## 20170414

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

Physics 205B, spring semester 2017
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.
"This section covered two magnetic fields exert forces on different test objects. Also, how magnetic fields are created by current-carrying wires."

"RHR2 is used to determine the direction of the B field created by a straight section of current-carrying wire. RHR3 is used to determine the direction of the B field created by a circular loop of current-carrying wire."

"The conceptual model for how magnetic fields and current function in the universe is best described as any wire loop in which current can run through that can be modeled as having a north and south pole. In this model we can use the vectors or direction of magnetic current and describe the nature of magnetic fields and how they impact moving charges and vice versa."

"Tonight reading assignment talked about the RHR2 and RHR3.The difference between RHR2 and RHR3 is that they the fingers represent different magnitudes. RHR3 is also used for circular loop of a current carrying a wire; when one curls their fingers in the direction of the current along the wire and the thumb points int the direction of the magnetic B field."

"Straight wires create a magnetic field that is circular around itself. Inside a loop, the magnetic field is perpendicular to the direction of the current."

"A current-carrying wire also produces a magnetic field of its own. As current travels through a wire it presents a circular pattern directing the magnetic field. A specific machine using these magnetic waves for a real-world application is with that of magnetic resonance imaging. A long solenoid is used to produce magnetic fields which exert forces on hydrogen atoms so prevalent in the body to to create an image of the physical location of internal structures."

"This section is on wires and loops. A source object creates a magnetic field everywhere around it. All wires have a magnetic loop around it even if they are closed or not."

"I understand how a magnetic field will exert a magnetic force on a test object such a straight section of current-carrying wire and also how to use the RHR 1. I get that the thumb is used either for the velocity direction of a positive charge, or for the direction of 'conventional current' along a straight section of wire."

"I know the right hand rules; that's really it."

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 was a little confused on how to know whether there would be magnetic forces on each other."

"I fill preety confident knowing I can comprehend an equation which works through these concepts and applies the different value to the formulas we will work with. I still have a little bit of trouble grasping the conceptual idea of magnetic fields and how for example north and south poles function in the real world."

"What I found a little confusing was the right-hand rules 2 and 3. I think that doing some examples actually using those hand gestures in class (like we did for right hand rule 1) would help very much!"

"Still somewhat confused on the hand rules. It seems that depending on which way you orient your hand you could have the same direction with your thumb and multiple possible directions for your B and F values."

"It is difficult to understand via online how to accurately use the RHR 2/3 so in class examples and explanation of this technique would be highly beneficial. I also don't really understand how the magnetic field works in terms of a straight wire and loops."

"I would like some more clarification on current-carrying wire examples."

"Can we go over more concepts in class? I don't even know what I'm confused on."

"I need to read the textbook."

"I just skimmed it."

"I think I'm okay this time around."

"All good!"

State/describe the symbol used for the "permeability of free space," and give its SI units.
0, where 'µ' is the symbol for micro and '0' is a subscript zero; Tesla·meters/Amperes."

"Permeability (symbolized by µ0) is measured in henries per meter, or newtons per squared-amperes."

"Didn't get to it yet."

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 [79%]
The end of a bar magnet, and a stationary charge: not possible [42%]
The end of a bar magnet, and a moving charge: possible [58%]
Current flowing through a wire, and a stationary charge: not possible [46%]
Current flowing through a wire, and a moving charge: possible [71%]
Current flowing through a wire, and another wire with current in it: possible [58%]

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 [88%]
Magnetic field B: curled fingers [88%]

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 [75%]
Magnetic field B: thumb [75%]

Explain the similarities/differences between a circular current loop, and a solenoid.
"There is no particular difference between the two, a solenoid is a large coil containing a large number of close turns and may be regarded as a current circular loop."

"A solenoid has a lot more turns than just a circular loop. Otherwise they're almost the same."

"A solenoid is essentially stacked current loops, so the magnetic effects stack as well, i.e. it gets more concentrated in the center."

"They are both a circular loop of wire with a current through it; a solenoid can act more like a magnet."

"I have no idea"

"I do not know what a solenoid is..."

"Not sure--I am kind of confused with this one!"

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"I didn't fully understand how the movement of a charge affects the capacity to exert magnetic forces. Does a charge only create a magnetic field if it is moving?" (Yes, and a magnetic field exerts a force only on a moving (test) charge.)

"Does the circular right hand rule work the same way on your left hand for a negative charge? (Only if you consider electron flow instead of (positive) current flow.)

"Are there any examples we do with the left hand? I hope not, because the right-hand rules are already confusing me!"

"I think a bit more practice with different hand rules examples would be helpful."

"Why do I feel mentally handicapped when reading this book?"

"Throwing up physics gang signs are fun!"