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.