A better way for kids to build circuits

I just learned about this from a fellow astronomer and part-time kid science instructor: squishy circuits.  It's worth watching the 4-minute video.  Next year, this is definitely the technology I'll use to get little hands to build circuits without frustration!

Whirled Series

Today at the elementary we explored series vs parallel circuits.  I
drew circuits on the board and part of my challenge to the kids was to
figure out how to build what I had drawn.  This led to some conceptual
breakthroughs; for example, some of them had not imagined that they
could twist the ends of three wires together at the same time to make
a more complicated circuit, but they were driven to this idea by the
challenge of building what I had drawn on the board.

The first pair of circuits compared parallel vs series switches rather
than light bulbs (of course there was still one light bulb in the
circuit so we could see when current was flowing).  This reinforced
the basic concept that you need a complete circuit from the positive
battery terminal to the negative one: with switches in series you need
both of them closed, but with switches in parallel you only need one
or the other.  As a corollary, an extra wire dangling off a closed
circuit (which is basically what you have in a parallel-switch
arrangement when one switch is closed and the other one is open)
doesn't hurt anything.

Next, we compared parallel vs series light bulbs.  Students saw that
two light bulbs in series (with a single battery) were quite dim, but
that in parallel they were much brighter.  (This one sentence
summarizes at least 10 minutes of work on their part!)  In thinking
about how to explain this beforehand, I thought a real explanation
involving current and voltage would just be too complicated, but I
didn't want to dumb it down too much either.  As it happened, when the
time came things were so hectic as I flitted from group to group that
I had to settle for something quick: with two paths in parallel we can
get twice the flow.  (This doesn't really explain anything because
there are still parts of the circuit which are NOT parallel...but I
think it was suitable for this age.)

Finally, we used two batteries to power our two light bulbs in series.
I had them compare two ways of connecting the batteries: top of one
touching bottom of the other, and bottoms together.  To explain the
result, I again shied away from voltage numbers and instead stuck to
the concepts they had thought about a lot: current wants to flow from
+ to -, so when you put them with bottoms together the two batteries
want the current to flow in opposite directions and nothing gets done.

Of course, it would have been instructive to compare two batteries in
parallel, but we did not have time.  Furthermore, to explain that one
I think they really would need a full treatment of current vs voltage.

One of the kids discovered how to make a short circuit, so I explained
that to the whole group.  That was something I should have planned do
even if no one had discovered it.

All in all, these activities were at just the right level in the sense
that they couldn't always predict what was going to happen, so that
the experiments did contribute to their understanding; yet the
experiments were not so far beyond their understanding as to be
frustrating.  There were elements of frustration, but they had more to
do with the assembly of the circuits.  Ease of assembly was much
improved over last time because I had soldered wires to many light
bulbs and batteries, but the circuits were also much more complicated.
Next year I will invest in real electronics kits with breadboards and
such.

Sparking an Atttraction

Today at the elementary we built electromagnets.  First I asked each table to prove that they were ready by getting a light bulb to light up given a battery and a few wires.  From their time with teacher Lorie they should have learned that electricity has to travel in a circuit, and most of them were able to do that fairly quickly.  To move on to the electromagnet, each table had to pass a test: if they switch the wires on the light bulb, will it still light up?  Many of them said no quite confidently, but regardless of what they predicted I made them do the experiment.  I then explained how the bulb (a small incandescent bulb for a flashlight or nightlight...the simplest, cheapest type of light bulb) works.  It heats up when electric current passes through, and gets so hot it glows.  This mechanism is so simple that it doesn't even matter which direction the current passes through.  Fancier light bulbs (LEDs) might be more persnickety.

For the electromagnet I told each table to wind a wire tightly many times around a 10d nail.  (Unlike the website linked to in the previous sentence, I didn't bother with switches; we just held things together with electrical tape and/or fingers.) As they finished, some of them asked me what to do next, and others just started trying things.  I gave only vague hints to those who asked, such as "Figure out how to pass electricity through it."  The trick is to make a complete circuit, just as with the light bulb, but they really had to think and experiment (and receive some more hints) before getting it.  This just shows how hard it is to transfer knowledge learned in one context to another context.

After the first session, I figured out that the best way to respond when asked what to do with the wire-wrapped nail is to ask them to check if it's magnetic yet.  This serves two purposes (1) makes them develop a procedure for checking if it's magnetic, which I wanted them to do at some point anyway (picking up a paper clip is the best test I know of); and (2) see that you need flowing current to make the electromagnet work.  After some current flows, the nail tends to get permanently magnetized, making it less obvious later on that current is necessary.  Therefore it's best to demonstrate early on that there is no magnetism.  Also, if you reuse materials from earlier experiments you might well get materials that have accidentally been permanently magnetized.  You'll want to check for this before handing materials out.

For tables which had time left after proving to me that their electromagnet functioned, I gave additional challenges.  Some I challenged to make 2 light bulbs light up.  To others I gave permanent magnets and asked them to figure out how to magnetize a nail without electricity.

All in all this was a pretty good 45-minute activity, with no lecture by me (although it is good to emphasize at some point that this demonstrates that there is a connection between electricity and magnetism).  The one downside was that any circuit more complicated than a single bulb or nail was impossible to keep together with tape and fingers, and one of the younger kids got very frustrated even with a single bulb.  If I were to do more circuit experiments, I would prep by soldering wires on to the light bulb and battery terminals.  This had crossed by mind beforehand, but I thought that this might be giving too much away; I wanted them to figure out the relevant parts of the light bulb.  So in the future I might have one set of bare bulbs for beginners, and one set of bulbs with leads for those who want to make more complicated circuits which don't fall apart.  (Those who don't like soldering might consider using sockets and wires with crimp connections, etc.)

More static electricity

Today I visited Primaria and did an abbreviated, simplified version of the static electricity work I did with the elementary.  The bare essentials are to demonstrate attraction (hair to rubbed balloon) and then show that, counterintuitively, two rubbed balloons do not attract doubly but actually repel.  This leads us to conclude that there must be two kinds of charge and that like repels like.  I also did the deflection-of-water demo because it's too cool to miss, and it demonstrates that all things contain two kinds of charge even if on balance they are uncharged.

I also emphasized that electricity is stronger than gravity (a wimpy balloon overcomes the entire Earth's pull of gravity on the hair) so they could be hurt if they play with it without a grownup around.

One thing I did differently is that I immediately made a connection to magnetism.  They had played with magnets with their regular teacher, so I thought they might be able to make the connection themselves.  But when I asked what other thing (other than static electricity, whose dual nature we had just established) sometimes attracts and sometimes repels, they drew a blank.  I reminded them of magnets and noted the similarity between +/- and north/south, saying that there is a deep connection but they might have to be older to understand it.

That led nicely to the last 5-10 minutes, in which they played with magnets and static electricity, doing their own experiments.  There are a lot of fun things they can build, like anti-gravity devices (opposing ring magnets threaded onto a vertical pencil, which keeps them from sliding sideways), magnet bombs (stacks of opposing magnets forced together by hand, then suddenly released), and remote-control devices (a magnet on top of a tray manipulated by an unseen magnet below the tray).  They just need a little bit of hinting to start exploring the possibilities.

Addendum: the effectiveness of the static electricity demos varies quite a bit from day to day depending on the humidity.  If you have any flexibility, save it for a dry day.

True North

Today at the elementary school we became familiar with magnets.  Grades 1-3 did more of an exploration than a specific lab or task.  I was amazed at how excited they were to play with magnets!  They discovered all kinds of creative things like an "antigravity" device and moving a magnet on a desk using another magnet under the desk, which could not be seen by casual spectators.  I tried to structure their exploration around some basic questions asking how magnets behave in ways similar to and different from charged objects, which was our previous session, and after a lot of exploration we discussed this as a group.    In addition to run-of-the-mill magnets, I brought in one really strong magnet, some small pieces of steel, a plastic case with iron filings, and some compasses.  I sketched how Earth has a magnetic field much like the one shown by the iron filings around a magnet, but we did not talk about compasses much more than that. (I know the kids had some training in the practical use of compasses at their field site. )  The compasses were used mostly as devices which could be spun like crazy by the handheld magnets!

For grades 4-6 I had planned a more directed activity after a shorter period of exploration, because most of them had already played with magnets at some point.  The activity was to build a compass by magnetizing a small piece of steel and then allowing the small piece to move freely.  The moving-freely part was supposed to be accomplished by floating it in water, which a small piece of steel can actually do thanks to surface tension (which formed a mini-lesson in itself).  It seemed pretty easy to get it to float when I practiced it at home, but it was very difficult at school.  The kids got discouraged and played with other aspects of magnets when I wasn't at their table to help them.  I still think it's a good lab, but next time I have to find even smaller pieces of steel, or some other way to remove friction.  I made an on-the-fly attempt at removing friction by hanging it on a string, but the string was too stiff to allow it to rotate freely.  Some people suggest sewing needles, but that seemed a bit dangerous.  I guess I could just blunt the needles before use.

This blog entry is a bit short, without the usual diagrammatic explanation of everything we discussed, but I have to save time this week.  Someday I hope to write a longer explanation of everything we talked about.  We did discuss in quite a bit of detail, because we had 45 minutes per group, which is now the norm since about mid-February.  This amount of time really facilitates thought and discussion.

All Charged Up

Continuing with the theme of different forms of energy, yesterday at
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.