I'm helping my son, in grade 2, with an experiment. We
took Rice Krispies and put them on a table, placed wooden blocks on
either side of the Rice Krispies and a Plexiglass on top of the blocks
(and rice Krispies). Next, we took a wool sweater and rubbed the
Plexiglass and the Rice Krispies started jumping and touching the
Plexiglass. How do I explain what is happening to my 7 year old?
I find that a little difficult to explain simply because
the experiment includes several steps happening in succession,
all of which may be new science to a young student:
1) Rubbing dissimilar materials together causes electrons to transfer.
"The wool steals electrons from the plastic" is a good phrase.
It might actually be the other way around,
with Plexiglass stealing electrons from the wool,
but all the subsequent effects are the same!
Positive and negative charges seem to act equivalently in most cases
even though they are opposites.
2) Electrons are negative. The place they left behind is positive,
and wants to get some electrons back.
So Negative and Positive attract each other over a distance.
This gentle attraction needs to be shown in a more isolated effect.
Perhaps you could show this by hanging a 1-inch patch of aluminum
foil from a 6-inch long strip of paper or scotch tape,
and turning your Plexiglass up 90 degrees, to face the hanging
foil about an inch away.
You may be able to see the pulling action reach out an inch or
more, getting stronger when closer.
3) Electrons can move through the wood or metal table, and the grains of
Rice Krispies, because they have water in them. But they cannot move through
air or Plexiglass. So electrons flow from the table into a grain
and across the little grain to the tip nearest the Plexiglass.
There they cannot go any farther, cannot jump into the air,
so they try to drag the grain upwards with them.
If there are enough of them, the grain of cereal can be lifted up.
4) When the grains touch the Plexiglass, these electrons jump onto the
plastic. Now they are not in the grain any more, so the grain can fall back
down to the table. And perhaps get more electrons there, and do it all again!
5) Eventually all the electrons you "stole" with the wool
are given back to the plastic through the table and jumping grains,
and other slow migrations.
6) The electrons in the rubbing-wool probably go to your body,
then to the table when you touch the table.
So the jumping grains are really a round-about way for kidnapped electrons
to "go home".
Hint- it does not actually matter if the exact same electrons "go home".
What matters is that "everybody" has the right number in the end.
If he asks what an electron looks like, tell him it is a tiny ghost,
smaller than an ant,
with a far-reaching attraction to its home (an atomic nucleus, positively
They are not rare, these tiny ghosts, all objects are full of them.
Only the ones far from home are rare.
Get a bag of marbles, or perhaps a stack of poke-mon cards. (whatever your 7
year old likes, I suppose.) Give some to him, and keep some for yourself.
explain that when you rub the wool against the Plexiglas, your taking
electrons away from it. (start dropping your marbles in a bowl off to one
side, or the cards on the table out of the way to demonstrate.)
Then, explain that the Plexiglas (you) wants as many electrons
(marbles/cards/whatever) as everything else. While explaining this, eye the
collection he has suspiciously or comically, and also explain that the rice
Krispies do not want to give theirs up. A tug-of-war should ensue, as you
each pull towards each other for the marbles or cards.
This explanation is kind of basic, and more than a little flawed, but to
explain the concept of attraction it should work well enough.
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Update: June 2012