## 20140416

### Online reading assignment: flux laws & devices

Physics 205B, spring semester 2014
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.
"The SI units are starting to stick."

"Dang. This is going to be a difficult section."

"I understand that the area enclosed by a rod-rail circuit that the magnetic field lines passes through increases, and therefore the magnetic flux increases and this area increases. That is why there is no magnetic flux if the rod is stationary. I can also apply this concept of increasing area to a rotating-coil generator."

"Lenz's law is responsible for the negative sign in Faraday's law, and this negative sign has an important meaning. There are going to be two possibilities for the direction of a current induced in a wire loop, due to the changes in magnetic flux ΦB through it. Lenz's law states that direction of this induced current must "oppose" the changes in magnetic ΦB."

"Faraday's law is a statement that an induced emf ε occurs in a wire loop while the magnetic flux ΦB through it changes, whether the magnetic field gets stronger or weaker, or by changing the orientation of the surface such that more or fewer magnetic field lines go 'through' the surface."

"Transformers: voltage increase causes a current decrease and vice versa. The magnetic field lines and a wire circle hate change and in order to keep the status quo the current in the wire will change accordingly."

"I understand that so long as the magnetic field is perpendicular to the surface area, the magnetic flux will have a maximum value. If on the other hand, the magnetic field is parallel to the surface area then the magnetic flux will equal zero."

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 am a little confused on the idea of a rotating-coil generator. It seems a little strange to me that the faster the coil rotates the more current and induced emf is produced. Is there a maximum that it will go or is it going to keep going forever?"

"Where do I start? How about....everything. Can we please go over this extensively in class?"

"I find most of the rest of the material confusing. I don't understand Faraday's law, Lenz's law, or transformers very well :/ I am going to need to pay really close attention in lecture tomorrow."

"I still don't understand the concept of flux very well. I understand the units, but is it only present in a moving system or can the area and field be stationary?"

"How does the coil 'know' the direction that the induced current must flow in order to resist changes in the external magnetic field? I don't get it or understand."

"It was a little hard to grasp how the induced current opposes the magnetic flux change."

State/describe the symbol used for magnetic flux, and give its SI units.
B is the product of a magnetic field B and an area A and its units are webers (Wb) or teslas·m2."

"Phi, webers or volt-seconds."

"Tesla's symbol is B."

For each situation involving magnetic flux and a wire loop, determine whether or not there would be an induced current in the loop.

If the magnetic flux through the loop is __________, this would __________ a current in the loop.

(Only correct responses shown.)
zero: not induce. [81%]
increasing in strength: induce. [81%]
decreasing in strength: induce. [70%]
a constant value: not induce. [56%]

Briefly describe what a transformer is supposed to "transform."
"A transformer allows voltage to either be stepped down or stepped up."

"Transformers alter the magnetic field of the secondary coil, inducing a greater or lesser emf into the circuit."

"A transformer uses what's known as 'induction' to change voltage. Transformers don't create energy, the overall amount of power remains the same."

"It transforms power."

Identify the conserved (or non-conserved) quantities that are put into the first coil, and output from the second coil of an ideal transformer.
(Only correct responses shown.)
Voltage: not conserved. [56%]
Current: not conserved. [44%]
Power: conserved. [61%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"If you don't change the flux is there a way to still induce an emf? (No.)

"So how did the aluminum plate levitate?" (A coil below changes the magnetic flux through the plate, which induces current in the plate. The induced current in the plate moves in the direction such that the magnetic field of the current pushes up on it. We can work out the Lenz's law and right-hand rules on this in class as time allows.)

"How can you tell when to use I instead of v for the thumb during RHR1?" (Use your right thumb for v for the direction of positive charges. Since current I is defined to be the flow of positive charges, then you can also use your right thumb for the direction of current as well.)