not be useful.
"It wouldn't be useful because it wouldn't change the voltage or the current at all."

"The difference in number of coils is what allows the step-down or step-up effect to occur."

"There would be a one-to-one relationship, which means that there would be no change in the induced current or voltage."

"Because they would nullify each other."

"I have no idea."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Dude I still don't understand the whole hand thing."

"Help me."

"Where does the flux capacitor show up in all of this?"

"I gotta say, the example relating to the induction forge made it really click how powerful magnetic fields are. I guess it sounds silly, since its one of the most fundamental and powerful forces in the universe, but for some reason that short clip made it click."