Lab03-PreLab - PHYS 2212 Lab Exercise 03: Gauss Law...

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1 PHYS 2212 Lab Exercise 03: Gauss’ Law PRELIMINARY MATERIAL TO BE READ BEFORE LAB PERIOD Counting Field Lines: Electric Flux Recall that an electric field (or, for that matter, magnetic field) can be difficult to visualize , since the only thing we can see is its effect on charges. To aid in the visualization process, we introduce the idea of a “ Field Map” which represents the field in terms of a set of continuous, directed “lines of force” or “field lines ( FL )”. For electric fields in particular, the basic rules for setting up the correct field map for a given collection of point source charges are as follows: 1. Field lines always begin on positive charges and end on negative charges. The number of field lines so constructed should be proportional to the magnitude of the charge. 2. In any particular region of space, the magnitudes of the electric field vectors are directly proportional to the “ density ” of field lines in that particular region. (Note that “field lines” and “field vectors” mean different things…) There are, of course, other rules as well, but we are not interested in them here. Refer back to the relevant section of the textbook if you’re interested. For present purposes, we simply note that the two rules above essentially describe a book-keeping process , where the counting of field lines conveys useful information, albeit of a qualitative nature. If we wish to be more precise and quantitative, we almost immediately run into some trouble, and the name of that trouble is “ artistic license ”: since a field map is mainly meant to convey an idea of the “shape” of the field, there are no hard-and-fast rules about how many field lines must be drawn in a particular circumstance. Put in other words, an individual needs to draw only as many field lines as he or she feels necessary in order to get a “feel” for the field. For example, if the field of a +1.0 nC point charge were being drawn, a physics prof. or a math expert might be able to get away with drawing a field map using only four field lines, as shown above. On the other hand, a less experienced Tech student might need to draw twice as many field lines (below left) before he or she feels “comfortable” in understanding what his or her drawing represents. Of course, it doesn’t end there. A really super-duper clueless person (such as a UGA student) might need to draw a lot more field lines before reaching any sort of comfort level with his or her field map (below right). Who’s right ? All of them, of course. As long as each one sticks with their chosen “# Field Lines per unit charge” ratio (i.e. 4 FL per nC for the prof., 8 FL per nC for the Techie, and fifty-gajillion FL per nC for the UGA student), they will all be able to construct valid field maps in subsequent sketches, and be able to draw qualitative— and even some crude quantitative —information from their sketches. The problem, though, is that these sketches are not cross-compatible.
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Lab03-PreLab - PHYS 2212 Lab Exercise 03: Gauss Law...

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