## 20180409

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

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 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.
"Source N-S poles exerts a force on test n-s poles. It can be attractive S-n or repulsive N-n."

"The basics of magnetism; I understand the 'with/against B field' due to NMR in chemistry."

"A source magnet with north and south poles creates a magnetic field everywhere around it. The magnetic field lines make closed loops coming out of the north pole and going to the south pole."

"A source magnet's poles exert force on a test magnet's poles. Also a B field exerts force on a test magnet."

"The source magnet creates B field, the B field exerts force on test magnet. Opposite ends attract, same ends repel."

"The two models of magnetism: direct and field. The direct model looks at how poles of magnets attract or repel each other depending on how they are oriented to each other. The field model is where test charges move around the magnetic field generated by a source magnet."

"Magnetic fields can exert forces on wires and moving charges. There are different hand gestures to remember the rules behind this concept. According to the field model, a source (either magnet or current I) creates a B field which exerts a force on a test current or a moving test charge."

"For a charge to experience a magnetic force when placed in a magnetic field the charge must be moving and the velocity of the moving charge must have a component that is perpendicular to the direction of the magnetic field."

"I understand what it's trying to explain, but I am not grasping it... It's just not visually making sense..."

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.
"Just the concept of B fields."

"The field model."

"Why do magnetic field lines go in a loop from north to south?"

"I don't quite understand the 'obeys/disobeys the B field' explanation."

"Not really sure what a rail gun is."

"Magnetic forces from fields. A lot of it came from probably not distorting my hand properly for RHR 1, but hopefully we will practice in class."

"The latter part of the presentation is challenging for me. I'm not super good with understanding magnetic field stuff."

"Nothing was too complicated, but the whole right hand rule would be good to review."

Outside of a bar magnet, magnetic field lines travel from the __________ pole to the __________ pole.
 north; south. [29] south; north. [0] (Unsure/guessing/lost/help!) [1]

Inside of a bar magnet, magnetic field lines travel from the __________ pole to the __________ pole.
 north; south. [5] south; north. [23] (Unsure/guessing/lost/help!) [2]

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

"B, newtons?"

"I, amps?"

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 [30%]
v finger and F finger: exactly 90° only [53%]

In general, the direction of the force of a magnetic field on a moving charged particle will be along the:
 particle's velocity vector. **** [4] the magnetic field line. ****************** [18] (Neither of the above choices.) *** [3] (Unsure/guessing/lost/help!) ***** [5]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"I just looked at the class schedule, and I am very relieved Quiz 5 was pushed back a bit!"

"I don't get understand when to use which right hand rule for magnetic fields. I understand that there are multiple right hand rules for magnetic fields but I don't know in what specific context to use them and if I can use them whenever. Some clarification on this would be greatly appreciated."

"Can we talk about the right hand rules in class?"