## 20150417

### 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.)