Physics midterm question: stationary loop near constant current wire

Physics 205B Midterm 2, spring semester 2019
Cuesta College, San Luis Obispo, CA

A square metal loop of resistance R is held stationary near a wire that carries a constant amount of current. Discuss whether or not there will be any induced current in the square metal loop, and explain why. Explain your reasoning using the properties of magnetic fields, forces, motional emf, Faraday's law and Lenz's law.

Solution and grading rubric:

• p:
  Correct. Explains how there would be no induced current in the square loop of wire because:
  1. from RHR2, the direction of current in the straight wire creates a magnetic field at the location of the square loop points into the page, creating a magnetic flux through the square loop that points into the page; and
  2. since the current in the straight wire is constant, the magnetic field it creates at the location of the square loop will have a constant magnitude (along with its constant direction), such that there is a constant, unchanging magnetic flux (magnitude and direction) through the square loop; so
  3. from Faraday's law and Lenz's law, since there is no change in magnetic flux through the square loop, there will be no induced emf and no induced current in the square loop.
  (May instead use RHR1 and discuss how the fictitious positive charges in each segment of the square loop are stationary with respect to the magnetic field of the wire, such that there is no force exerted on them to create an induced current.)
• r:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes:
  4. did not clearly indicate the direction of the magnetic field/flux through the square loop; or
  5. argues that there is no induced current in the square loop because there is no magnetic flux through the square loop (when there is a magnetic flux through the square loop, but it is constant); or
  6. argues that there is an induced current in the square loop even though the magnetic flux through the square loop is constant.
• t:
  Nearly correct, but argument has conceptual errors, or is incomplete.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some attempt at applying properties of magnetic fields, forces, motional emf, Faraday's law and Lenz's law.
• x:
  Implementation of ideas, but credit given for effort rather than merit. No clear attempt at systematically applying properties of magnetic fields, forces, motional emf, Faraday's law and Lenz's law.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 30882, 30883
Exam code: midterm02u7aH
p: 14 students
r: 15 students
t: 4 students
v: 3 students
x: 5 students
y: 0 students
z: 0 students

A sample "p" response (from student 0691):
Another sample "p" response (from student 4692):

Physics quiz archive: magnetism, induction

Physics 205B Quiz 6, spring semester 2019
Cuesta College, San Luis Obispo, CA
Sections 30882, 30883, version 1
Exam code: quiz06riQ1



Sections 30882, 30883 results

0- 6 :
7-12 :	***** [low = 9]
13-18 :	********
19-24 :	************** [mean = 21.4 +/- 5.5]
25-30 :	*********** [high = 30]

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.)

Physics quiz archive: magnetism, induction

Physics 205B Quiz 6, spring semester 2018
Cuesta College, San Luis Obispo, CA
Sections 30882, 30883, version 1
Exam code: quiz06Av3g


Sections 30882, 30883 results

0- 6 :	* [low = 6]
7-12 :
13-18 :	************
19-24 :	******************* [mean = 20.1 +/- 4.2]
25-30 :	** [high = 27]

Physics midterm question: loop entering, leaving magnetic field

Physics 205B Midterm 2, spring semester 2018
Cuesta College, San Luis Obispo, CA

Within a certain region (shown in gray) there is an external uniform magnetic field, and the magnetic field is zero everywhere outside of this region. A square metal loop of resistance R moves with constant velocity as it enters, then exits this magnetic field region. Discuss why the induced current flows in one direction around the loop as it enters the magnetic field region, then flows in the other direction around the loop as it exits the magnetic field region. Explain your reasoning using the properties of magnetic fields, forces, motional emf, Faraday's law and Lenz's law.


Solution and grading rubric:

• p:
  Correct. Explains how the induced current flows in one direction around the loop and in the other direction around the loop as it (a) enters and (b) exits the external magnetic field by using:
  1. RHR1, where the velocity direction of the loop (downwards) and external magnetic field direction (pointing into the page) result in a force on the fictitious positive charges to the right in both cases (a)-(b), but only in the horizontal portion of the loop that is inside the magnetic field, thus resulting in an induced current that is counterclockwise in (a) and clockwise in (b); or
  2. Faraday's law and Lenz's law, where moving the loop downwards would increase the magnetic flux going into the page through the loop in (a), and decreasing the magnetic flux going into the page through the loop in (b); and from Lenz's law, these changes in the magnetic flux through the loop will be "fought" by an induced current that must be flowing counterclockwise in (a), and clockwise in (b).
• r:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. Directions of induced current are clockwise in (a), and counterclockwise in (b), but otherwise still systematically applies RHR1 or Faraday's law and Lenz's law.
• t:
  Nearly correct, but argument has conceptual errors, or is incomplete.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some attempt at applying properties of magnetic fields, forces, motional emf, Faraday's law and Lenz's law.
• x:
  Implementation of ideas, but credit given for effort rather than merit. No clear attempt at systematically applying properties of magnetic fields, forces, motional emf, Faraday's law and Lenz's law.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 30882, 30883
Exam code: midterm02iFtW
p: 12 students
r: 12 students
t: 1 student
v: 5 students
x: 4 students
y: 0 students
z: 0 students

A sample "p" response (from student 1712):
Another sample "p" response (from student 1929):

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..."

Physics final exam question: induced current in loops surrounding vertical wire

Physics 205B Final Exam, spring semester 2017
Cuesta College, San Luis Obispo, CA


A vertical wire has a decreasing amount of current flowing upwards. Circular metal loops (of resistance R) oriented flat along the north-south direction are held by four different observers. Determine which observer (if any) will have a coil with an induced counterclockwise current, as seen in this perspective. Explain your reasoning using the properties of magnetic fields, forces, motional emf, Faraday's law and Lenz's law.

Solution and grading rubric:

• p:
  Correct. Determines that only the north observer will experience a counterclockwise induced current, by discussing:
  1. right-hand rule 2, where the thumb runs along a straight current-carrying wire, and fingers curled around the wire give the direction of the magnetic field surrounding the wire; and
  2. from Faraday's law, the east and west observers have coils that will not receive any magnetic flux from the wire's current, so they will not observe any induced current in their coils; while the north and south observers have coils that are oriented to receive the changing magnetic flux from the wire's decreasing current, so they will observe an induced current in their coils; and
  3. from Lenz's law (and right-hand rule 3), only the north observer will experience a counterclockwise induced current to counter the decreasing flux that points east-to-west through it (while the south observer will experience a clockwise induced current to counter the decreasing flux that points west-to-east through it).
• r:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. Correctly discusses (1)-(2), but has problems with discussing (3): typically may not have explicitly discussed why the magnetic field of the induced current in the north observer's loop must point in the same direction as the (decreasing) magnetic field from the wire, or determines that the south observer's coil will have a counterclockwise induced current.
• t:
  Nearly correct, but argument has conceptual errors, or is incomplete. At least some attempt at using magnetic forces and/or magnetic flux. Correctly discusses (1)-(2), but discussion of (3) is incomplete or missing.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some garbled attempt at applying theproperties of magnetic fields, forces, motional emf, Faraday's law and Lenz's law.
• x:
  Implementation of ideas, but credit given for effort rather than merit. No clear attempt at applying the properties of magnetic fields, forces, motional emf, Faraday's law and Lenz's law.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 30882, 30883
Exam code: finalmR3x
p: 1 student
r: 11 students
t: 2 students
v: 11 students
x: 1 student
y: 0 students
z: 0 students

A sample "p" response (from student 7117):

Physics midterm question: moving loop inducing current in stationary loop

Physics 205B Midterm 2, spring semester 2017
Cuesta College, San Luis Obispo, CA

A moveable circular metal loop with a constant counterclockwise current (from an ideal emf source, not shown) partially overlaps a stationary square metal loop of resistance R. In order to create a counterclockwise induced current in the square loop, discuss whether the circular loop should be moved to the left or moved to the right (or if it is not possible to induce a counterclockwise current in the square loop). Explain your reasoning using the properties of magnetic fields, forces, motional emf, Faraday's law and Lenz's law.

Solution and grading rubric:

• p:
  Correct. Explains how a counterclockwise current can be induced in the square loop by moving the circular loop to the right (and/or the square loop to the left) by discussing:
  1. the counterclockwise current in the circular loop creates a magnetic field that points out of the page everywhere within it, creating a magnetic flux that points out of the page in the overlapping region of the two loops; and
  2. moving the circular loop to the right means that the overlapping portion of the two loops will decrease in area, decreasing the outwards magnetic flux; and
  3. from Lenz's law this change in magnetic flux will through the square loop will be "fought" by an increasing outwards magnetic flux created by a counterclockwise current induced in the square loop.
(May also or instead discuss the weaker magnetic field and flux outside of the circular loop, and/or using RHR1 to show that fictitious positive charges in the right-hand side of the square loop will experience an upwards force exerted by the circular loop's magnetic field as the circular loop moves to the right (equivalently meaning that the square loop would be moving to the left, relative to the circular loop).
• r:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes.
• t:
  Nearly correct, but argument has conceptual errors, or is incomplete. At least recognizes direction of the circular loop's magnetic flux, but misapplies Lenz's law to determine direction of induced current in the square loop and/or direction that the circular loop must be moved.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some attempt at applying properties of magnetic fields, forces, motional emf, Faraday's law and Lenz's law. May say that this (static) outwards flux could be counteracted with the square loop's inwards flux, corresponding to a clockwise current, such that a counterclockwise current is not possible.
• x:
  Implementation of ideas, but credit given for effort rather than merit. Approach other than that of applying properties of magnetic fields, forces, motional emf, Faraday's law and Lenz's law.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 30882, 30883
Exam code: midterm02GruT
p: 4 students
r: 5 student
t: 10 students
v: 7 students
x: 3 students
y: 0 students
z: 0 students

A sample "p" response (from student 1921):
Another sample "p" response (from student 7117):

Physics quiz archive: magnetism, induction

Physics 205B Quiz 6, spring semester 2017
Cuesta College, San Luis Obispo, CA
Sections 30882, 30883, version 1
Exam code: quiz06LnDr


Sections 30882, 30883 results

0- 6 :
7-12 :
13-18 :	******** [low = 15]
19-24 :	************** [mean = 21.8 +/- 4.1]
25-30 :	******* [high = 30]

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."

Physics midterm question: loop descending into uniform magnetic field

Physics 205B Midterm 2, spring semester 2016
Cuesta College, San Luis Obispo, CA

A square metal loop of resistance R (seen edge-on) is dragged down into a region with an external magnetic field that points into the plane of this page. Discuss why there will be no induced current in the loop while it enters into this region. Explain your reasoning using the properties of magnetic fields, forces, motional emf, Faraday's law and Lenz's law.

Solution and grading rubric:

• p:
  Correct. Explains how there is no induced current in the loop using at least one of two similar arguments:
  1. plane of square metal loop is parallel to the magnetic field, such that there is zero magnetic flux through the loop, and since the magnetic flux is constantly zero, then there is no induced emf, and thus no induced current in the loop; or
  2. from using RHR1, the force on the fictitious positive charges in the square metal loop is to the right (in the +x direction), such that for the section of the loop closest to the viewer, this would induce a counterclockwise current (as viewed down into the magnetic field, along the −y direction), but in the section of the loop farthest from the view, a clockwise current is induced, such that there will be no (net) induced current in the loop.
• r:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes.
• t:
  Nearly correct, but argument has conceptual errors, or is incomplete.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some attempt at applying properties of magnetic fields, forces, motional emf, Faraday's law and Lenz's law.
• x:
  Implementation of ideas, but credit given for effort rather than merit. Approach other than that of applying properties of magnetic fields, forces, motional emf, Faraday's law and Lenz's law.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 30882, 30883
Exam code: midterm02Mc4s
p: 17 students
r: 7 student
t: 5 students
v: 10 students
x: 3 students
y: 0 students
z: 0 students

A sample "p" response (from student 1614), discussing the Lorentz force exerted on fictitious positive charges in the top and bottom segments of the wire loop:
A sample "p" response (from student 3214), using both the Lorentz force, and also applying Lenz's law to the changing flux through the wire loop:

Physics quiz archive: magnetism, induction

Physics 205B Quiz 6, spring semester 2016
Cuesta College, San Luis Obispo, CA
Sections 30882, 30883, version 1
Exam code: quiz06eL3k



Sections 30882, 30883 results

0- 6 :	*   [low = 6]
7-12 :	*****
13-18 :	*********************   [mean = 17.0 +/- 5.1]
19-24 :	**********
25-30 :	*   [high = 27]

Online reading assignment: flux laws & devices

Physics 205B, spring semester 2016
Cuesta College, San Luis Obispo, CA

Students have a bi-weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters and previewing presentations on flux laws and devices.


Selected/edited responses are given below.

Describe what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.

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

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

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

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

"Faraday's law is a statement that an induced emf ε occurs in a wire loop while the magnetic flux Φ_B through it changes, whether the magnetic field gets stronger or weaker, or by changing the orientation of the surface such that more or fewer magnetic field lines go 'through' the surface. If the magnetic flux Φ_B is constant or unchanging, then there is no induced emf in the wire loop."

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

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

"For any imaginary or actual area A in the presence of a (uniform magnitude and direction) magnetic field B, the magnetic flux Φ_B is the product of the magnetic field magnitude B and the area A. The perpendicular symbol "⊥" denotes that the maximum value for magnetic flux Φ_B occurs if the magnetic field lines are perpendicular to the surface and Φ_B would be zero if the magnetic field is parallel to the surface (as no magnetic field lines would actually go "through" the surface)."

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

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

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.

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

"Magnetic flux really has me in nowhereland."

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

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

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

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

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

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

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

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

"Pretty much everything to be honest, I read the blogs and I like to tell myself that it makes it easier to see it again in class because I don't understand anything while reading."

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

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

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

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

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

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

"Phi, webers, or something like that. #idk I was never in a frat."

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

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

"Circle with a line through it. Webers."

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

Constant zero magnetic flux: no induced current in loop [72%]
Constant non-zero magnetic flux: no induced current in loop. [47%]
Magnetic flux increasing in strength: induced current in loop. [72%]
Magnetic flux decreasing in strength: induced current in loop. [61%]

For an ideal transformer that "steps-down" voltage from its primary coils at 120 V to its secondary coils at 2.1 V, determine what happens to the current and to the power from its primary coils to its secondary coils.
(Only correct responses shown.)

Current: stepped-up (increases). [36%]
Power: no change. [39%]

For an ideal transformer that "steps-up" voltage from its primary coils at 1.5 V to its secondary coils at 220 V, determine what happens to the current and to the power from its primary coils to its secondary coils.
(Only correct responses shown.)

Current: stepped-down (decreases). [50%]
Power: no change. [44%]

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

"It needs to either step-up or step-down to be useful."

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

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

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

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

"Do the other instructors call you 'P-dog' as well?" (Only my graduate student teaching assistants at UC-Davis, and they meant it only ironically.)

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

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

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

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

"Can you go over magnetic flux?"

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

Physics midterm question: moving loop through opposing magnetic fields

Physics 205B Midterm 2, spring semester 2015
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Example 20.1, Conceptual Example 20.5

A square metal loop of resistance R is dragged from a region with an external magnetic field that points into the plane of this page, to a region with an external magnetic field that points out of the plane of this page. The magnitudes of the magnetic fields in these two regions are the same, only their directions differ. Discuss why the induced current in the loop while it is passing from one region to the other will be clockwise in direction. Explain your reasoning using the properties of magnetic fields, forces, motional emf, Faraday's law and Lenz's law.

Solution and grading rubric:

• p:
  Correct. Discusses/demonstrates that current induced in the square must be clockwise using either (or both) of the following (equivalent) arguments:
  1. right-hand rule 1 to show that the force on fictitious positive charges in the top of the square loop point to the right, while the force on the bottom points to left, resulting in a clockwise flow; or
  2. Faraday's (and Lenz's) law to the changing external magnetic flux through the square loop--as it moves downwards, the flux pointing into the page decreases (while the flux pointing out of the page increases), such that the current induced in the square loop must be clockwise in order to provide a counteracting flux that points into the page.
• r:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes.
• t:
  Nearly correct, but argument has conceptual errors, or is incomplete. At least some attempt at using magnetic forces and/or magnetic flux.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner.
• x:
  Implementation of ideas, but credit given for effort rather than merit. Approach other than that of applying properties of magnetic fields, forces, motional emf, Faraday's law and Lenz's law.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 30882, 30883
Exam code: midterm02m3tR
p: 31 students
r: 1 student
t: 11 students
v: 4 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 0550), discussing the Lorentz force exerted on fictitious positive charges in the top and bottom segments of the wire loop:
A sample "p" response (from student 8167), using both the Lorentz force, and also applying Lenz's law to the changing flux through the wire loop:

Physics quiz archive: magnetism, induction

Physics 205B Quiz 6, spring semester 2015
Cuesta College, San Luis Obispo, CA
Sections 30882, 30883, version 1
Exam code: quiz06t3SL



Sections 30882, 30883 results

0- 6 :	**   [low = 3]
7-12 :	*********
13-18 :	***************   [mean = 17.8 +/- 6.2]
19-24 :	****************
25-30 :	****   [high = 30]

Online reading assignment: flux laws & devices

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

Students have a weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters and previewing presentations on flux laws and devices.


Selected/edited responses are given below.

Describe what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.

"We look at flux laws and devices here; proving them using different laws. The first law is Faraday's law which begins with the magnetic flux Φ_B. The law states that the electric potential is equal to the number of coils N multiplied by the change in magnetic flux ∆Φ_B over the change in time ∆t.

"I honestly do not understand any of this stuff! I know it relates to what we learned Monday and Monday's stuff made sense for the most part but I feel like this is another language! Help!"

"I understand the laws used in the presentation. Lenz's law states that direction of this induced current must 'oppose' the changes in magnetic Φ_B."

"The basics of how transformers work: they take oodles of electricity and tone it down so it can actually be used."

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

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

"Let's be honest. What I understand is that this stuff is confusing. I could not repeat anything back in confidence just from reading the presentation."

"When the magnetic flux is constant or unchanging the there is no induced emf in the wire loop. I understand that the amount of induced emf can be compounded by the number of coil turns N in the wire loop."

"Magnetic flux relates an area to a magnetic field. The most magnetic flux is achieved when the area is exactly perpendicular to the magnetic field lines."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.

"I got somewhat confused on how the primary and secondary coils gets mixed up when strength is increasing and decreasing. also, I got a little confused from how primary coil voltage can be so high and secondary too low."

"I am still kind of confused as to what a magnetic flux is exactly and how it relates to generators."

"I'm having trouble understanding when a transformer steps down voltage it actually steps up the current."

"Pretty much everything to be honest, I read the blogs and I like to tell myself that it makes it easier to see it again in class because I don't understand anything while reading."

"I need to see some simple laid-out explanations of what is expected of us in terms of formula usage and some of the basic concepts."

"So magnetic flux is sideways current? Or sideways power? I'm still trying to fully understand what a magnet is and does. Flux, coil, inducing...something? None of it makes logical sense."

"I don't really understand what magnetic flux is? Is this just magnetic force?"

"Faraday's law doesn't seem to click conceptually."

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

"Φ_B represents magnetic flux."

"Funny alien looking symbol."

"Wb, T·m²."

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

Constant zero magnetic flux: no induced current in loop [77%]
Constant non-zero magnetic flux: no induced current in loop. [46%]
Magnetic flux increasing in strength: induced current in loop. [82%]
Magnetic flux decreasing in strength: induced current in loop. [59%]

For an ideal transformer that "steps-down" voltage from its primary coils at 120 V to its secondary coils at 2.1 V, determine what happens to the current and to the power from its primary coils to its secondary coils.
(Only correct responses shown.)

Current: stepped-up (increases). [41%]
Power: no change. [28%]

For an ideal transformer that "steps-up" voltage from its primary coils at 1.5 V to its secondary coils at 220 V, determine what happens to the current and to the power from its primary coils to its secondary coils.
(Only correct responses shown.)

Current: stepped-down (decreases). [38%]
Power: no change. [23%]

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

"The voltage stays constant."

"Because it would not transform anything, by looking at the equations we can see nothing would change. A transformer not transforming is like a heater not heating."

"There magnetic fields that are generated would be the same. So there would be no step up or down."

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

"The word 'flux' reminds me of the flux capacitor from the Back to the Future movies."

"Hellllppppppp."

"My mind is all fluxed up."

"Why is this class getting harder?"