Liquid nitrogen

Last Friday (May 25) Vera visited Primaria and brought liquid nitrogen, which is as cold as Uranus or Neptune.  Pluto might warm up close to liquid nitrogen temperatures during its summer, but in its fall it gets a lot colder and nitrogen is thought to freeze out of its atmosphere onto its surface.  Fun stuff you can do with LN2 includes: (1) freezing a banana hard enough to use it as a hammer and pound a nail into a piece of wood; (2) make a balloon completely flaccid as the air inside cools...blow on it to warm it up and it pops back into its normal state; (3) freeze a racquetball and watch it shatter when thrown on the ground; (4) make ice cream instantly (Vera didn't do that one); (5) freeze a flower and see how it shatters when frozen; (6) freeze anything the kids suggest.  Vera also related it to the infrared camera I showed the kids earlier this year.  What color would LN2 appear on an infrared camera?

This is also a great demo for elementary (and older) kids, although we haven't done it there yet.   Keep in mind, it is a demo, not a hands-on activity, although you can definitely involve the kids in thinking of what to freeze next and making predictions for what will happen,

Planets and their orbits

Today I brought the coin funnel to Primaria and we learned how orbits work.  One four-year-old came up with Kepler's 3rd Law all by himself!  With 6-8 kids in each group I organized it by having each child raise a quiet hand, tell me something they know about planets or the Sun, and then I gave that child a marble to roll in the well.  That gave me a chance to comment on what we could learn from each marble.  Depending on how it was thrown, the lesson could have been about escape velocity, about planets not falling straight into the Sun, about all the planets going around in the same direction, etc. Most of the time we pretended the Sun was at the center but if you want to make it more exciting you can pretend it's a black hole. 

Then  we shifted gears and built a scale model of the solar system: I showed a basketball representing the Sun, and I laid out various objects (a tomato, a grape, a small candy, etc) for them to choose which one they thought was the right size to represent Jupiter, the Earth, etc.   The correct answers are truly amazing: learn more at this well-written site. We didn't have time to go outside and put the correct distances between the planets, but I did describe the highlights of that aspect: Earth would be roughly in the office if we were with the Sun in the P1 room.

Aside on teaching and learning: one aspect of orbits is that all objects in a given orbit go at the same speed, regardless of their mass (technically, as long as the mass is much smaller than the mass of the thing being orbited).  To reinforce that, I dropped two marbles of very different mass and asked which one would hit the ground first.  Of course, many had the misconception that the heavy one would hit first.  I remark on it now because I had done the same demo with the same kids eight weeks ago in a slightly different context, so it's clear that they forgot.  These misconceptions are persistent!

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.