## 20180411

### 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."

0 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."