## 20140407

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

Physics 205B, spring semester 2014
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.
"I understand that magnets exert forces on one another, and that a magnet creates a magnetic field around itself that travels in a complete loop from the north to south pole. I also understand that the north poles of the two magnets and the south poles of the two magnets will repel each other while the south pole of one magnet and the north pole of the other magnet will attract."

"I thought the arrows on all the compasses in the blog was really cool! They all follow the magnetic field of the magnet!"

"Magnetic fields exert a force on magnetic charges that are placed in that field, much in the same way that electric fields exert forces on electric charges that are placed in their fields. The direction of the force on the charge in the field can be found by using a right-hand rule ('Hidden Dragon'), unless the charge is negative in which case a left-hand rule must be used."

"A magnetic field will exert a force on a current-carrying wire, where the magnitude is given by the amount of current I (in units of coulombs per second, or amps), the (straight) segment of wire L within this magnetic field (in units of meters), the strength of the magnetic field B (in units of teslas), and the sine of the angle θ measured between the magnetic field line and the direction of current."

"Magnets must have at least one north and one south pole. Magnetic field lines are always closed loops. the direction of magnetic force on a charged particle depends in its velocity and is perpendicular to the velocity of the particle."

"North and south poles attract, while North and north poles repel. Field lines can be drawn to represent the magnetic field along a source magnet. A force is exerted along the this line from the source's 'N' pole. A test magnet's 'n' pole will obey and 's' pole will disobey."

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'll need to see the equation used a few times. I know I'll have to practice getting my hand into the correct orientation. I'll need to know what to look for to determine which values given are test charge, field charge, etc. I'll also need some discussion on the units. What is a tesla? I'd like to see how they all 'cancel' to equal newtons."

"I don't understand the right hand rule thing. I really just don't understand most of the assigned reading."

"I think this chapter was not too bad compared to most!"

Outside of a bar magnet, magnetic field lines travel from the __________ pole to the __________ pole.
 north; south. ********************************* [33] south; north. * [1] (Unsure/guessing/lost/help!) ** [2]

Inside of a bar magnet, magnetic field lines travel from the __________ pole to the __________ pole.
 north; south. ****** [6] south; north. *************************** [27] (Unsure/guessing/lost/help!) *** [3]

In general, the direction of the force of a magnetic field on a moving charged particle will be __________.
 along the particle's velocity vector. ****** [6] along the magnetic field line. ********************** [22] (Neither of the above choices.) *** [3] (Unsure/guessing/lost/help!) ***** [5]

State the symbol used for the magnetic field, and the SI units for magnetic field strength.
"B with an arrow over it, strength is measured in T (teslas)."

"I didn't get to it yet."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"You lost me at The Governator."

"In the 'what-if' circuit questions, what is the difference between a light bulb geing removed versus replaced by a wire?" (Removing a light bulb leaves the socket open, and the circuit is broken, such there is an infinite resistance for current to flow there. Removing a light bulb, and replacing it with a wire makes zero resistance for current to flow there.)