Online reading assignment: flux laws & devices

Physics 205B, spring semester 2019
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.

"Magnetic flux (Φ_B) is the product of the magnetic field magnitude B and area A. Units of T·m² or webers."

"Magnetic flux is the product of the magnetic field magnitude and the area. Faraday's law states that an electromagnetic field (emf) occurs in a wire loop while the magnetic flux changes through the wire, while if the magnetic flux is constant, or unchanging, then there is no induced emf in the wire meaning that in order to produce an emf, then the magnetic flux must have changed. Lenz's law shows that the direction of current must oppose the magnetic flux."

"Magnetic flux increases with more external magnetic field lines pass through the area of an object. Lenz's law explains that the magnetic field, created by the induced current, points in the opposite direction of the external magnetic field lines that cause a change in magnetic flux."

"Magnetic flux is the product of magnetic field magnitude and area. Faraday's law says that an induced emf occurs when the magnetic flux going through a circuit area changes."

"If flux is constant then there is no induced emf, and in order to induce an emf in a wire loop the magnetic flux must change."

"This section covered Faraday's law and Lenz's law and their connections to magnetic flux. Magnetic flux deals with the magnetic field and the enclosed loop area it passes through. Faraday's law says that in order to induce an emf in a wire loop, the magnetic flux must be changed. Lenz's law says the induced current always opposes change."

"Transformers increase or decrease voltage and current. They are changed by the magnetic fields surrounding them. The amount of coils in them have an effect."

"Transformers are used to step-down or step-up voltage and current into another circuit by the property of induction. This is a very useful property in electric engineering and everyday appliances. Inducing current with reduced or increased voltage can be applied to several different appliances and components."

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'm barely getting the RHR1 and LHR1 so I'm doing my best to keep up with this new stuff."

"Lenz's law."

"I do not really understand transformers at all and could use clarification on magnetic flux and how to use Faraday's law."

"Lenz's law and how RHR3 is applied to these cases or how the magnetic field will affect the magnetic flux. I don't really understand the brick example (of inertia) in your presentation."

"The section on Lenz's law. I do not understand how the current and magnetic field work to oppose magnetic flux change. Seems to be a lot going on."

"Transformers and the step-up/step-down concept. I don't really get why there are coils with different amount of turns and how these effect each other. I want to know how all this stuff works because it has real-life applications but its not clicking for me."

"I was pretty confused by most of this section, but the part that really threw me for a loop (no pun intended) was the whole part about transformers. That really made no sense to me and I have no idea what the parts are doing."

"The equations were confusing. The examples explained a lot, but might need more clarification in lecture tomorrow."

"Equations."

"I didn't really get what each term means and how to use them."

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

"Phi with a 'B' subscript, units are [Wb] or [T·m²]."

"Symbol: Φ_B. SI unit the weber (Wb), or in derived units: volt seconds)."

"The symbol is like an O with a vertical line through it, with subscript B, and is measured in SI unit webers (Wb)."

"It kind of looks like Mike Wazowski from Monsters Inc. It's in Teslas·meters². So fancy."

"Not sure."

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 [84%]
Constant non-zero magnetic flux: no induced current in loop. [52%]
Magnetic flux increasing in strength: induced current in loop. [84%]
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). [23%]
Power: no change. [16%]

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. [16%]

Explain why a transformer that has the same number of primary coils and number of secondary coils would not be useful.

"Transformers are designed to transform voltages, if the primary and secondary coils have the same number of turns, it's not doing its one job because the voltages won't be different."

"The difference of them is related to the ratio of secondary coil to primary coil. If you have the same amount of turns in the coils you won't transform anything, but instead lose energy in the process of heat."

"The primary and secondary coil with the same number of coils is not useful. because transformer with different number of coils allow voltages to be stepped-down or stepped-up, and same number can not."

"I would love to be able to tell you that... But I can't. Give me some time listening in class and it will probably make sense to me, but until then, I got nothing."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"Not going to lie, this section is dense."

"Not the best section for me. I am pretty confused with the multiple coils and their effect on each other. A real life example would help a lot."

"I'm not quite understanding the second part of this section involving the transformers."

"I found the concept of voltage step-up and step-down confusing. If the primary coil has a greater number of rotations than the secondary coil, then the voltage between the primary and secondary drops. So then if a trickle of current flows through the primary how does this current step up to a large value of current in the secondary?" (Energy must be conserved, or rather power (energy transferred per time) must be conserved. So the power going in (current times voltage) must equal the power going out (current times voltage). So if the current in the primary coil is small, and gets "stepped up" to a large current in the secondary coil, then the voltage must compensate, so the voltage gets "stepped down" to a smaller value in the secondary coil.)

Online reading assignment: flux laws & devices

Physics 205B, spring semester 2018
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.

"I am beginning to understand generators better now, and better understand how moving a magnet creates energy than when I did the last reading assignment."

"The magnetic flux Φ_B is the product of the magnetic field magnitude B and the area A. The maximum magnetic flux occurs if the magnetic field is perpendicular to the surface."

"Magnetic flux is an area multiplied by a magnetic field. Faraday's law says that an induced emf occurs in a wire loop when the magnetic flux through it changes."

"Magnetic flux Φ_B is the product of the magnetic field magnitude B and the area A. Units of T·m² or webers."

"Induced current opposes Φ_B change."

"Induced emf, which is produced by changing magnetic flux."

"The slide-rail generator and Faraday's law. The faster the rod moves, the more area there is, thus more emf is produced. In addition, if magnetic flux is constant, an emf can not be produced."

"Faraday's law states that an induced emf in a wire loop occurs while the magnetic flux through it changes. If flux is constant there is no emf. Induced current always opposes the magnetic flux. Differing primary and secondary coil turns allow emf to be stepped up or down."

"How transformers work to step up or down the voltage from the primary loop to the secondary loop."

"I'm not sure I really understand any of this lesson."

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.

"The slide-rail generator--how the force generated on charges through the rod makes the ends charged."

"I'd like clarification on what magnetic flux is, I'm still confused by it after reading the examples."

"How magnetic flux works with the induced current."

"How and when to apply Lenz's law."

"I am confused in the situational uses of these equations. More in-class assistance and instruction would be helpful."

"I found the section on transformers pretty confusing. Also the step-down vs. step-up stuff didn't make a lot of sense to me."

"A little bit of everything is confusing. I just need to make the connections between the different piece...Lenz's law is not yet understood."

"I definitely need a lot of explaining on this stuff I cannot grasp the concepts from just reading the lectures online."

"What is Lenz's law? I don't know what is used for and what context it is useful. Really some explanation for me here would go a long way."

"How to incorporate RHR3 to Lenz's law."

"I'm pretty confused about most of this lesson."

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

"Magnetic flux Φ_B is the product of the magnetic field magnitude B and the area A. Units of T·m² or webers."

The symbol looks like a circle with a vertical line through it and it is the product of a magnetic field and an area."

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 [87%]
Constant non-zero magnetic flux: no induced current in loop. [39%]
Magnetic flux increasing in strength: induced current in loop. [83%]
Magnetic flux decreasing in strength: induced current in loop. [52%]

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). [30%]
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). [30%]
Power: no change. [35%]

Explain why a transformer that has the same number of primary coils and number of secondary coils would not be useful.

"There would be no change in emf as the ratio of N₂ to N₁ would be 1."

"The transformer would not be able to regulate voltage to step it up or down. The primary coil and secondary coil turns cannot be the same amount."

"The whole point is that they have a different number of turns in order for voltages to be stepped up or down."

"Because nothing is being transformed."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"Following this pace is getting hard."

"Please go over these examples I am very lost on this subject!"

"Help..."

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 Φ_B 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 Φ_B is the product of the magnetic field B and the area is A. The perpendicular sign '⊥' means the maximum value for the magnetic flux Φ_B."

"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.

"Φ_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."

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 Φ_B 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 Φ_B 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 Φ_B is the product of the magnetic field magnitude B and the area A. The perpendicular symbol "⊥" denotes that the maximum value for magnetic flux Φ_B occurs if the magnetic field lines are perpendicular to the surface and Φ_B 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."

"Φ_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."

"I HATE MY LIFE RIGHT NOW BECAUSE I'M STILL STUCK ON CIRCUITS!"

"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).)

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 Φ_B. The law states that the electric potential is equal to the number of coils N multiplied by the change in magnetic flux ∆Φ_B 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 Φ_B."

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

"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.

"Φ_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?"

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 Φ_B through it. Lenz's law states that direction of this induced current must "oppose" the changes in magnetic Φ_B."

"Faraday's law is a statement that an induced emf ε occurs in a wire loop while the magnetic flux Φ_B 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.

"Φ_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.)

Online reading assignment: flux laws & devices

Physics 205B, spring semester 2013
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 something you found interesting from the assigned textbook reading or presentation preview, and explain why this was personally interesting for you.

"It was interesting to read about how transformers work because they are a part of our everyday lives and are important."

"I like how this is kind of a different way to figure out what we just did."

"I thought that the units given from a magnetic field passing through a perpendicular area was Webers. I have a family of friends called the Webers!! I love them!"

"I liked the floating candelabra, although I don't understand it."

"I thought that it was super interesting when the AAA battery lit up the 220 V bulb with a transformer."

"I really liked the idea of magnetic flux and Faraday's Law, they completely made sense to me."

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.

"Faraday's law was a little hard to follow the way they had it in the textbook."

"When looking at the equations, I still don't quite get what 'Wb' is. Watts?"

"I down quite understand the transformers step-up and step-down business."

"Faraday's law is weird. I think I get it, But what exactly is being induced?"

"I still don't fully understand what happens in transformers or how generators turn magnetic fields passing through areas into emf."

"I honestly tried to read and understand the online presentation and I read the book but I was just lost for a majority of it. I don't feel like I understood much. I would really benefit going over this in class."

"'However, the current induced in a loop (because the external magnetic flux through it is changing over time) will be in the direction that creates a magnetic field that opposes the changes in the external magnetic flux.' I cannot wrap my head around this, I think I just need to see it."

"I think what is difficult has been keeping up with everything so far with magnetic fields and being able to recall fast enough to serve me well with what we are currently working with."

Describe what happens to a wire coil while the magnetic flux through it is changing.

"A coil experiences an induced current when the magnetic field passing through it varies."

"If the magnetic flux is changing then there is an induced emf of the wire coil."

"Heats."

Briefly describe what a transformer is supposed to "transform."

"Voltage up and down as a step-up or step-down transformer."

"A transformer transforms one level of voltage and current into another. A step up transformer raises voltage and lowers current, usually to make long power lines more efficient, and a step down transformer raises current and lowers voltage, usually to make it safe for homes and appliances."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"The levitating chandelier was cool. After further reading the blog post, I was reminded of wireless charging, which I think is induced charging rather than direct charging. This is why the bulbs light up, right?" (Right. You can even buy an aftermarket wireless charging kit.)

"How many cats do you have?" (Right now, only one--and she has her own Facebook page.)

Presentation: flux laws & devices

Look at this levitating aluminum plate. Just look at it. And the glowing light bulbs in this candelabra mysteriously not plugged into any electrical outlet or battery. Just look at them. (Video link: "Levitating Barbecue! Electromagnetic Induction.")

In contrast to the previous presentation where we analyzed the behavior of generators using right-hand rules, in this presentation we will analyze the behavior of generators using two magnetic flux laws.

First, Faraday's law.

In order to discuss Farady's law, we need to introduce 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 Φ_B is the product of the magnetic field magnitude B and the area A. The perpendicular symbol "⊥" denotes that the maximum value for magnetic flux Φ_B occurs if the magnetic field lines are perpendicular to the surface (for the maximum amount of magnetic field lines passing "through" the surface); and Φ_B would be zero if the magnetic field is parallel to the surface (as no magnetic field lines would actually go "through" the surface).

The units on the right side of this equation are the product of the units of magnetic field B (in teslas) and area A (in meters²), which is defined to be the units of magnetic flux Φ_B, in T·m² or webers.

(There is a similar construct of electric flux Φ_E (defined as the measure of electric field lines passing through a surface) that we have omitted from our discussion of electromagnetism. As it turns out, what happens to the magnetic flux Φ_B is a much richer topic in terms of practical applications, such as building generators and other devices.)

Faraday's law is a statement that an induced emf ε occurs in a wire loop while the magnetic flux Φ_B 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 Φ_B is constant or unchanging, then there is no induced emf in the wire loop.

The amount of induced emf can be compounded by the number of coil turns N in the wire loop. If there are more turns, then there is a proportional increase in the induced emf, for a given change in magnetic flux.

The main idea of Faraday's law is that in order to induce an emf in a wire loop (such that current begins to flow), the magnetic flux Φ_B must somehow be changed.

(The negative sign on the right side of this equation is explained by Lenz's law later in this presentation.)

To give you an idea of how Faraday's law works, let's revisit two types of generators from the previous presentation that we analyzed using only right-hand rules of magnetic fields exerting forces on moving charges.

In the slide-rail generator, we have a rod moving to the right, along rails that complete a circuit, in the presence of a magnetic field that points into the plane of this page. In our previous discussion, moving the rod creates an induced emf in the rod itself, which induces current in the rest of the rod-rail circuit. The faster the rod moves, the more induced emf (and current) is produced; for a stationary rod there is no induced emf (and current).

In terms of Faraday's law, we simply note that the amount of area enclosed by the rod-rail circuit that magnetic field lines passes "through" increases steadily as the rod is moved to the right. This means that there is more and more magnetic flux Φ_B (as the area increases in Φ_B = B_⊥·A), which creates an induced emf (and current) in the rod-rail circuit. The faster the rod moves, the faster increase there is in area and flux, and the more induced emf (and current) is produced; for a stationary rod, there is no change in area and flux, so there is no induced emf (and current).

Let's apply Faraday's law again, to a rotating-coil generator. This was not easily analyzed using right-hand rules to determine how induced current would flow while the coil flips over and over in the presence of an external magnetic field. However, Faraday's law notes that the magnetic flux Φ_B is constantly changing, as the amount of magnetic field lines that pass "through" the square wire coil changes as the coil is perpendicular or sideways to the magnetic field lines, and thus an induced emf (and current) is produced in the coil. The faster the coil rotates, the faster the flux changes over time, and the more induced emf (and current) is produced; for a stationary coil, there is no change flux, so there is no induced emf (and current).

Second, Lenz's law.

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 Φ_B through it. Lenz's law states that direction of this induced current must "oppose" the changes in magnetic Φ_B.

First, recall from a previous presentation how the third right-hand rule (RHR3) relates the magnetic field created by the current in a wire loop. Keep in mind that is the magnetic field created by the induced current in the wire loop, and not the external magnetic field that is responsible for the constant/changing magnetic flux Φ_B that passes "through" the loop.

However, the current induced in a loop (because the external magnetic flux through it is changing over time) will be in the direction that creates a magnetic field that opposes the changes in the external magnetic flux. If the flux through the coil is increasing because the external magnetic field lines are getting stronger, then the resulting induced current in the coil will "fight" this change by creating magnetic field lines in the opposite direction, to "cancel" out the strengthening external magnetic field. If the flux through the coil is decreasing because the external magnetic field lines are getting weaker, then the resulting induced current in the coil will "fight" this change by creating magnetic field lines in the same direction, to "boost" the weakening external magnetic field.

How does the coil "know" the direction that the induced current must flow in order to resist changes in the external magnetic field? Consider trying to change the motion of a heavy brick, by increasing its speed by throwing it, or decreasing its speed by catching it--how does the brick "know" how hard and which direction to press back on you as you try to speed it up, or slow it down? The brick doesn't really know, but it is merely resisting changes in its motion due to its mass (which Newton's law is that?). In an analogous manner, the coil doesn't really know which direction the induced current should flow, it is merely resisting changes in its magnetic flux...and that is Lenz's law.

For purposes of this discussion, we can personify the coil as "hating change," such that the current induced in it "kills" or "boosts" increasing or decreasing flux through it.

A dramatic application of this is where the current in the outer copper coils is ramped up and down rapidly. This creates an increasing and decreasing magnetic field and flux through the metal object (effectively a coil or loop), and this changing flux creates an induced current inside of the metal object. This induced current rapidly heats up the metal object to its melting point. (Video link: "High power induction heater owns ball bearing.")

The clip at the beginning of this presentation has a similar set-up, where there is large coil in the pedestal where current is ramped up and down rapidly, creating a changing magnetic field. This creates a continuously changing magnetic flux through the aluminum plate (effectively a coil or loop), and we can see how the induced currents in the plate continuously "fights" the changing external magnetic field of the pedestal as it levitates in the air. Also the light bulbs in the chandelier are also effectively coils that have currents induced in them due to the changing magnetic flux from the pedestal, which makes the bulbs light up.

Next time turn on your blender next to your radio. While the rapidly changing currents in your blender won't levitate your radio, it will cause the magnetic flux to continuously change through your radio circuitry, inducing currents that will be picked up as rude static.

Third, transformers.

The essential parts of a transformer are the primary coil and secondary coils, each with different numbers of turns. The metal housing ensures that all of the magnetic field created by the primary coil passes through and creates a magnetic flux through the secondary coil. Rapidly changing the current in the primary coil (as is typically done in household alternating current) creates a continuously changing magnetic field that varies the magnetic flux passing through the secondary coil. This means that the secondary coil will then have an induced emf and current produced in it. (Video link: "How to Make The Metal Melter.")

Because the primary and secondary coils have different numbers of turns (N₁ and N₂, respectively), then the voltage in the primary coil, and induced emf in the secondary coil will be different, making the transformer very useful in allowing voltages to be stepped-down, or stepped-up. This does not violate energy conservation, as the amount of energy supplied per time in the primary coil (that is, power) is ideally equal to the energy given per time to the secondary coil, such that any step-down or step-up in voltage will result in a corresponding step-up or step-down in current.

(Keep in mind that these are all time-averaged values, which seem constant, but the instantaneous values of voltages and currents are all continuously changing per time.)

If the primary coil has more N₁ turns compared to the fewer N₂ turns in the secondary coil, then supplying household alternating current with an emf of 120 V to the primary coil (again, this is a time-averaged value) will be stepped-down to an induced emf of 2.1 V (time-averaged reading from the voltmeter) in the secondary coil. This can be very dangerous, as the trickle of current in the primary coil will then be stepped-up to a very large current in the secondary coil, here to be used as a rudimentary arc welder. (Video link: "How to Make The Metal Melter.")

On the other hand, if the primary coil has fewer N₁ turns compared to the many N₂ turns in the secondary coil, then supplying varying current with an emf of 1.5 V to the primary coil (from a AAA battery) will be stepped-up to an induced emf of 220 V in the secondary coil, in order to light up a compact fluorescent light (CFL) bulb. The larger current in the primary coil will be stepped-down to a trickle of current in the secondary coil, but the voltage (and power) requirement of the CFL is still met after stepping-up. (Video link: "Lighting an 11W 220 V CFL using a 1.5 V AAA battery and a CVS disposable camera flash circuit (SD).")

(Note that the battery only provides a direct current, which by itself could not vary the primary coil magnetic field, and not vary the magnetic flux through the secondary coil, so there would be no induced emf in the secondary coil. However, an oscillating circuit will rapidly vary the otherwise direct current from the battery, and the resulting abrupt changes in the current in the primary coil then can create a fluctuating magnetic field and a fluctuating magnetic flux through the secondary coil, which has more windings, and thus a corresponding stepped-up emf. In practice, salvaging these circuits from disposable cameras should be exercised with caution, due to the amount of charge that can be stored in the capacitors within.)

Physics final exam question: magnetic force of wire on moving charge

Physics 205B Final Exam, spring semester 2010
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problems 19.65, 19.66

A positive charge q moves upwards in the presence of a current-carrying straight wire coming out of the plane of this page. Determine the direction of the wire's magnetic field at the location of the charge, and the resulting magnetic force on the charge. Explain your reasoning using the properties of magnetic fields and forces.

Solution and grading rubric:

• p:
  Correct. Applies RHR2 to the wire (thumb out of page, fingers curling counterclockwise in the plane of the page) to find that B field is to the right in the plane of the page at the location of the positive charge. Then applies RHR1 to the charge (thumb up along plane of the page, index finger to the right in the plane of the page, middle finger into the page) to find that the force on the charge is into the page.
  
• r:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. Has at least only one RHR completely correct.
  
• t:
  Nearly correct, but argument has conceptual errors, or is incomplete.
  
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner.
  
• x:
  Implementation/application of ideas, but credit given for effort rather than merit.
  
• y:
  Irrelevant discussion/effectively blank.
  
• z:
  Blank.
  

Grading distribution:
Section 31988
p: 3 students
r: 5 students
t: 4 students
v: 0 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 1987):