Energy

The elementary school has now moved into its new site, and Friday I
taught in a bona fide science room for the first time!  It was good to
see the kids in grades 1-3, whom I hadn't seen in a while, and Teacher
Cara asked to try a new way of splitting into groups: instead of three
groups of seven, I would be with the seven in grades 4-6 for half an
hour, then the 14 in grades 1-3 for half an hour.  This gave me more
time with each group, which I believe was very useful as described
below.  And I was pleasantly surprised at the manageability of this
large group; the more formal school setting seems to have drawn out more
serious behavior.

The students are starting a big unit on energy.  As planned by Lorie,
the emphasis is on ecology: how all living things get their energy
ultimately from the Sun, how it is passed from plants to plant-eaters
to carnivores, and how we can take advantage of the Sun's energy more
directly by building things like solar ovens.  I plan to come in and
do the more physics-y aspects.

So Friday I started with an overview of different forms of energy.  I
elicited kids' ideas about energy and the forms it comes in, so we
covered these concepts as they came up, in a different order in each
group:

--energy of motion is called kinetic energy

--the energy in food is called chemical energy and is no different
  from the chemical energy in gasoline ("how did you get to school
  today?") or coal/gas-burning power plants ("where does the
  electricity in the wall come from?").  I burned a few chemicals as a
  demonstration: first a potato chip, which burns quite vigorously,
  revealing it has a lot of chemical energy (a chance to slip in a few
  words of nutrition advice); a piece of whole-wheat spaghetti, which
  burns much less vigorously (it may not have that much less energy,
  but it is released more slowly, which is good for their bodies); a
  candle; and alcohol.  For the candle I made an analogy to the human
  body: your body is slowly burning the food, so you don't get very
  hot but your body is warm to the touch.  The alcohol was a hit; it
  burns so cleanly that you can't see any smoke or flame, but you can
  really feel the heat.  (Advice to food-burners: it's much harder
  than it looks.  You should practice everything at home.  Few foods
  really ignite, even some, like sugar, which you would think would be
  easy.  I can't get alcohol to ignite with a lighter, so I bring a
  torch which impresses the kids.  Thus the alcohol should be in a
  bowl rather than a glass, so the torch can reach it.  Etc.  You
  really have to practice!)

  The flame gives off two other kinds of energy:

--heat.  Our bodies turn chemical energy into energy of motion and
  heat.  What are some ways you can turn heat energy into other forms
  of energy? (A hot air balloon, perhaps.)

--light. These kids were very surprisingly familiar with the concept
  that light is a form of energy; it turned out that Lorie had already
  discussed that with them.  But even so, I had a nice demo that light
  can push on things and start them in motion, using a very simple
  device called a Crooke's radiometer.  Shining light on a Crooke's
  radiometer causes it to spin, which demonstrates the conversion of
  light energy into energy of motion.  (Maven alert: this conversion
  is not really in one step, but I glossed over that for the sake of a
  good demo.)  The solar oven they will build will demonstrate the
  conversion of light to heat.

--potential energy.  A ball on the edge of a desk has the potential to
  fall and gain kinetic energy, so it has potential energy from
  gravity.  Another example is a stretched rubber band: it has the
  potential, if I let go, to go flying off somewhere.

--electricity.  Plugging the light into the wall to spin the
  radiometer was a clear demonstration of the conversion of
  electricity into light, which suggests that electricity is a form of
  energy.  I asked them where it comes from, which led to interesting
  discussions.  If they suggest solar panels, that's a chance to
  highlight the conversion of light to another form of energy (and a
  chance to disabuse them of the notion that most of our power is
  clean).  Fuel-burning power plants convert chemical energy into
  electricity.  Hydro plants convert gravitational potential energy
  into electricity.

I led them to many different examples of one form of energy converting
to another.  For example, a red-hot electric stove burner converts
some of its heat energy into light.  I swung a pendulum from my
finger.  A pendulum converts back and forth between potential and
kinetic but eventually loses them both.  Where did the energy go?
Into heat, by stirring up the air in the room ever so slightly and by
rubbing my finger.  How do you feel when exercising vigorously?  Hot!
So your body also "loses" energy by converting some to heat which
escapes.  It turns out that you never really lose energy when doing
these conversions, but heat energy is only useful if it's
concentrated, like right around a flame.  Once it's spread out, it's
still there but difficult to capture and make use of.  That's why
people say we "use up" energy when, in a strict physics sense, we
don't.

With all these different kinds of energy, I wanted to come back to a
unifying theme.  (The unifying themes of science tend to get lost when the
details are lost, which is a shame because the unity of scientific knowledge is
a beautiful thing.) The fact that we can do all these conversions means that
all these things are, down deep, just different manifestations of the
same thing: energy.  A good analogy is money: we can convert a dollar
bill into four quarters, or ten dimes, or two quarters and five dimes,
etc, and these things all look different but are really the same
thing.  (For adults we could also mention stocks, bonds, derivatives,
credit default swaps, etc.)

In the last ten minutes, the ten minutes I never had with the old
schedule, I had the kids use their creativity to think of and draw as
many energy-converting devices as they could think of.  The more and
the more unexpected, the better, just as in a Rube Goldberg machine.
This turned out very well; I had a chance to circulate and consult
with each child one-on-one, and they had a chance to put the new
concepts into practice using familiar skills and choosing their own
focus.  Here are some results:

The drawing above incorporates elements of the classroom (Vaca the rabbit): this is basically a rabbit-petting machine.

This one (above) was continued on the back, with the electricity from the hydro plant going to school and people using it.

Note the creative elements above: the potential energy of the tub of water will be released when its wooden support burns, and the pinwheels at left are like windmills, but are turned by water flowing downhill.  A large amount of potential energy is stored when constructing the machine, by piling dirt on a removable cover over a pit.

I like this one because the student considered practical matters.  There is a ball return device so the machine can keep going without manual reloading.  (I helped the younger students put labels on parts of their drawings.)