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: generators

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 generators.


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.

"A moving charge feels force from a magnetic field, the direction of which can be determined using the RHR1 or the LHR1 depending on the sign of the charge. A stationary charge does not feel the force from a magnetic field."

"How motion can be converted into electrical current. I know that single-pass generators have to be reset, but continuous generators do not."

"Single-pass generators can only be used once before being reset, while continuous can continuously provide electricity without a need for being reset. Motional emf requires a rod in motion to generate emf."

"Single-pass generators are those which can be used once and then have to be reset. In order to reset these systems you have to bring them back in the opposite direction which will create a motional emf and current but with opposite polarity."

"When a rod moves through a magnetic field the electrons within the rod experience a force and the rod becomes positively charged on one end and negatively charged on the other, and that is how it becomes a battery."

"I seem to have a pretty decent understanding of how a generator works."

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.

"Where do I start, I'm really having a hard time trying to figure out how to use the hand symbols such as 'Hidden Dragon, Crouching Tiger.' I don't really understand how do you use the field model."

"How to orient my hand to not snap my wrist."

"RHR2 and RHR3."

"Motional emf and electric energy. I do not understand how to use the right-hand rule to find the direction of force."

"I don't know what 'motional emf' means. And I don't understand the right-hand rules either."

"Pretty confused on how to use the right-hand rule to find the direction of the force from a single-use generator."

"Overall, this whole chapter is really confusing. It's taking me a long time to get the hang of it."

"I was confused by the rail and hoop-drag generators."

"I don't understand the differences or main parts of the examples of single-use generators. It felt like I was reading gibberish."

"I think I would just need some explanation and demonstration of the different generators to get a better grasp on what they actually do and how they work. Other than that, I felt like this wasn't too hard to wrap my head around."

"I am not quite sure how changing the area of a coil can induce an emf."

A metal rod moves to the right along a magnetic field that points into the page. The direction of the magnetic force on (fictitious) positive charges in the rod is:

up ↑.	************* [13]
down ↓.	********** [10]
left ←.	[0]
right →.	****** [6]
into the page ⊗.	** [2]
out of the page ⊙.	* [1]
(No direction, as this quantity is zero.)	* [1]
(Unsure/guessing/lost/help!)	*** [3]

A metal rod pivoted at one end rotates counterclockwise in a magnetic field that points out of the page. The direction of the magnetic force on (fictitious) positive charges in the rod is:

in towards the center of rotation.	******** [8]
out away from the center of rotation	***************** [17]
into the page ⊗.	* [1]
out of the page ⊙.	** [2]
(No direction, as this quantity is zero.)	[0]
(Unsure/guessing/lost/help!)	******** [8]

Explain what a generator is supposed to "generate."

"A potential difference."

"An electrical current."

"A generator coverts mechanical energy (usually rotational) into electricity (emf)."

"Generators generate a motional electromotive force and current."

Explain the meaning of "motional" in the term "motional emf."

"Well, the moving bar is a source of an electromotive force, called motional emf. An emf is generated by the motion of the bar. "

"When a rod is moved through a magnetic field in order to gain a charge. The motion and magnetic field causes the rod to have electrons (and the fictitious positive charges) moved to one side which essentially creates a battery."

"The moving bar is the motional part that generates emf."

"The bar has to stay in motion!"

"I don't understand what this means."

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

"When using RHR1, does the force (represented by the middle finger) point out the way your finger tip is pointing. Or does the force point in towards your palm?" (For positive charges, the force on it points the way your middle finger tip points, out away from your palm.)"

"I could use more help on the right-hand rules."

"Generators are interesting because they turn mechanical energy into electricity."

"Seeing the man on the bicycle-driven generator took me back to my childhood days when I was watching Pokémon. I remember an episode where a man was generating power by using a stationary bicycle. On another note, I need to do more research to understand this topic."

"I need to study."

"I think I know why you're so ecstatic about the zombie apocalypse. You either know when it's going to happen or you will bring it about, but you won't tell us. You may tread on thin ice if you make light of a serious topic." (I am the Night King. #winteriscoming #whatisdeadmayneverdie)

Online reading assignment: magnets, magnetism, and magnetic forces from fields

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 magnetism and magnetic forces from fields.


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 understanding a lot better the relationships between current and the resistance in series and parallel circuits after going over and over it for the quiz. I also find greater understanding of the measurements of the ammeters and voltmeters."

"Magnetic fields of a magnet leave through the north pole and form a looping path and return through the south pole."

"In a direct model a source magnet exerts a force on the test magnet. It will attract if the poles are opposite and repel when they are the same."

"There are two ways to approach the direction of magnets. For the direct way, the source magnet exerts a force on the test magnet. The indirect approach requires two steps, the source magnet creates a magnetic B field everywhere around it, and then the B field exerts a force on the test magnet."

"We label the two magnets that exert forces on each other as 'source' and 'test' poles, where the source magnet is said to be exerting a force on the test magnet (with a north and a south pole). For a field model, instead of a source magnet directly exerting a force on a test magnet, in this two-step model, the source magnet is said to create a magnetic B field everywhere around it, and it is this magnetic field that exerts a force on a test magnet."

"North repels north, but attracts south. Vice versa. Source magnet creates B field, B field exerts force on test magnet. If another magnet enters the field the force is exerted opposite of the lines of the field."

"Magnetic fields are created by magnetic sources and that these fields behave similarly to electric fields in the way that they affect test magnets, currents, and charges. Magnetic field lines move away from the north pole, towards the south pole, and through the magnet back out of the north pole."

"Magnetic force is always perpendicular to both magnetic field and the velocity. Whereas the force applied by an electric field is always parallel or antiparallel to the field direction. "

"I understand the right-hand rule now that I actually wrote them on my right hand rather than the left. I understand that this right-hand rule shows the direction of the magnetic force and that the middle finger will show the direction properly if the thumb is pointing on the velocity vector and the index is on the magnetic field lines, but I think this only works for positive particles. The left-hand rule is used for for magnetic forces on negative charges."

"Hidden Dragon. Crouching Tiger. Hidden Dragon. Crouching Tiger. And 'very bad finger.'"

"Single-pass generators can only be used once before having to be reset, while continuous generators do not have to be reset and they then provide continuous emf and current. A Faraday disk is an example of a continuous generator when cranked."

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 was a bit confused on the magnetic fields. I am just having trouble with figuring out where the magnetic field is and how to find it."

"Why magnetic field lines loop from north to south. The right hand rule."

"I find confusing the RHR1 and RHR2 based on how to use them!"

"I do not understand the hand things."

"Everything was relatively easy to understand except the RHR1, I didn't understand that."

"I didn't understand how field models work."

"I would benefit from a walkthrough on field models just to make sure I understand the concept."

"I just need a class lecture on this to understand better. I'm confused on how to point which finger where."

"Can you explain the right-hand rules a little bit more? I would really benefit from that in class."

"Not super sure what the right-hand rules symbolize."

"I'm so confused on angle theta and what the angle is measuring. Tomorrow I really want to get through what theta is and what RHR1 means for the 'Hidden Dragon, Crouching Tiger,' because trying it at home made no sense for me."

"I'm still confused on how to properly use the 'Hidden Dragon, Crouching Tiger' hand signs."

Outside of a bar magnet, magnetic field lines travel from the __________ pole to the __________ pole.

north; south.	************************** [36]
south; north.	* [1]
(Unsure/guessing/lost/help!)	*** [3]

Inside of a bar magnet, magnetic field lines travel from the __________ pole to the __________ pole.

north; south.	********** [10]
south; north.	************************* [25]
(Unsure/guessing/lost/help!)	***** [5]

State the symbol used for the magnetic field, and the SI units for magnetic field strength.

"B; teslas [T] or [N·s/C·m]."

"B; teslas [T] or gauss, where 1 tesla = 10,000 gauss."

"The symbol used for magnetic field is F with an arrow over the top; the SI units used is newtons?"

When properly using right-hand rule 1, in general there can be any angle between the v thumb and B finger (depending on the situation). In general, specify the angles between the other fingers used in right-hand rule 1.
(Only correct responses shown.)

B finger and F finger: exactly 90° only [35%]
v finger and F finger: exactly 90° only [35%]

In general, the direction of the force of a magnetic field on a moving charged particle will be along the:

particle's velocity vector.	** [2]
the magnetic field line.	****************************** [30]
(Neither of the above choices.)	** [2]
(Unsure/guessing/lost/help!)	****** [6]

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

"I need more explanation on right-hand rules please."

"I think we need several in-class examples using the right-hand (and left-hand) rules."

"Physics gang signs... (laughing inside)."

"Magnets are cool! :)"

"I get a little confused when it comes to the magnetic fields and the whole idea around it."

"I just think this material is more difficult to understand from the readings so hopefully class will clear up some confusion."

"After finishing the resistance lab last week, we tried to use a number of batteries from our calculators to see how bright the light bulbs can get and home many bulbs (connected in series) we could light with the batteries that we have."

"YOUR CLASS IS TOO DIFFICULT."

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: generators

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 generators.


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.

"A single-pass generator cannot indefinitely continue to slide the rod along the rails to generate a constant motional emf and current."

"More common generators have a coil that rotates between the north pole and south pole of an external magnet. This also generates a motional emf that can be measured with a voltmeter, or made to generate current, but the values of the motional emf (and current) will fluctuate over each cycle of rotation, or even change direction."

"A Faraday disk is an example of a continuous generator, while motional emf are examples of single-pass generators."

"Motional emf is generated when a metal rod is moved through a magnetic field. The way to calculate motional emf is via multiplying the length of the rod by the speed of the rod by the magnetic field through which it is going through."

"Motional emf arises because a magnetic force acts on the charges in a conductor that is moving through a magnetic field."

"Single-pass generators can only be used once before having to be reset. Hoop-drag and rail generators are two types of single-pass generators."

"There are different kinds of generators. Some need to be reset (single-pass) before they can be used again, while some generators are continuous and can keep providing current and emf."

"There are many different types of generators that work in different ways. We are still using the right-hand rules for these generators."

"Single-pass generators can only be used once before having to reset them. In a uniform magnetic B field, electrons in a metal rod experience a downwards pointing force, and as long as the rod is made to move through the field, its bottom will be negative and the top positive. Therefore, moving this rod in a B field makes it a battery. Continuous generators do not need to be reset to provide emf and current. An example of this is a Faraday disk, where the north and south pole lie between the external magnet. All you do is crank it to generate emf. Rotating coils are also commonly used."

"How to use the right-hand rule when applying it to generators and all kinds of magnetic fields."

"Learned more on how to use my hands to solve problems."

"I understand that I don't know how generators work."

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 would like to learn more on the motional emf."

"How to determine the direction of magnetic force for a rod that is being rotated from one end."

"Still not really intuitively understanding how generators work and what the 'resetting' is about. I get the idea of having to 'move the snowboard back up the rail,' but I don't really understand what is meant by resetting in a generator, especially for the rotating rod (Faraday disk)."

"More explanation on how to use the right-hand rules for the generators would be helpful."

"It is hard for me to use the right-hand rule to determine where the forces are going with a rod moving through magnetic fields for single-pass generator diagrams."

"The motional emf animation, and how the variables relate as well as the units."

A metal rod moves to the right along a magnetic field that points into the page. The direction of the magnetic force on (fictitious) positive charges in the rod is:

up ↑.	*************** [15]
down ↓.	******** [8]
left ←.	*** [3]
right →.	* [1]
into the page ⊗.	[0]
out of the page ⊙.	*** [3]
(No direction, as this quantity is zero.)	[0]
(Unsure/guessing/lost/help!)	** [2]

A metal rod pivoted at one end rotates counterclockwise in a magnetic field that points out of the page. The direction of the magnetic force on (fictitious) positive charges in the rod is:

in towards the center of rotation.	***** [5]
out away from the center of rotation	******************* [19]
into the page ⊗.	*** [3]
out of the page ⊙.	** [2]
(No direction, as this quantity is zero.)	[0]
(Unsure/guessing/lost/help!)	*** [3]

Explain what a generator is supposed to "generate."

"Generators generate a motional emf."

"Emf and current."

"Generates a current and motional emf, eventually creating power."

"Electric potential energy."

"Electricity?"

Explain the meaning of "motional" in the term "motional emf."

"That the emf is created through motion."

"Energy is created via movement of a metal rod through a magnetic field."

"It means something is constantly moving to produce the emf."

"The motion of the bar is what creates the emf, thus 'motional emf.'"

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

"This is all becoming pretty foreign to me, any worksheets and practice in class will definitely benefit me!"

"what are the benefits of making a single-pass generator over a continuous generator?" (Single-pass generators are not very practical (as you must stop producing electricity in order to "reset" the system), but are much easier to understand conceptually. More practical, continuous generators (that don't have to, or automatically "reset" by returning to their starting point) are a bit more difficult to understand.)

"Aren't you still technically putting in some energy for the system to reset a continuous generator, just as in the single-pass generator?" (You actually put energy into all generators (they just convert your kinetic energy into electric potential energy), but you wouldn't have an abrupt break trying to reset a continuous generator (and can generate electricity uninterrupted), compared to a single-pass generator like a slide-rail generator, where you would have to pick up the rod at the end of a (finite) set of rails, pick it up to put it back to the beginning (or you would need to stop the rod, and start sliding it backwards along the rails.)

"I'm excited that we are learning useful stuff for the zombie apocalypse." (I'm excited, too.)

Physics quiz question: magnetic force of magnet on current-carrying wire

Physics 205B Quiz 6, spring semester 2015
Cuesta College, San Luis Obispo, CA

The direction of the force on the current-carrying wire near the north end of this magnet is:
(A) up ↑.
(B) down ↓.
(C) left ←.
(D) right →.
(E) into the page ⊗.
(F) out of the page ⊙.
(G) (Some other (diagonal) direction not listed above.)
(H) (No direction, as the magnetic force is zero.)

Correct answer (highlight to unhide): (F)

For the field model of magnetism, the source object is the magnet, which generates a magnetic field around itself. This magnetic field (generated by the magnet) then exerts a force on the test object: the flow of positive charges of the current in the wire.

At the position of the wire, the magnetic field just above the north end of the magnet points up along the plane of the page, as magnetic field lines come out of the north end of magnets (while magnetic field lines go into the south end of magnets).

Using the first right-hand rule (RHR1), where the thumb points along the direction of the current in the wire, which is the velocity direction v of positive charge flow: to the right in the plane of the page; the index finger along the direction of the magnetic field B: up along the plane of the page; the middle finger then must point perpendicularly out of the page of the page, which is the direction of the force F on the flow of positive charges for the current in the wire.

Sections 30882, 30883
Exam code: quiz06t3SL
(A) : 6 students
(B) : 3 students
(C) : 4 students
(D) : 1 student
(E) : 12 students
(F) : 18 students
(G) : 2 students
(H) : 0 students

Success level: 34%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.62

Online reading assignment: magnets, magnetism, and magnetic forces from fields

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 magnetism and magnetic forces from fields.


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.

"Source N-S poles exerts a force on test n-s poles. It can be attractive S-n or repulsive N-n."

"The basics of magnetism; I understand the 'with/against B field' due to NMR in chemistry."

"A source magnet with north and south poles creates a magnetic field everywhere around it. The magnetic field lines make closed loops coming out of the north pole and going to the south pole."

"A source magnet's poles exert force on a test magnet's poles. Also a B field exerts force on a test magnet."

"The source magnet creates B field, the B field exerts force on test magnet. Opposite ends attract, same ends repel."

"The two models of magnetism: direct and field. The direct model looks at how poles of magnets attract or repel each other depending on how they are oriented to each other. The field model is where test charges move around the magnetic field generated by a source magnet."

"Magnetic fields can exert forces on wires and moving charges. There are different hand gestures to remember the rules behind this concept. According to the field model, a source (either magnet or current I) creates a B field which exerts a force on a test current or a moving test charge."

"For a charge to experience a magnetic force when placed in a magnetic field the charge must be moving and the velocity of the moving charge must have a component that is perpendicular to the direction of the magnetic field."

"I understand what it's trying to explain, but I am not grasping it... It's just not visually making sense..."

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.

"Just the concept of B fields."

"The field model."

"Why do magnetic field lines go in a loop from north to south?"

"I don't quite understand the 'obeys/disobeys the B field' explanation."

"Not really sure what a rail gun is."

"Magnetic forces from fields. A lot of it came from probably not distorting my hand properly for RHR 1, but hopefully we will practice in class."

"The latter part of the presentation is challenging for me. I'm not super good with understanding magnetic field stuff."

"Nothing was too complicated, but the whole right hand rule would be good to review."

Outside of a bar magnet, magnetic field lines travel from the __________ pole to the __________ pole.

north; south.	[29]
south; north.	[0]
(Unsure/guessing/lost/help!)	[1]

Inside of a bar magnet, magnetic field lines travel from the __________ pole to the __________ pole.

north; south.	[5]
south; north.	[23]
(Unsure/guessing/lost/help!)	[2]

State the symbol used for the magnetic field, and the SI units for magnetic field strength.

"The symbol is B, and the SI units are teslas."

"B, newtons?"

"I, amps?"

When properly using right-hand rule 1, in general there can be any angle between the v thumb and B finger (depending on the situation). In general, specify the angles between the other fingers used in right-hand rule 1.
(Only correct responses shown.)

B finger and F finger: exactly 90° only [30%]
v finger and F finger: exactly 90° only [53%]

In general, the direction of the force of a magnetic field on a moving charged particle will be along the:

particle's velocity vector.	**** [4]
the magnetic field line.	****************** [18]
(Neither of the above choices.)	*** [3]
(Unsure/guessing/lost/help!)	***** [5]

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

"I just looked at the class schedule, and I am very relieved Quiz 5 was pushed back a bit!"

"I don't get understand when to use which right hand rule for magnetic fields. I understand that there are multiple right hand rules for magnetic fields but I don't know in what specific context to use them and if I can use them whenever. Some clarification on this would be greatly appreciated."

"Can we talk about the right hand rules in class?"

Physics quiz question: magnitude of magnetic force on moving charge

Physics 205B Quiz 6, spring semester 2014
Cuesta College, San Luis Obispo, CA

A q = +4.0 µC charge moves at a speed of 4.0×10⁴ m/s through a uniform magnetic field with magnitude 4.50 T. The magnitude of the magnetic force on the charge is:
(A) 0 N.
(B) 0.51 N.
(C) 0.72 N.
(D) 1.0 N.

Correct answer (highlight to unhide): (B)

The magnitude of the force of this magnetic field on this moving charge is given by:

F_B = |q·v·B·sinθ|,

where the angle θ between the charge's velocity, and the magnetic field is 45°, such that:

F_B = |(+4.0×10^–6 C)·(4.0×10⁴ m/s)·(4.50 T)·sin(45°)|,

F_B = 0.5091168825... N,

or to two significant figures, the magnitude of the magnetic force on the charge is 0.51 N.

(Response (C) is just |q·v·B|; response (D) is |q·v·B/sin(45°)|.)

Sections 30882, 30883
Exam code: quiz06Lp4s
(A) : 0 students
(B) : 24 students
(C) : 13 students
(D) : 0 students

Success level: 65%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.55

Physics quiz question: direction of magnetic force on moving charge

Physics 205B Quiz 6, spring semester 2014
Cuesta College, San Luis Obispo, CA

A q = +4.0 µC charge moves at a speed of 
4.0×10⁴ m/s through a uniform magnetic field with magnitude 4.50 T. The direction of the magnetic force on the charge is:
(A) up ↑.
(B) down ↓.
(C) left ←.
(D) right →.
(E) into the page ⊗.
(F) out of the page ⊙.
(G) (Some other (diagonal) direction not listed above.)
(H) (No direction, as the magnetic force is zero.)

Correct answer (highlight to unhide): (F)

Using the first right-hand rule (RHR1), where the thumb points along the direction of the (positive) charge's velocity v: diagonally in the plane of the page; the index finger along the direction of the magnetic field B: up along the plane of the page; the middle finger then must point perpendicularly out of the page of the page, which is the direction of the force F on this (positive) charge. (The force would be in the opposite direction given by the middle finger if the charge was instead negative.)

Sections 30882, 30883
Exam code: quiz06Lp4s
(A) : 5 students
(B) : 2 students
(C) : 2 students
(D) : 2 students
(E) : 3 students
(F) : 18 students
(G) : 5 students
(H) : 0 students

Success level: 49%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.55

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."