Lab Exercise 03:
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
, since the only thing we
can see is its
To aid in the visualization process, we introduce the idea of a “
represents the field in terms of a set of continuous, directed “lines of force” or “field lines (
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
Field lines always begin on positive charges and end on negative charges.
field lines so constructed should be proportional to the magnitude of the charge.
In any particular region of space, the
of the electric field
proportional to the “
” of field
in that particular region.
(Note that “field lines” and “field vectors” mean
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
, where the counting of field lines conveys useful information, albeit of a
If we wish to be more precise and quantitative, we almost immediately run into some
trouble, and the name of that trouble is “
since a field map is mainly
meant to convey an idea of the “shape” of the field, there are no hard-and-fast rules
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
more field lines before
reaching any sort of comfort level with his or her field map (below right).
All of them, of course.
As long as each one sticks
with their chosen “# Field Lines per unit charge” ratio
per nC for the prof., 8
per nC for the Techie, and fifty-gajillion
per nC for the UGA student), they will all be able to construct
valid field maps in subsequent sketches, and be able to draw
and even some crude
The problem, though, is that these sketches are not cross-compatible.