## 20170410

### Online reading assignment: magnets, magnetism, and magnetic forces from fields

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 re-reading textbook chapters and reviewing presentations on magnetism and magnetic forces from fields.

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.
"Right hand rule 1 is used to determine the direction of a magnetic force exerted by a magnetic field on a positive moving particle; the direction is reversed if determining the force of a magnetic field on a moving negative particle."

"Magnetic field lines travel from a north pole to a south pole."

"The direct model. It's easy to tell if it's repelling or attracting."

"One can look at two different models at magnetism. They are known as direct and field magnetism. When dealing with direct magnetism one is looking at the poles and seeing how two magnets can attract one another or repel one another depending on how they are oriented to one another. The field model, is when the magnetic fields surround the source magnet from its poles, north to south."

"A magnet creates a magnetic field everywhere around it and the magnetic field exerts a force on either another magnet, moving charge, or current carrying wire. The force of the magnetic field is determined by the amount of current, the segment of wire within this magnetic field, the strength of the magnetic field, and the sine of the angle measured between the magnetic field line and the direction of current."

"The direct model and how the magnets attract or repel."

"The first right hand rule has to do with the direction of a positive magnetic force. I also understand to use the first left hand rule for negative magnetic forces."

"The force magnitudes of a magnetic field acting on a current carrying wire or charge can be calculated using the given equations, and the directions of different vectors can be found using the 'Crouching Tiger, Hidden Tiger' mnemonic."

"Magnets attract (N-s) and (S-n) and repel when (N-n) and (S-s)."

"Magnetic fields are closed loops and move from the north pole to the south pole outside the magnet. How it interacts with a test n-s is determined by orientation (opposites attract, same sides repel)."

"I understand the basics of magnetism. The magnetic field forms closed loops from the N end to the S end. I also understand that a current can create a magnetic field and that magnetic field can exert a force on a test current or a moving charge."

"In the two-step model, the source magnet is said to create a magnetic B field everywhere around it."

"I understand that the source N is attracted to the test s and and the source S is attracted to the test n. The source N is also repulsed by the test n and the source S is repulsed by the test s. I also learned that the source magnet, with a north pole N and a south pole S creates a magnetic field everywhere around it and that the direction of all magnetic fields make closed loops, each coming out of the N pole, or the 'source' and going into the S pole, or the 'sink.'"

"I am lost."

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.
"What I didn't understand from the reading was the field model. I dont see how you can tell if it repels or attracts."

"The force of a magnet affecting a flowing current. It doesn't make sense to me."

"Dealing with the fingers."

"I don't understand the right-hand rule and what the directions mean."

"I'm not sure where the B field is in relation to magnets."

"I am still confused over the right-hand rule (what it means, how to apply, etc.)"

"I don't understand how to position my hand for the right-hand rules that were talked about on the blog. I am sure that once I see it done correctly it will make more sense. The equations seem to be ok, but doing some practice is always good."

"When placing other magnets along the field line, does strength of the magnet matter?"

Outside of a bar magnet, magnetic field lines travel from the __________ pole to the __________ pole.
 north; south. *************** [15] south; north. **** [4] (Unsure/guessing/lost/help!) * [1]

Inside of a bar magnet, magnetic field lines travel from the __________ pole to the __________ pole.
 north; south. ******** [8] south; north. ************ [12] (Unsure/guessing/lost/help!) [0]

State the symbol used for the magnetic field, and the SI units for magnetic field strength.
"B; teslas."

"Magnetic field is measured in teslas (T)."

"Wb?"

"I'm kind of lost."

When properly using right-hand rule 1, in general there can be any angle between the v thumb and B finger (depending on the situation). In general, specify the angles between the other fingers used in right-hand rule 1.
(Only correct responses shown.)
B finger and F finger: exactly 90° only [25%]
v finger and F finger: exactly 90° only [45%]

In general, the direction of the force of a magnetic field on a moving charged particle will be along the:
 particle's velocity vector. *** [3] the magnetic field line. ************* [13] (Neither of the above choices.) *** [3] (Unsure/guessing/lost/help!) * [1]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"I didn't quite understand the orientation of the fingers in RHR1, especially the thumb direction. Does the thumb need to be placed vertically and the other fingers oriented around it? Is there a situation where the thumb does not point straight up?" (That is the conventional way to hold your fingers using RHR1; but in other situations when your thumb (the direction of a positive charge's velocity vector) points in other directions, then the orientation of your fingers must remain the same, even if you need to twist your wrist/arm in weird directions.)

"I'm a little confused on the process of why the magnetic forces exist. Is it because the magnet creates a B field that exerts a force on the magnet. What does the B field consist of?" (A "field" (whether an electric field, or a magnetic field) can be interpreted as a fictional construct that "conveys" the force from a source object to a test object across empty space. There will be other interpretations of what exactly a field "is" at then end of this semester.)

"What kind of material is required for a rail gun projectile?" (Any conductive material will do, ideally with a high melting temperature.)

"I'm pretty interested to hear about what role magnetic fields of a current play in electronics and circuits."

"Does the B field surround the magnet?" (Yes, and is also inside the magnet as well.)

"Please go over the right-hand rule stuff."