## 20140409

### 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."

0, 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.)