Online reading assignment: magnetism

Physics 205B, spring semester 2013
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 reading textbook chapters and previewing presentations on magnetism concepts.

Selected/edited responses are given below.

Describe something you found interesting from the assigned textbook reading or presentation preview, and explain why this was personally interesting for you.

"That Earth's magnetic north pole is south, and that magnetic bacteria are cool."

"I am really confused on magnetism, and therefore didn't find anything interesting."

"The compasses and the way they reacted to the bar magnet."

"I thought that it was very interesting that they have never found a magnetic monopole and how crazy weird and cool it would be to do that research."

"Magnets are interesting--I've always been fascinated with how they work. They are also fun to play with."

"I found it interesting that magnets have inside and outside magnetic field lines. I thought that magnetic only had field on the outside not the inside."

"There have been theoretical predictions of the existence of the magnetic monopoles, but years of experiments have yet to turn up a single one. I seem to like learning about the things we don't know in the universe more than the things we know."

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.

"The source of magnetism was not detailed. What makes an item in nature magnetic?"

"Everything was confusing, I don't really understand the two-step model, and how the B field comes into play."

"Whats really going on in a magnet? How come you can cut it in half and still have both poles? Is it made of the same medium? Can you reverse the poles?"

"Keeping track of what each letter and symbol stands for."

"I am confused about B force lines and could use some clarification."

"Why is it that magnetic monopoles can't exist or if they do are very rare? If one does exist what would be an example of one?"

"I would benefit from going over the field models. I don't understand what you mean by 'source' and 'sink.'"

"I found it confusing to understand why a magnetic monopole can violate the closed loop rule in a magnetic field. I couldn't wrap my head around the idea of the rule being violated."

"The magnetic field vector at any point is tangent to the field line and the magnitude of the dield is proportional to the number lo lines per unit area perpendicular to the lines."

Explain why a magnetic monopole would violate the rule that "magnetic field lines are always closed loops."

"A magnetic monopole does not exist because the magnetic field is created between two poles and does not exist with one north or one south pole, they must be interacting. Therefore, if a monopole existed, it would not create a loop but simply send out a north or south pole magnetism."

"A monopole would have an open loop because it only has one pole, not two. You cant just cut a bar magnet in half and have two monopole magnets."

"If there is a monopole, there is no opposite pole to close the loop. Therefore there is no pole and even dipoles that are split become two dipoles. As described in the text, if monopoles do exist they are extremely rare in this universe."

"There would be no place for the field lines to begin or end."

"If there are no magnetic monopoles there is no place for the field lines to begin or end, so they must be close loops."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"Which way does the magnetic field travel within a bar magnet? I just assumed it would complete a loop and go back to the north from the south." (Yes.)

"Can you explain monopoles more thoroughly? Thanks!" (First, let me know when you find one.)

Presentation: magnetism

Look at these tiny compasses. Just look at them. And look at what happens to them when a bar magnet is brought nearby. Just look at them move around. (Video link: "PH MD SC TUTE 70031A V0541 Nuclear Magnetic Resonance NMR Model.")

In this presentation we discuss the attraction and repulsion of bar magnets in terms of magnetic fields, in parallel with a previous presentation discussing the attraction and repulsion of electric charges in terms of electric fields.

First, a "direct" model of magnetic forces.

By convention we label the two magnets that exert forces on each other as "source" and "test" poles, where the source magnet (with a north N and a south S pole) is said to be exerting a force on the test magnet (with a north n and a south s pole).

Throughout this discussion, don't worry about the magnitude of the forces these magnets exert on each other, as will focus only the direction of these magnetic forces. This force is attractive if the ends of the source magnet and test magnet face each other with opposite poles, and repulsive if the ends of the source magnet and test magnet face each other with like poles.

Note the convention where the source bar magnet (held stationary) has square ends, while the test bar magnet, which would be free to turn about a fixed center, is drawn like a compass with pointy ends.

Second, a more sophisticated "indirect" or two-step model of magnetic forces.

Instead of a source magnet directly exerting a force on a test magnet, in this two-step model, the source magnet is said to create a magnetic B field everywhere around it, and it is this magnetic field that exerts a force on a test magnet.

In order to visualize the magnetic field created by a source magnet, let's imagine filling space with many test magnets, all of them small enough that the only significant force exerted on them is due to the source magnet, and not due to the test magnets on each other. The directions of all of these test magnets shows us how the "influence" of the source magnet at each and every location in space--it is this influence throughout space that is the magnetic field of the source magnet.

Instead of drawing tiny test magnets at each and every location in space to represent the magnetic field of a source magnet, we place a series of test magnets end-to-end, and trace this magnetic field line. We can then replace the line of test magnets with magnetic field lines everywhere, where the direction of these lines denotes the direction of the north ends of the test magnets.

Let's focus on the first step of this two-step model. The source magnet, with a north pole N and a south pole S creates a magnetic field everywhere around it. (Again, don't worry about the magnitude of this magnetic field). The direction of all magnetic fields make closed loops, each coming out of the N pole (which is the "source" of magnetic field lines), and going into the S pole (which is the "sink" of magnetic field lines).

Notice how these magnetic field lines form closed loops, coming out of the N pole of the bar magnet, and coming into the S pole of the bar magnet.

Now what? If there is another magnet anywhere in the presence of this magnetic field, this magnetic field will exert a force on the test bar magnet's north pole n and south pole s. (Again, don't worry about the magnitude of this magnetic force). The direction of the magnetic force on the n pole is along the direction of magnetic field lines, and the force on the s pole is directed against the direction of magnetic field lines. As a result, the test magnet will often twist and move around corresponding to the forces exerted on its n and s poles.

Here, from before, we show the magnetic field lines filling in all space surrounding a source magnet with N and S poles. This magnetic field will cause a magnetic force to be exerted along a field line on the test magnet's n pole, and a magnetic force to be exerted against the field line on the test magnet's s pole. If we hold the middle of these test magnets stationary, but allow them to twist around, they will all align themselves accordingly where n poles "obey" and s poles "disobey" the field lines. (In any case, as a check the direction of the force on any test magnet poles should be attractive or repulsive depending on whether it is the opposite or same end as the source magnet N and S poles.)

Note that all of these magnets have both a north pole and a south pole. While there is speculation on the existence of magnetic monopoles, we will only consider magnetic dipoles, such as these bar magnets.

Found physics: ferromagnetic paper clip, staples

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http://www.flickr.com/photos/waiferx/4463902740/
Originally uploaded by Waifer X

U-magnet, paper clip, and staples demonstration of ferromagnetism. Photo by Cuesta College Physical Sciences Division instructor Dr. Patrick M. Len.