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.)