## 20150413

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

Physics 205B, spring semester 2015
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 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.
"There are the two models of looking at magnetism; direct and field. With direct, we look at the poles and how two magnets can attract or repel each other depending on how they are oriented to each other. The field model is kind of how it sounds, where magnetic forces resonate out and around the source magnet from the poles N to S."

"In RHR1 the angle between v and F should be 90°. A negative moving charge will have the opposite force direction when given with RHR1."

"Magnetic fields (symbol B) are created by charges in motion (currents). Magnetic fields are force fields like electric fields but the magnetic force only acts on charges in motion."

"A magnetic dipole has both a north pole and a south pole. The field lines created by these poles are always closed loops."

"The force of magnetism is attractive if the ends of the source magnet and test magnet face each other with opposite poles, and they repel each other if the ends of the source magnet and test magnet face each other with similar poles. This is very similar to the electric charges, like charges repel and different charges attract."

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.
"The field model is pretty weird."

"When to use LHR. When to use RHR2."

"Noting was too complicated, but the whole right hand rule stuff would be good to review, which I am sure we will be doing on Monday."

"For once, I think I have a handle on all the equations. But I can't quite visualize the RHR1 and how to use it. Except in a street-fight situation."

"The whole thing with the fingers doesnt make any sense to me. Hopefully we can go over it in class and it will make sense. I just can't grasp how it is supposed to tell us which direction it goes or anything!"

Outside of a bar magnet, magnetic field lines travel from the __________ pole to the __________ pole.
 north; south. ****************************** [30] south; north. *** [3] (Unsure/guessing/lost/help!) ****** [6]

Inside of a bar magnet, magnetic field lines travel from the __________ pole to the __________ pole.
 north; south. *********** [11] south; north. ********************** [22] (Unsure/guessing/lost/help!) ****** [6]

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

"B is used for the magnetic field, and the SI units for magnetic field strength is newtons."

"B, J/s."

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 [31%]
v finger and F finger: exactly 90° only [38%]

In general, the direction of the force of a magnetic field on a moving charged particle will be along the:
 particle's velocity vector. * [1] the magnetic field line. ********************* [21] (Neither of the above choices.) ****** [6] (Unsure/guessing/lost/help!) *********** [11]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"We're getting into some pretty deep stuff here."

"I'm unsure what B exactly represents. Also, no idea what these hand things mean. Trying to figure it out, but I don't get it."

"Can you go over the right-hand rule? It is very confusing to understand from the images."

"How was your spring break?" (Mainly just trying to keep up with Mrs. P-dog.)