## 20160415

### Online reading assignment: magnetic fields of current-carrying wires and loops

Physics 205B, spring semester 2016
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.
"How to do RHR2 and RHR3. Yay! (Palm straight, fingers curled, and thumb up for both. For RHR2 the thumb represents v or L, the line of the palm represents r, and the tips of the fingers represent B. For RHR3, the thumb represents B, the line of the palm still represents r, and the tips of the fingers represent current I.)"

"A source object creates a magnetic field everywhere around it: B, and it exerts a force on a test object."

"Current carrying wires that are straight will exhibit a force field B that is circular around it; while a coiled or looped wire exhibits a force field B that kinda comes from the middle of the loop and spreads outwards."

"The second right hand rule is used to determine the direction of the field lines in a straight current carrying wire. The third right hand rule is used to determine the direction of the field lines for a circular loop of current carrying wire."

"I understand that a magnetic B field will exert magnetic forces on a test object, while a source object (a straight section of current-carrying wire, or a closed loop of current) will create a magnetic B field everywhere around itself."

"RHR2 and RHR3 are used for the direction of the magnetic field of wires. RHR2 specifically is for straight wires and has thumb as current, palm as distance from wire to location and curl of fingers as the direction of magnetic field B. RHR3 is for circular loop of current carrying wire and the thumb = direction magnetic field B, curl = direction of current."

"The direction of magnetic force is given by the first right-hand rule (RHR1, or 'hidden dragon'), where the thumb is used either for the velocity direction of a positive charge, or for the direction of 'conventional current' (positive charge flow) along a straight section of wire."

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 having trouble convincing myself that 'two of the sections will have forces exerted on them by the magnetic field that will result in the loop turning until it is perpendicular to the magnetic field.' When I do it, I get F pointing towards me, not down or up. I think my RHR1 is broken."

"I found the majority of this presentation confusing, specifically how to use the hand rules."

"I am still struggling with what a solenoid is. I don't know what I am supposed to be applying it to."

"hat I found confusing about the reading was completely grasping the loop-rule. I can understand how a magnetic field will circle around a given straight section of current-carrying wire in conjunction with the north and south poles of the magnet. But I just do not understand how the magnetic field will loop around a current-carrying wire at a fixed distance around a wire. Wouldn't the magnetic field circle around a wire at distance approaching infinity? even though the force might be very weak. Is the equation to determine the magnitude of a magnetic field similar to solving the magnitude of an electric field? Because I think that would clear it up for me."

"I'm pretty sure I find everything confusing. I've consistently missed lectures on circuits and things relating to this. I think I may be screwed."

"Too many things to remember!"

"Does the direction of the magnetic field for a loop of current wire go only in the direction of the thumb in RHR3?"

"honestly have no idea what is going on. I felt like I kind of understood the right hand rule in class but then when I looked at the homework I actually have no idea what's happening at all. And then don't even get me started trying to add the other two rules to that."

"I don't get magnetism."

"I didn't find any of the concepts particularly confusing. Most of the problems I encountered involve the implementation of the three right hand rules."

"How to use the R1H1 rule for the second part of the magnetism. I'm still not sure if I understand the whole R1H1 and RHR2 concept that well."

"The RHR2 and RHR3 hand gestures. And I will benefit from some explanation in class to clarify."

"I get the RHR2 and 3 because they are relatively simple, but still questioning RHR1."

State/describe the symbol used for the "permeability of free space," and give its SI units.
"'Mu nought,' µ0 = 4π×10–7 T⋅m/A."

"Henries per meter (H/m), amount of resistance encountered when forming a magnetic field."

"The symbol is the one that looks like a fancy m, pronounced as 'mew' and its units are N per A squared."

0, kinda super confused."

"Greek 'm,' µ0 = henries/meter or newtons per ampere squared."

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 [60%]
The end of a bar magnet, and a stationary charge: not possible [30%]
The end of a bar magnet, and a moving charge: possible [63%]
Current flowing through a wire, and a stationary charge: not possible [17%]
Current flowing through a wire, and a moving charge: possible [60%]
Current flowing through a wire, and another wire with current in it: possible [57%]

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 [83%]
Magnetic field B: curled fingers [83%]

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 [77%]
Magnetic field B: thumb [80%]

Explain the similarities/differences between a circular current loop, and a solenoid.
"A solenoid is basically 'an array of connected [circular] current loops.'"

"We use RHR2 for a circular current loop and then RHR3 for a solenoid."

"Circular current loop: Creates a concentrated magnetic field in the circle of the center of the loop compared to the outside of the loop. Solenoid: Controlled magnetic field similar to an inductor."

"Coil is a series of loops and solenoid is a long tightly wound loop."

"I still can't grasp what a solenoid is..."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"I'm still lost on how to use my hand for the right hand rule."

"CONFUSED!"

"Why did I feel like the semester was over when we left for spring break? We only have six weeks left but they're going to be tough."

"My hand cramps doing the RHRs."

"The hand...I am lost."