the elementary we did some static electricity experiments. I knew
some students in the upper grades had studied electricity before, but
I decided to start at a pretty basic level here, to make sure everyone
really understood what they thought they understood. Along the way I
hoped to add some physics context which would be lacking in most
elementary experiences of this topic. You can do all of these at home
too.
We started with the classic: rubbing a balloon on someone's hair. Of
course this makes the hair stand on end, but I expect few people will
have thought about it this way: the mass of the entire Earth is
pulling down on that hair with gravity, yet it only takes a few rubs
with a balloon to get the balloon to exert a stronger upward force on
the hair. That demonstrates how remarkably strong electric forces can
be, compared to gravity. (By the end of the activity we'll see why
they aren't always stronger.)
Why is the hair attracted to the balloon? The older kids will shout
out some version of "the balloon has negative charge" but I make it
clear that giving it a name doesn't explain anything by itself. We
could just as well call it magic if all we want is a name for it. So
let's proceed through some other experiments to see if we can learn
more about it. I wrote "Observations" on the board and jotted down
the result of each experiment as we did it.
One experiment we can do is see if other things besides hair are
attracted to the balloon. I brought some ground pepper and shook some
out on the table, and a well-rubbed balloon will make that pepper just
jump up and if the kids are quiet they will hear a nice kind of
raining sound as the pepper hits the balloon. (Warning: the balloon
loses its "magic" over time, so you need to give some good rubs before
each experiment. If you get tired of rubbing people's hair, come
equipped with some tools for it. Rabbit fur is usually recommended,
but if you find that difficult to come by you can google for
alternatives. It's a good idea to bring a few pieces for the kids to
share when they do their own experiments. Otherwise, it's unfair to
the girls with long hair!) Apparently sawdust is another good material
to try, but I have not tried it.
Next, we did a ping-pong ball. The balloon doesn't lift it up, but
the kids knew right away that's because the ball is heavier. But the
balloon can pull the ball sideways across the table. You can wave the
balloon back and forth and make the ball dance, or keep pulling the
ball in one direction clear off the table.
For fun, I did soap bubbles too. Blow some bubbles and put the
charged balloon above one. The bubble goes up rapidly and dashes
itself on the balloon. With some practice, you can pull the bubble up
without breaking it instantly.
So is the rubbed balloon always attractive? If we rub two balloons
then maybe they attract each other even more strongly? I had a second
balloon ready, tied to a string so I could hold the string with the
balloon hanging straight down. Any attraction would then be visible
to the whole class by seeing the string depart from perfectly
vertical. I had two students rub the two balloons, then brought them
near each other. They repel! They do not attract. So how do we
explain that?
If two similar things repel, then we might think that opposites
attract. Kids can come up with this idea just as well as the
18-century geniuses who provided the foundation for electromagnetism.
We can call these opposites positive and negative, or up and down, or
blue and red, or whatever. The basic picture is that in normal matter
these two kinds coexisting closely, so that from the outside they
appear to cancel out and have no net charge. But the balloon, when
rubbed on hair or fur, tends to tear off and acquire one kind (which
we happen to call negative) more easily than the other. This makes
the balloon negative and the hair net positive, and they attract. But
two rubbed balloons, each being negative will repel. (By this logic
the hair of two rubbed people will repel, which is an interesting
experiment I did not think of at the time.) I drew all this out on
the board, with a bunch of mixed + and - signs initially, moving some
- signs away to show how the hair is net positive, etc.
(By the way, this +/- picture explains why gravity appears to be more relevant than electric forces in, say, holding you down to the earth. Electric charges can be much stronger when one kind is isolated, but usually the two kinds are mixed and deliver no net effect.)
So far, so good. Now I wanted to challenge the kids. I rubbed the
balloon vigorously and stuck it on the wall, where it stayed. How do
you explain that, when I didn't rub the wall? (Astute observers will
note that I didn't rub the pepper, the ping-pong ball, or the soap
bubbles either, but I didn't remind them of that, just to minimize
confusion.) Explanations were offered, but none that really worked.
Is it something about the wall? I went to the sink, ran a thin stream
of water, and brought the balloon close (but not enough to get it
wet). The balloon attracts the water! This is pretty cool and you
should do it at home if you've never seen it. So now we have many
different types of material (including the pepper, the ping-pong ball,
and the soap bubbles) which, even in the absence of rubbing (i.e.
presumably uncharged; we would never get a spark from them, for
example), are attracted to a rubbed (i.e. charged) balloon.
This stumped the kids, but it turns out we don't need a radically new
model of how charge works; we just need to think in more detail about
the implications of our existing model. Our model is that water (or
the wall, the pepper, etc) contains a mixture of + and -, mixed so
thoroughly that from the outside we experience it as uncharged. But
as the water nears the balloon, maybe the balloon can push the -
particles toward the far side of the water stream and attract the +
particles to the near side of the stream:
With the + part of the water nearer the balloon, the water has a net
attraction to the balloon. Yes, the - part of the water is repelled
from the balloon, but more weakly than the + part is attracted.
Therefore, the stream of water moves toward the balloon. Physicists
say that the water is polarized by the charge in the balloon. Not all
materials are polarizable, but apparently many are.
Grades 1-3 asked me some very good questions about this. They asked
if I could get the balloon to repel the water by rearranging the
charge in the water. I said I guess so, but I don't know how you'd
prepare the water with the negatives on the balloon side. As I said
that, I did think of a way, but thought it was too complicated to explain. Then
a girl raised her hand and suggested the same thing I had thought of:
prepare a stream with negatives on, say, the left-hand side by
passing it near a balloon on the right-hand side (as shown above). Then pass
that stream by a balloon on the left-hand side and see if it repels. I
was floored. This was pretty good thinking for a second-grader! It
illustrates one of the main ways science progresses, by using the
results of one experiment to set up a more elaborate experiment. And
in retrospect, it demonstrates that she really understood the model of
how charges behave. She had not just memorized the buzzwords.
With about 15 minutes left, I set the kids free to work on an
experiment of their choice. The one that I recommended was building
an electroscope. I demonstrated a sturdy one: a vertical piece of
metal branching into two vertical pieces of aluminum foil, in a
protective glass container. This is just a more sensitive version of
the repelling balloon demo. When any charge is brought near the piece
of metal, the aluminum foil lifts up. I showed them how to build a
cheaper version out of a clay base, a flexible straw support, and
pieces of aluminum foil attached with string and tape. They could
take those home. Other students chose to try to find unpolarizable
materials, experiment with charged balloons and running water, examine
the motion of pepper across a charged balloon, etc. They seemed to be
very engaged in these activities.
All in all, this activity was a hit.
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