Physics quiz question: step-up transformer output voltage

Physics 205B Quiz 6, spring semester 2019
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 20.34

The primary coil of a transformer has 400 turns; the secondary coil has 1,000 turns. An alternating current is sent through the primary coil with an input emf of 17 V. The output emf in the secondary coil is:
(A) 6.8 V.
(B) 10 V.
(C) 26 V.
(D) 43 V.

Correct answer: (D)

Given: N₁ = 400 turns, N₂ = 1,000 turns, and ε₁ = 17 V.

Since ε₂/ε₁ = N₂/N₁ = I₁/I₂, then:

ε₂ = ε₁·(N₂/N₁) = (17 V)·(1,000/400) = 42.5 V,

which was expected to be higher than the input emf, as this is a step-up transformer (more turns in the secondary coil than the primary coil).

(Response (A) is ε₁·(N₁/N₂); response (B) is ε₁*(N₂ – N₁)/N₂; response (C) is ε₁·(N₂ – N₁)/N₁.

Student responses
Sections 30882, 30883
Exam code: quiz06riQ1
(A) : 4 students
(B) : 1 student
(C) : 2 students
(D) : 31 students

Success level: 82%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.25

Online reading assignment: magnetic fields of current-carrying wires and loops

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 magnetic fields of current-carrying wires and loops.


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 force is attractive if the ends of the source magnet and test magnet face each other with the opposite poles, and repulsive if it ends of the source magnets and test magnet see each other with the same poles. Magnetic field lines are everywhere and the point out from the north end of a source magnet."

"A circle with an × going through it (⊗) is a vector going into the the page ('innie'). A circle with a dot in the center (⊙) is a vector coming out of the page ('outie')."

"What the fingers represent in the first right-hand rule, and how to position them. I also understand that source objects create a magnetic field, which in turn effects a test object."

"I think that I have a pretty good grasp on the basic concepts of magnets and how they interact with each other. After last class, I now understand the first right-hand rule, and I think that I have a slight grasp on the other hand rules, but I know that after we talk about them in class I will fully understand them."

"The first right-hand rule is used to see where the force on the test charge is pointing based on the direction of the B field and the angle of the velocity v. We now see the crouching tiger in its natural habitat."

"I now know how to use the 'Crouching Tiger, Hidden Dragon' right-hand rule signs."

"In the two-step model of magnetism, the source object creates a magnetic B field everywhere around it, and then it's the field that exerts a force on the test object."

"How to use the first right-hand rule in order to find the direction of magnetic field forces. From the other right-hand rules, for a circular loop of wire the fingers curl in the direction of the current and for the straight section of wire the fingers curl in the direction of the magnetic field."

"That RHR2 is used when current is flowing in a straight wire), and models a circular magnetic field around the wire. RHR3 is used when for circular current-carrying wire, and it models the closed circular magnetic fields created around the loop."

"The second and third right-hand rules determine the direction of the magnetic field by current-carrying wires. Current traveling through a wire loop creates a magnetic field within its diameter that get stronger with more loops. "

"We use RHR2 to determine the direction of the field lines for a straight current-carrying wire, and we use RHR3 to determine the direction of the field lines for a circular loop of current-carrying wire."

"That I should use RHR2 and RHR3 to determine the direction of the magnetic field that the source loop/ wire creates. I understood that I should use RHR1 to determine the direction of the force exerted by the magnetic field. "

"For an infinitely long, straight wire the magnetic field will be stronger near the wire where the r is small. Also the magnetic field outside a solenoid is not constant and is weaker than the interior field."

"How a current-carrying wire also produces its own field. A long straight wire produces its own field, as does a circular loop, as does a solenoid."

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.

"How the direction of the magnetic force on a magnet's north pole is along the direction of the magnetic field lines and the force of a magnet's south pole is directed against the direction of magnetic field lines. As well as how to tell when to use 'Crouching Tiger and Hidden Dragon.'"

"I'm confused on the right-hand rules because its hard to see them in all the different scenarios on paper and try to figure them out in real life."

"How the second and third right-hand rules work."

"I just don't understand why B is the curled fingers for the second right-hand rule."

"I need a bit more help on using the new right-hand rules RHR2 and RHR3."

"Solenoids and circular loops. RHR2 and RHR3. I understood RHR1 in class, so I will pay attention tomorrow."

"I would benefit from more discussion on the field models and problems with those concepts."

"I don't have any immediate concerns, I'd just like to practice the rules and go over examples in class like we always do. What are the requirements of an object being a source object versus a test object? Is this something we'll cover? That seems like it would be interesting!"

"I'm confused on a little bit of everything."

State/describe the symbol used for the "permeability of free space," and give its SI units.

"Symbol: µ₀. SI unit = [T·m/A]."

"m·kg/(s·A)²."

"Henries per meter."

"Webers per ampere-meter."

"No clue."

State whether it is possible or not possible for the following pairs of objects to exert magnetic forces on each other.
(Only correct responses shown.)

The ends of two bar magnets: possible [67%]
The end of a bar magnet, and a stationary charge: not possible [30%]
The end of a bar magnet, and a moving charge: possible [79%]
Current flowing through a wire, and a stationary charge: not possible [39%]
Current flowing through a wire, and a moving charge: possible [70%]
Current flowing through a wire, and another wire with current in it: possible [55%]

For the magnetic field created by current in a long straight wire, indicate which right-hand finger(s) point along which directions.
(Only correct responses shown.)

Current I in long straight wire: thumb [88%]
Magnetic field B: curled fingers [88%]

For the magnetic field created by a current in a circular loop of wire, indicate which right-hand finger(s) point along which directions.
(Only correct responses shown.)

Current I in circular loop of wire wire: curled fingers [82%]
Magnetic field B: thumb [82%]

Explain the similarities/differences between a circular current loop, and a solenoid.

"A current loop is kinda self explanatory, current following through a closed loop. A solenoid is a coiled wire with multiple current loops stacked together."

"A circular current loop electricity creates a magnetic field which is more concentrated in the center of the loop than outside the loop, while stacking multiple loops concentrates the field even more which makes it a solenoid. Both serve to create a magnetic field within their diameter by current flowing around, however solenoids are stronger fields due to more loops stacked on each other."

"A solenoid is a bunch of parallel loops."

"I don't know the difference. I Googled it and it says that a solenoid is a cylindrical coil of wire acting as a magnet when carrying electric current. A circular current loop creates a magnetic field which is more concentrated in the center of the loop than outside the loop. but that doesn't really do me any good. I still don't understand."

"I'm honestly not sure about the similarities/differences between a circular current loop and a solenoid."

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

"So. Why does the current split to a 10 ohm light bulbs and a 8 ohm light bulb in parallel? Why not all of the current just going through the 8 ohm light bulb considering it's less resistance?" (Yes, more of the current will go through the 8 ohm light bulb (56%) and less will go through the 10 ohm light bulb (44%), and you should work out how much goes through each light bulb--there are no "all or nothing" shortcuts, unless one of the light bulbs has zero, or an infinite resistance.)

"I'd like to get a explanation on the solenoid and circular current loop. Based from what I understood, theres no particular differences between solenoid and current loop. So, I'd like something to explain their differences." (Yes, there is no substantive difference between them other than the solenoid a stack of many circular current loops.)

"Looks like I'll be doing Khan Academy this weekend."

Online reading assignment: magnetic fields of current-carrying wires and loops

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 magnetic fields of current-carrying wires and loops.


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.

"if there is a circle with an × going through it (⊗) then it is a vector going into the plane of the page, and if there is a circle with a dot in the center (⊙) then it is a vector coming out of the plane of the page."

"When an electric charge moves through a magnetic field, there is a magnetic force."

"You can use the RHR1 to determine the F on a moving positive charge by simply lining your other fingers correctly to the direction of v and B, but the result is reversed if the charge is negative."

"The second right-hand rule is used to determine the direction of magnetic field on a straight wire, whereas the third right-hand rule is used on a closed loop. Solenoids, or electromagnets, are densely looped coils of wire that have important properties that slightly differ from other narrower looped coils."

"Source objects do not act directly on test objects, they create a magnetic field that exerts a force on the test object. When current is passing through a straight wire, the magnetic field is circulated around the wire (RHR2). When wire is in a closed loop or solenoid form, the magnetic field circulates up through the center, around the wire, and up through the bottom."

"The magnitude of the B magnetic field produced by an infinitely long, straight wire is directly proportional to the current I and inversely proportional to the radial distance r from the wire."

"Most of the content from this section is slightly confusing and I would benefit from further examples and explanations in class."

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 application of RHR2. Your example for the first right-hand rule in class was helpful so I'm sure this will be as well."

"How a magnetic force cannot exist when the electrical charge is not moving through a magnetic field."

"I don't understand the physics behind the section of current-carrying wire. Specifically I don't quite understand the direction of the B field and B vectors."

"The solenoids and circular loops were quite confusing to me."

State/describe the symbol used for the "permeability of free space," and give its SI units.

"The symbol used is 'mu-nought' and is in T·m/A."

"µ₀ is the symbol, SI unit is T·(m/A)."

"Henrys per meter."

"Newtons per square-amperes."

"Don't know."

"I'm not sure."

State whether it is possible or not possible for the following pairs of objects to exert magnetic forces on each other.
(Only correct responses shown.)

The ends of two bar magnets: possible [76%]
The end of a bar magnet, and a stationary charge: not possible [48%]
The end of a bar magnet, and a moving charge: possible [88%]
Current flowing through a wire, and a stationary charge: not possible [40%]
Current flowing through a wire, and a moving charge: possible [60%]
Current flowing through a wire, and another wire with current in it: possible [80%]

For the magnetic field created by current in a long straight wire, indicate which right-hand finger(s) point along which directions.
(Only correct responses shown.)

Current I in long straight wire: thumb [92%]
Magnetic field B: curled fingers [92%]

For the magnetic field created by a current in a circular loop of wire, indicate which right-hand finger(s) point along which directions.
(Only correct responses shown.)

Current I in circular loop of wire wire: curled fingers [80%]
Magnetic field B: thumb [80%]

Explain the similarities/differences between a circular current loop, and a solenoid.

"A circular current loop is a loop of where with a current flowing through it. A solenoid is a coiled wire that acts as a magnet when carrying electrical current."

"A solenoid is essentially a device that creates a field via magnets. It's like a circular current loop that repeats."

"A solenoid is basically a whole bunch of circular current loops right next to each other, and they are often called electromagnets."

"I feel a little lost right now..."

"I have no idea."

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

"I'm having a hard time understanding this topic."

"Magnetic fields make my head hurt. Can we talk about this a little more the book did not help me."

"Can we review the right-hand rules again?"

"Thank you for pushing back the quiz."

Online reading assignment: magnetic fields of current-carrying wires and loops

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 magnetic fields of current-carrying wires and loops.


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.

"This section covered two magnetic fields exert forces on different test objects. Also, how magnetic fields are created by current-carrying wires."

"RHR2 is used to determine the direction of the B field created by a straight section of current-carrying wire. RHR3 is used to determine the direction of the B field created by a circular loop of current-carrying wire."

"The conceptual model for how magnetic fields and current function in the universe is best described as any wire loop in which current can run through that can be modeled as having a north and south pole. In this model we can use the vectors or direction of magnetic current and describe the nature of magnetic fields and how they impact moving charges and vice versa."

"Tonight reading assignment talked about the RHR2 and RHR3.The difference between RHR2 and RHR3 is that they the fingers represent different magnitudes. RHR3 is also used for circular loop of a current carrying a wire; when one curls their fingers in the direction of the current along the wire and the thumb points int the direction of the magnetic B field."

"Straight wires create a magnetic field that is circular around itself. Inside a loop, the magnetic field is perpendicular to the direction of the current."

"A current-carrying wire also produces a magnetic field of its own. As current travels through a wire it presents a circular pattern directing the magnetic field. A specific machine using these magnetic waves for a real-world application is with that of magnetic resonance imaging. A long solenoid is used to produce magnetic fields which exert forces on hydrogen atoms so prevalent in the body to to create an image of the physical location of internal structures."

"This section is on wires and loops. A source object creates a magnetic field everywhere around it. All wires have a magnetic loop around it even if they are closed or not."

"I understand how a magnetic field will exert a magnetic force on a test object such a straight section of current-carrying wire and also how to use the RHR 1. I get that the thumb is used either for the velocity direction of a positive charge, or for the direction of 'conventional current' along a straight section of wire."

"I know the right hand rules; that's really it."

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 little confused on how to know whether there would be magnetic forces on each other."

"I fill preety confident knowing I can comprehend an equation which works through these concepts and applies the different value to the formulas we will work with. I still have a little bit of trouble grasping the conceptual idea of magnetic fields and how for example north and south poles function in the real world."

"What I found a little confusing was the right-hand rules 2 and 3. I think that doing some examples actually using those hand gestures in class (like we did for right hand rule 1) would help very much!"

"Still somewhat confused on the hand rules. It seems that depending on which way you orient your hand you could have the same direction with your thumb and multiple possible directions for your B and F values."

"It is difficult to understand via online how to accurately use the RHR 2/3 so in class examples and explanation of this technique would be highly beneficial. I also don't really understand how the magnetic field works in terms of a straight wire and loops."

"I would like some more clarification on current-carrying wire examples."

"Can we go over more concepts in class? I don't even know what I'm confused on."

"I need to read the textbook."

"I just skimmed it."

"I think I'm okay this time around."

"All good!"

State/describe the symbol used for the "permeability of free space," and give its SI units.

"µ₀, where 'µ' is the symbol for micro and '0' is a subscript zero; Tesla·meters/Amperes."

"Permeability (symbolized by µ₀) is measured in henries per meter, or newtons per squared-amperes."

"Didn't get to it yet."

"Can't find this answer."

State whether it is possible or not possible for the following pairs of objects to exert magnetic forces on each other.
(Only correct responses shown.)

The ends of two bar magnets: possible [79%]
The end of a bar magnet, and a stationary charge: not possible [42%]
The end of a bar magnet, and a moving charge: possible [58%]
Current flowing through a wire, and a stationary charge: not possible [46%]
Current flowing through a wire, and a moving charge: possible [71%]
Current flowing through a wire, and another wire with current in it: possible [58%]

For the magnetic field created by current in a long straight wire, indicate which right-hand finger(s) point along which directions.
(Only correct responses shown.)

Current I in long straight wire: thumb [88%]
Magnetic field B: curled fingers [88%]

For the magnetic field created by a current in a circular loop of wire, indicate which right-hand finger(s) point along which directions.
(Only correct responses shown.)

Current I in circular loop of wire wire: curled fingers [75%]
Magnetic field B: thumb [75%]

Explain the similarities/differences between a circular current loop, and a solenoid.

"There is no particular difference between the two, a solenoid is a large coil containing a large number of close turns and may be regarded as a current circular loop."

"A solenoid has a lot more turns than just a circular loop. Otherwise they're almost the same."

"A solenoid is essentially stacked current loops, so the magnetic effects stack as well, i.e. it gets more concentrated in the center."

"They are both a circular loop of wire with a current through it; a solenoid can act more like a magnet."

"I have no idea"

"I do not know what a solenoid is..."

"Not sure--I am kind of confused with this one!"

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

"I didn't fully understand how the movement of a charge affects the capacity to exert magnetic forces. Does a charge only create a magnetic field if it is moving?" (Yes, and a magnetic field exerts a force only on a moving (test) charge.)

"Does the circular right hand rule work the same way on your left hand for a negative charge? (Only if you consider electron flow instead of (positive) current flow.)

"Are there any examples we do with the left hand? I hope not, because the right-hand rules are already confusing me!"

"I think a bit more practice with different hand rules examples would be helpful."

"Why do I feel mentally handicapped when reading this book?"

"Throwing up physics gang signs are fun!"

Online reading assignment: magnetic fields of current-carrying wires and loops

Physics 205B, spring semester 2016
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 magnetic fields of current-carrying wires and loops.


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.

"How to do RHR2 and RHR3. Yay! (Palm straight, fingers curled, and thumb up for both. For RHR2 the thumb represents v or L, the line of the palm represents r, and the tips of the fingers represent B. For RHR3, the thumb represents B, the line of the palm still represents r, and the tips of the fingers represent current I.)"

"A source object creates a magnetic field everywhere around it: B, and it exerts a force on a test object."

"Current carrying wires that are straight will exhibit a force field B that is circular around it; while a coiled or looped wire exhibits a force field B that kinda comes from the middle of the loop and spreads outwards."

"The second right hand rule is used to determine the direction of the field lines in a straight current carrying wire. The third right hand rule is used to determine the direction of the field lines for a circular loop of current carrying wire."

"I understand that a magnetic B field will exert magnetic forces on a test object, while a source object (a straight section of current-carrying wire, or a closed loop of current) will create a magnetic B field everywhere around itself."

"RHR2 and RHR3 are used for the direction of the magnetic field of wires. RHR2 specifically is for straight wires and has thumb as current, palm as distance from wire to location and curl of fingers as the direction of magnetic field B. RHR3 is for circular loop of current carrying wire and the thumb = direction magnetic field B, curl = direction of current."

"The direction of magnetic force is given by the first right-hand rule (RHR1, or 'hidden dragon'), where the thumb is used either for the velocity direction of a positive charge, or for the direction of 'conventional current' (positive charge flow) along a straight section of wire."

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 having trouble convincing myself that 'two of the sections will have forces exerted on them by the magnetic field that will result in the loop turning until it is perpendicular to the magnetic field.' When I do it, I get F pointing towards me, not down or up. I think my RHR1 is broken."

"I found the majority of this presentation confusing, specifically how to use the hand rules."

"I am still struggling with what a solenoid is. I don't know what I am supposed to be applying it to."

"hat I found confusing about the reading was completely grasping the loop-rule. I can understand how a magnetic field will circle around a given straight section of current-carrying wire in conjunction with the north and south poles of the magnet. But I just do not understand how the magnetic field will loop around a current-carrying wire at a fixed distance around a wire. Wouldn't the magnetic field circle around a wire at distance approaching infinity? even though the force might be very weak. Is the equation to determine the magnitude of a magnetic field similar to solving the magnitude of an electric field? Because I think that would clear it up for me."

"I'm pretty sure I find everything confusing. I've consistently missed lectures on circuits and things relating to this. I think I may be screwed."

"Too many things to remember!"

"Does the direction of the magnetic field for a loop of current wire go only in the direction of the thumb in RHR3?"

"honestly have no idea what is going on. I felt like I kind of understood the right hand rule in class but then when I looked at the homework I actually have no idea what's happening at all. And then don't even get me started trying to add the other two rules to that."

"I don't get magnetism."

"I didn't find any of the concepts particularly confusing. Most of the problems I encountered involve the implementation of the three right hand rules."

"How to use the R1H1 rule for the second part of the magnetism. I'm still not sure if I understand the whole R1H1 and RHR2 concept that well."

"The RHR2 and RHR3 hand gestures. And I will benefit from some explanation in class to clarify."

"I get the RHR2 and 3 because they are relatively simple, but still questioning RHR1."

State/describe the symbol used for the "permeability of free space," and give its SI units.

"'Mu nought,' µ₀ = 4π×10^–7 T⋅m/A."

"Henries per meter (H/m), amount of resistance encountered when forming a magnetic field."

"The symbol is the one that looks like a fancy m, pronounced as 'mew' and its units are N per A squared."

"µ₀, kinda super confused."

"Greek 'm,' µ₀ = henries/meter or newtons per ampere squared."

State whether it is possible or not possible for the following pairs of objects to exert magnetic forces on each other.
(Only correct responses shown.)

The ends of two bar magnets: possible [60%]
The end of a bar magnet, and a stationary charge: not possible [30%]
The end of a bar magnet, and a moving charge: possible [63%]
Current flowing through a wire, and a stationary charge: not possible [17%]
Current flowing through a wire, and a moving charge: possible [60%]
Current flowing through a wire, and another wire with current in it: possible [57%]

For the magnetic field created by current in a long straight wire, indicate which right-hand finger(s) point along which directions.
(Only correct responses shown.)

Current I in long straight wire: thumb [83%]
Magnetic field B: curled fingers [83%]

For the magnetic field created by a current in a circular loop of wire, indicate which right-hand finger(s) point along which directions.
(Only correct responses shown.)

Current I in circular loop of wire wire: curled fingers [77%]
Magnetic field B: thumb [80%]

Explain the similarities/differences between a circular current loop, and a solenoid.

"A solenoid is basically 'an array of connected [circular] current loops.'"

"We use RHR2 for a circular current loop and then RHR3 for a solenoid."

"Circular current loop: Creates a concentrated magnetic field in the circle of the center of the loop compared to the outside of the loop. Solenoid: Controlled magnetic field similar to an inductor."

"Coil is a series of loops and solenoid is a long tightly wound loop."

"I still can't grasp what a solenoid is..."

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

"I'm still lost on how to use my hand for the right hand rule."

"CONFUSED!"

"Why did I feel like the semester was over when we left for spring break? We only have six weeks left but they're going to be tough."

"My hand cramps doing the RHRs."

"The hand...I am lost."

Online reading assignment: magnetic fields of current-carrying wires and loops

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 magnetic fields of current-carrying wires and loops.


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 source object creates a magnetic field everywhere around it. Which exerts force on test object."

"We use RHR3 for a circular loop of current carrying wire, the direction of your fingers curl in the direction of the current along the wire, and the thumb points in the direction of the magnetic B field. RHR2 and RHR3 are similar but represent different magnitudes on the fingers."

"I understand that wires will have a magnetic loop around them whether they are in a closed loop or not. RHR2 and RHR3 predict directions of magnetic field for both cases."

"After the first day of class this week, along with reading the lecture, I'm starting to get a lot more comfortable using the RHR. I understand that the force is always perpendicular to the magnetic force."

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 had a hard time understanding the RHR2 and RHR3 and how they can be applied to the field model. Also, is the source here the wire? contrary to the test objects section?"

"I'm not sure when magnetic forces are possible. RHR2 and RHR3 are confusing."

"I am finding the right-hand rules extremely confusing and could definitely benefit from more examples in class."

"I need clarification on the RHR2 and RHR3 rules and how they work. Also the similarities and differences between a circular current loop and a solenoid."

"So after last class I understood how to utilize the right-hand rule. However, now I'm lost on how to use the new right hand rules. Could we please go over these two new rules together in class along with a couple practice problems using the two different rules? Thank you!"

"The loops are throwing me for a loop. It's really hard to get a good grasp of what is going on by just reading this. I need oral explanations and examples."

State whether it is possible or not possible for the following pairs of objects to exert magnetic forces on each other.
(Only correct responses shown.)

The ends of two bar magnets: possible [64%]
The end of a bar magnet, and a stationary charge: not possible [36%]
The end of a bar magnet, and a moving charge: possible [72%]
Current flowing through a wire, and a stationary charge: not possible [33%]
Current flowing through a wire, and a moving charge: possible [56%]
Current flowing through a wire, and another wire with current in it: possible [44%]

State/describe the symbol used for the "permeability of free space," and give its SI units.

"μ, (H)(m^-1)."

"'Mu,' henries per meter, or newtons per ampere-squared."

"I am not sure."

"Magnetic field (B)."

"I can't find this."

For the magnetic field created by current in a long straight wire, indicate which right-hand finger(s) point along which directions.
(Only correct responses shown.)

Current I in long straight wire: thumb [77%]
Magnetic field B: curled fingers [77%]

For the magnetic field created by a current in a circular loop of wire, indicate which right-hand finger(s) point along which directions.
(Only correct responses shown.)

Current I in circular loop of wire wire: curled fingers [74%]
Magnetic field B: thumb [74%]

Explain the similarities/differences between a circular current loop, and a solenoid.

"No clue what a solenoid is."

"A solenoid is multiple circular current loops."

"They are similar because they are both loops that have magnetic fields flowing between them. They are different because solenoids are uniform and are long in the middle, while circular current loops are more like rings."

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

"I can testify first-hand the effect of coiling electric leads near one's work while welding. If long leads are coiled near the work, the magnetic field compounds and has undesirable effects on a DC welding arc, causing the arc to wander out of control. One of those things you remember after you have already done it."

"I am having a lot of trouble understanding these chapters and feel like I have a lot of work to do in these next few weeks in order for me to do well. First I need to focus on this quiz we have Friday."

"I'm really confused about the different hand rules/when to use them?"

"Wow, it just keeps getting more and more confusing. You are killing me."

"Need some more explanations!"

"I love bacon." (I love turtles.)

Online reading assignment: magnetic fields of current-carrying wires and loops

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 re-reading textbook chapters and reviewing presentations on magnetic fields of current-carrying wires and loops.

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 understand that the second right hand rule should be used for a straight section of current carrying wire and the third right hand rule should be used for a circular loop of current carrying wire. The magnetic field for the second right hand rule will be in the direction of the curled fingers whereas in the third rule, the thumb will point in the direction of the magnetic field."

"It's cool how the iron fillings in oil line up parallel to the magnetic field around them."

"I get the concept of the magnetic field model where the source object creates a magnetic field and it exerts a force on a test object (a lot of similar concepts from the electric field model)."

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.

"Pretty much all of it."

"I'm thinking that remembering which fingers go where in a specific situation will be the toughest thing for me at this point. Of course, nothing serves as a better remedy as habitual and continued practice, so I should get the hang of this soon."

"Nothing yet."

"I could use some further explanations on source objects and the differences and similarities between straight wires and loops."

"I found the idea of solenoids to be confusing. I understand how they are very similar to circular current loops but I don't really understand this concept."

State/describe the symbol used for the "permeability of free space," and give its SI units.

"The symbol is a weird Latin-ish looking lowercase 'u' (the same letter used as the prefix for 'micro' in unit conversions) followed with a '0' in subscript. It's units are in T·m/A."

"Unsure what this is."

"µ₀, henries per meter."

"'mew,' newtons per ampere squared."

For the magnetic field created by current in a long straight wire, indicate which right-hand finger(s) point along which directions.
(Only correct responses shown.)

Current I in long straight wire: thumb [97%]
Magnetic field B: curled fingers [97%]

For the magnetic field created by a current in a circular loop of wire, indicate which right-hand finger(s) point along which directions.
(Only correct responses shown.)

Current I in circular loop of wire wire: curled fingers [83%]
Magnetic field B: thumb [83%]

Explain the similarities/differences between a circular current loop, and a solenoid.

"Solenoid has a longer length compared to its diameter. The Distance between two loops in a solenoid is longer."

"I'm not really grasping what a solenoid is."

"Both have looping field lines that pass through the middle and then loop back outside and around to complete their circuit. A solenoid is a helix of loops such that the field lines in the middle of the helix are near parallel."

"A solenoid has a large number of circular current loops. The field lines emerge from one side of the current look and solenoid and reenter the other side for both."

"The 'solenoid' is from Greek word for 'channel.' It is temporary strong magnet, when connected through external source like battery or when electricity passed through it. It is also known as an electromagnet."

"I'm lost."

"VFPt_Solenoid_correct2.svg"
http://en.wikipedia.org/wiki/File:VFPt_Solenoid_correct2.svg
Geek3

Determine which solenoid end (*.html) corresponds with which magnetic pole.
(Only correct responses shown.)

Left end: south pole [69%]
Right end: north pole [69%]

Briefly explain your reasoning for choosing which solenoid end corresponds with which magnetic pole.

"Confused!"

"The magnetic field lines exit out the right end and return via the left."

"In seeing the direction of the magnetic field lines they go on the outside from right to left in a N/S bar magnet the lines go from the north end to the south on the outside."

"The field by the solenoid is similar to that of a short bar magnet."

The B field comes out of the north pole and into the south pole."

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

"What is flux?" (You mean, 'what the flux?' More seriously, we'll cover this next week.)

"How safe are electromagnets? Well, a home-made electromagnets? I kind of made one by accident a year ago." (They shouldn't be dangerous, as long as you keep them away from magnetic media (credit cards, hard drives), and make them not as powerful as superconducting magnets in MRI (Magnetic Resonance Imaging) machines.)

Online reading assignment: magnetic fields of current-carrying wires and loops

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 magnetic fields of current-carrying wires and loops.

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.

"The movie is awesome! I wish the human eye could see magnetic fields like that, although that might be a little distracting."

"Being able to use my fingers to predict which direction the current, magnetic field, or force is going."

"It was interesting how the iron filings aligned themselves with the magnetic field."

"I found many of the concepts hard to picture, so I don't think I have enough insight to find anything too interesting."

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

"Using the hand rules for cross products--I'm thinking the textbook just overcomplicates it."

"The third right-hand rule is giving me a little trouble. it says your thumb points in the direction of the magnetic field in the interior of the loop, I'm having some trouble visualizing how to move my hand."

"I get confused on when to use a right-hand rule and a left-hand rule."

"Its interesting that the magnetic field in a closed loop is so concentrated in the center of the loop; I suppose that is similar to a bar magnet in that the lines are closest near the magnet but for each line in the center, doesn't there have to be just as many farther out?"

"Even though it isn't confusing when reading about it on your blog, I have a feeling I am going to get easily confused when trying to determine which right-hand rule to use on any given problem!"

Explain the similarities/differences between a circular current loop, and a solenoid.

"The similarities are that the magnetic field lines are concentrated within the loop or loops of wire and so a solenoid is like a series of circular loops. The difference is that a solenoid magnetic field is not dependent upon the radius of the apparatus as seen with a loop of wire."

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

"The magnetic field of a solenoid and a loop do still make closed loops, right?" (Yes.)

"Will you explain the difference between the circular current loop and a solenoid?" (Will do.)

Presentation: wires & loops

Look at those magnetic field lines. Just look at them. F-exerting magnetic fields--how do they work? (Video link: "Magnetic Movie.")

In a previous presentation we looked at the magnetic fields of bar magnets, with both a north pole and a south pole. Here we will consider magnetic fields created by various configurations of current-carrying wires.

Recall that instead of objects directly exerting magnetic forces on other objects, in the two-step model of magnetism, a source object creates a magnetic B field everywhere around it, and it is this magnetic field that exerts a force on a test object.

First, magnetic fields exerting forces on test objects: current-carrying wires.

This is the second step of the two-step model of magnetism. A magnetic B field will exert a magnetic forces on a test object such a straight section of current-carrying wire, or a closed loop of current. Here we will emphasize the direction of the magnetic force on the test object, exerted by the magnetic B field.

The direction of this magnetic forces is given by the first right-hand rule (RHR1, or "Hidden Dragon"), where the thumb is used either for the velocity direction of a positive charge, or for the direction of "conventional current" (positive charge flow) along a straight section of wire.

Any closed loop of current can be modeled as a square (or at the very least, quadrilateral), with four straight sections of current. In this example the loop is slightly askew with respect to a uniform (constant direction, constant magnitude) magnetic field. Convince yourself using RHR1 that two of the sections will have forces exerted on them by the magnetic field that will result in the loop turning until it is perpendicular to the magnetic field. (The other two sections also have forces exerted on them, but they will not contribute to turning the loop.)

Second, source objects that create magnetic fields: current-carrying wires.

On the left is the second right-hand rule (RHR2, or "Crouching Tiger"), and the closely related third right-hand rule RHR3, both of which are used to determine the direction of the magnetic field created by current-carrying wires.

This is the first step of the two-step model of magnetism. A source object such a straight section of current-carrying wire, or a closed loop of current will created a magnetic B field everywhere around itself. Here we will emphasize the direction of the magnetic field created by the test object.

For a straight section of current-carrying wire, RHR2 is used, where the thumb points along the direction of (conventional) current I along the wire, the top of the palm represents the distance r from the wire to a specific location in space, and the direction of the magnetic B field at that location is indicated by the direction of the "curl" of the tips of your fingers. Here for every point at the same distance r from the wire, the magnetic B vectors all point in the same direction around the current-carrying wire, such that the magnetic field line (which must form a closed loop) circles around in the direction given by the curl of your right hand fingertips.

To demonstrate the circular magnetic field lines that surround a straight section of current-carrying wire (perpendicular to the plane of this page), here iron filings suspended in oil are initially aligned in random directions near a straight section of wire with no current, and then when the current is turned on, these iron filings will behave as tiny test magnets, aligning themselves parallel to the magnetic field at each of their locations. (Video link: "PH EM MF DEMO 70001A V1025 2D Magnetic Field Demonstrations Simple Wire Coils.")

For a circular loop of current-carrying wire, RHR3 is used, where instead the direction of the tips of your fingers curl in the direction of the (conventional) current I along the wire, and the thumb points in the direction of the magnetic B field anywhere inside of the loop. Note that the magnetic field circles around, since magnetic field lines must form closed loops.

To demonstrate the magnetic field lines that surround a circular loop of current-carrying wire, again iron filings suspended in oil are initially aligned in random directions when there is no current in the loop, and then when the current is turned on, these iron filings will behave as tiny test magnets, aligning themselves parallel to the magnetic field at each of their locations. (Video link: "PH EM MF DEMO 70001A V1025 2D Magnetic Field Demonstrations Simple Wire Coils.")