Online reading assignment: flux laws & devices

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 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.
"For an ideal transformer, power in = power out."

"We are using two magnetic flux laws to analyze generators: Faraday's and Lenz's."

"A current is induced through a wire loop when the magnetic flux through it is changing (as described by Faraday's law). The current induced in the loop will be in the direction that creates a magnetic field opposing the change in magnetic flux (Kind of like how an object resists a change in motion). A transformer is made of a primary coil and a secondary coil; by rapidly changing the current in the primary coil the magnetic flux through the secondary coil is constantly changing which induces an emf on it."

"I do understand how to apply Faraday's law and the magnetic flux maybe."

"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. If the magnetic flux ΦB is constant or unchanging, then there is no induced emf in the wire loop."

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

"I understand the concept behind the sliding rail generator. As the rod moves it creates an induced emf and the faster it moves the greater the emf but when it is stationary no emf is produced."

"For any imaginary or actual area A 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 perpendicular symbol "⊥" denotes that the maximum value for magnetic flux ΦB occurs if the magnetic field lines are perpendicular to the surface and ΦB would be zero if the magnetic field is parallel to the surface (as no magnetic field lines would actually go "through" the surface)."

"Magnetic flux is determined by the magnetic field and area and this value is greatest when magnetic field is perpendicular to surface. There is no induced emf when a magnetic flux is constant or unchanging, and the amount of emf depends on the number of coil turns."

"Magnetic flux is the product of the magnetic field and the area."

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.
"Why does the magnetic flux have to not be constant to induce an emf?"

"Magnetic flux really has me in nowhereland."

"I found the induction forge example a little confusing. I'm having a hard time grasping the concept of magnetic flux and how it creates heat in this context."

"What exactly is magnetic flux, and how is it different from a magnetic field or magnetic force?"

"I'm not sure if I really understood Lenz's law."

"I am having some confusion in the explanation of the two rules are applied to the objects pictured in the presentation of Lenz's law and Faraday's law."

"What I had trouble with in this reading was understanding Faraday's law, and what can be explained from using it. I can see that as time changes a magnetic flux may change through a loop of wire, but I am very much confused with where to go from there. The most definitive thing I can say about Faraday's law is that uses the average time rate of change of the flux that passes through a loop."

"I have no idea what is going on in class and that is purely my fault. Hope I get caught up."

"I do not get the Lenz's law and the transformers section of the online presentation."

"I really thought that this presentation was interesting to read, I honestly need to read into it more and do more of the example problems to get a better understanding of the concepts in it though."

"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'm still confused on how to interpret the rotating-coil generator, I'm just not sure how to apply the RHR's to these scenarios correctly. I always seem to be off in the placement/ orientation of my palm. I'll have my fingers correct but my palm will be facing the wrong direction, which changes the orientation of at least one of the units marked on my fingers."

"I am unsure about the 'step-down and step-up' operations of transformers. My roommates tried to explain it to me (they are engineers). But I need more clarification or applicable knowledge about the use of transformers."

"I'm pretty confused on what exactly magnetic flux is. That being said I'm also confused on the whole concept of Faraday's law."

"I started to get really confused starting at the generators part and then was totally lost by the end of the presentation. I really need to see some practice problems for this section."

"I'm not sure I understand what 'step-down/step-up' means. I could use some clarification and maybe visuals of what is supposed to be happening with that."

State/describe the symbol used for magnetic flux, and give its SI units.
"Phi, webers, or something like that. #idk I was never in a frat."

B is the symbol, which is the product of the magnetic field magnitude B and the area A."
"'Phi sub B,' teslas times meters squared, or webers."

"Units are webers, Wb. And the symbol is an oval with a vertical line through it with a subscript B."

"Circle with a line through it. 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 [72%]
Constant non-zero magnetic flux: no induced current in loop. [47%]
Magnetic flux increasing in strength: induced current in loop. [72%]
Magnetic flux decreasing in strength: induced current in loop. [61%]

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). [36%]
Power: no change. [39%]

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). [50%]
Power: no change. [44%]

Explain why a transformer that has the same number of primary coils and number of secondary coils would not be useful.
"It needs to either step-up or step-down to be useful."

"The induced emf would just be the same if there was the same number of coils."

"It would not be useful because nothing would be transformed. There would be no difference."

"It won't allow to stepped-down or stepped-up. Making it to different numbers will be very useful for the transformer."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Do the other instructors call you 'P-dog' as well?" (Only my graduate student teaching assistants at UC-Davis, and they meant it only ironically.)

"This just seems to be getting harder and harder every time!"

"Explain why transformers are more efficient at high frequency as in an inverter welder." (A constant magnetic flux does not induce an emf. Changing the magnetic flux induces an emf. Changing it rapidly (with a high frequency) will induce more emf.)

"I was sort of confused on the questions regarding the stepping-up and stepping-down of a transformer."


"Can you go over magnetic flux?"

"Why are electrons affected by magnets? Like, what is happening in a permanent magnet that pulls an electron like that?" (Electrostatics is how stationary charges exert forces on each other. On the other hand, magnetism is how moving charges exert forces on each other. A single moving charge, or current flowing through a wire, or the unpaired electron spins in the outermost atomic shell in a permanent magnet (like ↑↓       ) create magnetic fields (step 1 of the two-step process). Then magnetic fields exert force on a single moving charge, or current flowing through a wire, or on the unpaired electron spins in a permanent magnet (step 2 of the two-step process).)

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