Grades 1-3 wouldn't have been able to handle the molecule-size
estimation lab I did with grades 4-6 today, so I did the classic
vinegar-and-baking-soda experiment.
I started by eliciting what they already knew about atoms and
molecules, and trying to organize the barrage of facts they gave me.
Some knew very advanced things like a carbon atom binds with four
other atoms, but they didn't necessarily have a clear idea of how to
define atom or molecule, or who thought of the idea of atoms and why.
(Note to self: bringing in the ancient Greeks doesn't help because
they don't know who the ancient Greeks are.) Most simply, if you take
a substance like gold and divide it into smaller and smaller pieces,
an atom is the the smallest possible unit which still behaves like
gold: you might be able to split that atom, but then the split pieces
will not behave like gold.
For other substances, like water, we call the smallest unit a
molecule. Why? It turns out that we can make water by uniting two
other substances, hydrogen and oxygen, and we can get the hydrogen and
oxygen back by splitting the water. We can't make hydrogen or oxygen
by any analogous process, so the smallest unit of water does not have
the same status as the smallest unit of hydrogen, oxygen, or gold.
Hydrogen, oxygen, and gold are therefore called elements whose
smallest unit is an atom, and water is called a compound whose
smallest unit is a molecule, which in turn is composed of more than
one atom, usually atoms of different types. (We also reviewed states
of matter---solid, liquid, gas---to avoid any confusion there. Just
about any substance can be put into any of these states with enough
heat.)
Making water from hydrogen and oxygen is a bit dangerous because it
releases a lot of energy, and splitting water requires electricity,
which doesn't sound like a good lab for 1-3 graders. So we looked at
a reaction between vinegar and baking soda. First, I had them combine
the two and just observe what happens: it bubbles violently for a time
and then settles down, and there seems to be less baking soda at the
end of the process. This is a chemical reaction: the baking soda and
the vinegar each donated some atoms to make a new compound which
escaped because it's a gas at room temperature. Chemical reactions
are naturally explained by the atomic nature of matter, whereas if we
didn't believe in atoms we would have to stretch to explain all these
reactions.
Next, I had them measure how much mass was lost to the atmosphere in
the reaction. They redid the reaction, this time carefully weighing
the plastic cups, the reagents, and the products. At the simplest, we
wanted to compare the total mass of "one cup with some vinegar and one
cup with some baking soda" with "the cup with the product of the
reaction plus the empty cup." Groups found a difference of 1-3 grams,
easily measurable. I had given each about 1.5 teaspoons of baking
soda and a similar shot of vinegar, for a total of about 40 grams of
reagents. The plastic cups were about 6 grams each.
With a bit of time left, I told them that the gas given off was carbon
dioxide. Could they think of any reactions which take things from the
atmosphere rather than give things to the atmosphere? I steered the
discussion toward something they knew about: plants. Plants take in
lots of carbon from the atmosphere; that's basically most of their
mass. When you grow a plant in a pot, it takes some trace elements
from the soil but it basically doesn't use up the soil even as it gets
pretty big. It gets most of its bulk from the atmosphere. It's
especially important to be aware of that because the reactions we
cause when we drive a car or indirectly cause when we turn on the
lights put carbon in the atmosphere, which causes global warming.
Planting trees can help take some carbon back out, and of course we
should use less to begin with.
Note: The kids needed more time than expected with the arithmetic
(just subtracting the initial mass from the final mass). This is more
a reflection on me than on the kids; they are 1-3 graders after all.
Friday, March 30, 2012
Estimating the size of a molecule
The elementary kids have been learning about molecules, and they
learned about area and volume earlier in the year, so I thought it
would be great to use area and volume to estimate the size of a
molecule. This is an appropriate challenge for 4-6 graders, but
beyond the 1-3 graders, so I did a different molecule-related
experiment with them (see next post).
I introduced the basic idea with a bunch of blocks (which happened to
be Post-it notepads, but could have been anything). When the blocks
are bunched up together as close as possible (a 2x2 wide times 4-high
stack in this case), the bunch has a certain volume which does not
change when the blocks are rearranged. If I spread them out only
1-high, they cover a 4x4 area on the table, so their "footprint" has
quadrupled. But the height of the arrangement now has to be 1/4
of its former height so that the total volume ("footprint" times height)
is the same. So we can infer, or indirectly measure, the relative heights
of the two arrangements after we directly measure the relative areas of the footprints.
In equations, it looks like this:
volume before rearranging = volume after
but volume = footprint times height so that
volume before = footprint after times height after
so, dividing both sides by the footprint after rearranging, we get
volume before divided by footprint after = height after
If we manage to spread something out so that it's only one molecule
thick, the "height after" is just the thickness of the molecule, which
is exactly what we wanted to estimate. Oil spreads out very thinly on
water, so that's what our experiment will involve. We can't see
directly that it spreads out only one molecule thick, so the estimate
we get this way will be an upper limit on the size of the molecule;
each molecule could be smaller if the thin layer actually consists of
several molecules. But these kinds of limits are very important in
science. Often scientists can't measure some small effect but can put
an upper limit on it which is useful for reasoning about the causes
and consequences of that effect. And the upper limit we will get from
this method is impressively small for a tabletop experiment. (Maven
alert: oil is hydrophobic so the molecules actually stand on end like
blades of grass. Thus, our upper limit will actually be on the length
of the oil molecule.)
Before getting the kids all messy with water and oil, I wanted to run
through a practice calculation with a collection of macroscopic
objects, which I thought would clarify the idea for them and make the
oil calculation easier when the time came. I brought 30+ tennis balls
in a bag and had them determine the volume of the bag, then I poured
out the tennis balls one-deep on a table (with trays to bound the
area) and had them measure the area after spreading. The ratio of
"volume before" to "area after" should equal the height of the tennis
ball, which we can measure directly at the end to confirm the
soundness of the method.
The problem was, the kids were overwhelmed with the math. I think
they were rusty on area and volume, and also not used to thinking
about so many variables. There's a "before" and "after" version of
any variable you care to mention: volume, area, height, length, width.
Although these are related in simple ways which allow us to solve the
problem by thinking only about the "volume before" and "area after",
this does not mean that it was easy for kids to restrict their
thinking to these two variables, nor that I made it as clear as I
could have. We spent most of our time trying to work through this
problem. Calculators might have helped, as kids were in cognitive
overload trying to do the arithmetic while simultaneously struggling
to understand what it meant. So I would hear students proudly come up
with answers like "10" and I'd have to say "do you really think a tennis
ball is 10 inches high?" In retrospect, I should have asked
Teacher Chris to conduct a review of area and volume on Thursday so
the kids would not be rusty on those topics.
Another idea, in retrospect, would be to get rid of the tennis balls and
have them estimate the thickness of a post-it note, given just a pad
of post-it notes and a ruler. This seems like a better analogy,
because you can't directly measure the thickness of a post-it note,
and the strategy of dividing the thickness of the pad by the number of
sheets is pretty clear. To make the most direct analogy, I would
implement the counting strategy by having the kids stick the notes in
an array on a tabletop, which is less boring than just counting them,
and then multiplying out the length of the array by the width of the
array.
In any case, only two groups got to spread oil on water, and only one
really got numbers out of it, but at the end we looked at the result
all together. A small drop of oil was estimated to be 3 mm
across, for a volume of roughly 27 cubic mm, and when spread out that
drop covered an area of 100x130 mm, or 13,000 sq. mm. The ratio of
27/13,000 then gives 0.002 mm or 0.00008 inches or 0.000002 meters.
That's a bit large because I think the drop size was overestimated,
but not too far off, and an impressive accomplishment for fifth
graders.
Summary: this was too much for a self-contained 45-minute session. I
should have coordinated with the teacher to have the kids brush up on
the math, and I should have offloaded the cognitive burden of
calculating by either supplying calculators or requiring rounding
(9x9x5 inches should be rounded to 10x10x5 inches for the bag of
tennis balls). And I could have used post-it notes instead of tennis
balls as an analogy. But next time, with these things in place, it's
going to be a fantastic activity for the 4-6 graders.
Link: here is a nicely illustrated page on this topic.
learned about area and volume earlier in the year, so I thought it
would be great to use area and volume to estimate the size of a
molecule. This is an appropriate challenge for 4-6 graders, but
beyond the 1-3 graders, so I did a different molecule-related
experiment with them (see next post).
I introduced the basic idea with a bunch of blocks (which happened to
be Post-it notepads, but could have been anything). When the blocks
are bunched up together as close as possible (a 2x2 wide times 4-high
stack in this case), the bunch has a certain volume which does not
change when the blocks are rearranged. If I spread them out only
1-high, they cover a 4x4 area on the table, so their "footprint" has
quadrupled. But the height of the arrangement now has to be 1/4
of its former height so that the total volume ("footprint" times height)
is the same. So we can infer, or indirectly measure, the relative heights
of the two arrangements after we directly measure the relative areas of the footprints.
In equations, it looks like this:
volume before rearranging = volume after
but volume = footprint times height so that
volume before = footprint after times height after
so, dividing both sides by the footprint after rearranging, we get
volume before divided by footprint after = height after
If we manage to spread something out so that it's only one molecule
thick, the "height after" is just the thickness of the molecule, which
is exactly what we wanted to estimate. Oil spreads out very thinly on
water, so that's what our experiment will involve. We can't see
directly that it spreads out only one molecule thick, so the estimate
we get this way will be an upper limit on the size of the molecule;
each molecule could be smaller if the thin layer actually consists of
several molecules. But these kinds of limits are very important in
science. Often scientists can't measure some small effect but can put
an upper limit on it which is useful for reasoning about the causes
and consequences of that effect. And the upper limit we will get from
this method is impressively small for a tabletop experiment. (Maven
alert: oil is hydrophobic so the molecules actually stand on end like
blades of grass. Thus, our upper limit will actually be on the length
of the oil molecule.)
Before getting the kids all messy with water and oil, I wanted to run
through a practice calculation with a collection of macroscopic
objects, which I thought would clarify the idea for them and make the
oil calculation easier when the time came. I brought 30+ tennis balls
in a bag and had them determine the volume of the bag, then I poured
out the tennis balls one-deep on a table (with trays to bound the
area) and had them measure the area after spreading. The ratio of
"volume before" to "area after" should equal the height of the tennis
ball, which we can measure directly at the end to confirm the
soundness of the method.
The problem was, the kids were overwhelmed with the math. I think
they were rusty on area and volume, and also not used to thinking
about so many variables. There's a "before" and "after" version of
any variable you care to mention: volume, area, height, length, width.
Although these are related in simple ways which allow us to solve the
problem by thinking only about the "volume before" and "area after",
this does not mean that it was easy for kids to restrict their
thinking to these two variables, nor that I made it as clear as I
could have. We spent most of our time trying to work through this
problem. Calculators might have helped, as kids were in cognitive
overload trying to do the arithmetic while simultaneously struggling
to understand what it meant. So I would hear students proudly come up
with answers like "10" and I'd have to say "do you really think a tennis
ball is 10 inches high?" In retrospect, I should have asked
Teacher Chris to conduct a review of area and volume on Thursday so
the kids would not be rusty on those topics.
Another idea, in retrospect, would be to get rid of the tennis balls and
have them estimate the thickness of a post-it note, given just a pad
of post-it notes and a ruler. This seems like a better analogy,
because you can't directly measure the thickness of a post-it note,
and the strategy of dividing the thickness of the pad by the number of
sheets is pretty clear. To make the most direct analogy, I would
implement the counting strategy by having the kids stick the notes in
an array on a tabletop, which is less boring than just counting them,
and then multiplying out the length of the array by the width of the
array.
In any case, only two groups got to spread oil on water, and only one
really got numbers out of it, but at the end we looked at the result
all together. A small drop of oil was estimated to be 3 mm
across, for a volume of roughly 27 cubic mm, and when spread out that
drop covered an area of 100x130 mm, or 13,000 sq. mm. The ratio of
27/13,000 then gives 0.002 mm or 0.00008 inches or 0.000002 meters.
That's a bit large because I think the drop size was overestimated,
but not too far off, and an impressive accomplishment for fifth
graders.
Summary: this was too much for a self-contained 45-minute session. I
should have coordinated with the teacher to have the kids brush up on
the math, and I should have offloaded the cognitive burden of
calculating by either supplying calculators or requiring rounding
(9x9x5 inches should be rounded to 10x10x5 inches for the bag of
tennis balls). And I could have used post-it notes instead of tennis
balls as an analogy. But next time, with these things in place, it's
going to be a fantastic activity for the 4-6 graders.
Link: here is a nicely illustrated page on this topic.
Friday, March 16, 2012
Kindergarteners in Motion
Today at Primaria I did Newton's first and second laws of motion, much
as I did them at the elementary last fall. If you read that entry, I
followed that plan up to and including how the Moon is attached to the
Earth by gravity.
Before doing that, I did a quick advertisement for the
tornado-in-a-bottle building activity I want to do next time. We had
earlier, in the context of pressure, shown how water will not fall out
of a straw if you hold your finger over the top. This is because for
the water to get out, air would have to get in to fill the space. But
air can't get in when your finger is over the top. So today I brought
in a giant size version of this: two 2-liter soda bottles screwed into
a connector, with one half-full of water. Turn it upside down, and
the water doesn't fall because the air need to switch places but get
in each other's way. Now the cool part: swish the water around and it
forms a "tornado" which allows the air to go up through the center
while the water swirls down around the outside.
I bought plenty of the connectors and my goal is to collect enough
bottles to allow each kid to make one at school (with the option of
food coloring and glitter in the water!) and take it home. Parents,
please bring in empty 2-liter bottles!
I bought the connectors (40 for $40) at teachersource.com. I
recommend you try somewhere else because they took forever to ship,
and when I got them I found out that they leak. I was able to prevent
leakage by wrapping the bottle threads with masking tape first, but I
shouldn't have to do that. I went cheap because I wanted a large
quantity. If you just want one or a few, Artec Educational has a
clear one (so you can see what's happening through the connector) for
$2 each.
as I did them at the elementary last fall. If you read that entry, I
followed that plan up to and including how the Moon is attached to the
Earth by gravity.
Before doing that, I did a quick advertisement for the
tornado-in-a-bottle building activity I want to do next time. We had
earlier, in the context of pressure, shown how water will not fall out
of a straw if you hold your finger over the top. This is because for
the water to get out, air would have to get in to fill the space. But
air can't get in when your finger is over the top. So today I brought
in a giant size version of this: two 2-liter soda bottles screwed into
a connector, with one half-full of water. Turn it upside down, and
the water doesn't fall because the air need to switch places but get
in each other's way. Now the cool part: swish the water around and it
forms a "tornado" which allows the air to go up through the center
while the water swirls down around the outside.
I bought plenty of the connectors and my goal is to collect enough
bottles to allow each kid to make one at school (with the option of
food coloring and glitter in the water!) and take it home. Parents,
please bring in empty 2-liter bottles!
I bought the connectors (40 for $40) at teachersource.com. I
recommend you try somewhere else because they took forever to ship,
and when I got them I found out that they leak. I was able to prevent
leakage by wrapping the bottle threads with masking tape first, but I
shouldn't have to do that. I went cheap because I wanted a large
quantity. If you just want one or a few, Artec Educational has a
clear one (so you can see what's happening through the connector) for
$2 each.
Saturday, March 10, 2012
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.
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.
Epistemology 101
A little side note on yesterday's class: I knew some students in the upper
grades had studied electricity before, but I decided to start at a
pretty basic level with static electricity, to make sure everyone
really understood what they thought they understood. At some point a
student volunteered an answer to one of my questions, and I asked,
"How do you know that?" with the intention of highlighting how the
student's conclusion followed from the things we had just observed.
But the student said she knew it because a teacher (in a previous
year) had told her, so I asked how the teacher knew that. The
response: that teacher had a teacher at some point. So where did THAT
teacher learn it? The whole class was eagerly getting in on the act
and shouting out different answers by this point, but one answer was
"From scientists." So how did scientists learn it? Eventually we came
back to the idea of doing experiments and learning from them.
I think this was really useful because too many people are stuck at
the first stage of epistemology: knowledge comes from an authority,
and that's that. Of course, it's normal at this age (grades 4-6), but
I'd like to do whatever I can do to move the kids on through the next
stages. It goes to the very nature of science: is it just a set of
results, or is it the process? It's both, of course, but the process
too often gets short shrift in education. It's difficult to
teach---it can't be a unit by itself, rather it has to be built in to
every science unit, which makes the logistics very difficult---and
it's difficult to write a test question about it. But it has to be
done.
If you're interested in what thoughtful people have discovered about
the stages of epistemology, you might start with this quick summary of William G. Perry's research.
The second group, grades 1-3, would have missed out on this except
that at some point I said, "Here's what I think is going on," and one
student said. "You're the teacher, you should KNOW what's going on" or
something like that. So that was a good chance to have a similar
discussion with that group.
grades had studied electricity before, but I decided to start at a
pretty basic level with static electricity, to make sure everyone
really understood what they thought they understood. At some point a
student volunteered an answer to one of my questions, and I asked,
"How do you know that?" with the intention of highlighting how the
student's conclusion followed from the things we had just observed.
But the student said she knew it because a teacher (in a previous
year) had told her, so I asked how the teacher knew that. The
response: that teacher had a teacher at some point. So where did THAT
teacher learn it? The whole class was eagerly getting in on the act
and shouting out different answers by this point, but one answer was
"From scientists." So how did scientists learn it? Eventually we came
back to the idea of doing experiments and learning from them.
I think this was really useful because too many people are stuck at
the first stage of epistemology: knowledge comes from an authority,
and that's that. Of course, it's normal at this age (grades 4-6), but
I'd like to do whatever I can do to move the kids on through the next
stages. It goes to the very nature of science: is it just a set of
results, or is it the process? It's both, of course, but the process
too often gets short shrift in education. It's difficult to
teach---it can't be a unit by itself, rather it has to be built in to
every science unit, which makes the logistics very difficult---and
it's difficult to write a test question about it. But it has to be
done.
If you're interested in what thoughtful people have discovered about
the stages of epistemology, you might start with this quick summary of William G. Perry's research.
The second group, grades 1-3, would have missed out on this except
that at some point I said, "Here's what I think is going on," and one
student said. "You're the teacher, you should KNOW what's going on" or
something like that. So that was a good chance to have a similar
discussion with that group.
Friday, March 2, 2012
Kindergarten Energy
Today I discussed energy with the pre-K/K kids. I followed the same basic plan as I did when I discussed energy with the elementary kids (minus the last three paragraphs). I know they've studied the water cycle quite a bit, so at the end I related it to the water cycle: water in the clouds has potential energy, water in the river has energy of motion, damming the river stores the energy, letting water our of the dam turns a turbine which generates electricity, etc.
In the remaining time, instead of having the kids draw pictures of different forms of energy as I did with the elementary kids, I let them play with the lights, prisms and lenses which were so popular last week. The primaria kids loved them too. They were very disappointed when science time was up; in fact, one of them cried so much that I decided to leave all the materials at the school so that they could play with them in afterschool care as well. This is a good thing for the future of science: girls crying for more science time! Apparently this activity was a big hit in aftercare as well.
In the remaining time, instead of having the kids draw pictures of different forms of energy as I did with the elementary kids, I let them play with the lights, prisms and lenses which were so popular last week. The primaria kids loved them too. They were very disappointed when science time was up; in fact, one of them cried so much that I decided to leave all the materials at the school so that they could play with them in afterschool care as well. This is a good thing for the future of science: girls crying for more science time! Apparently this activity was a big hit in aftercare as well.
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