A
Van de Graaff electrostatic generator and a batch of fearless sixth grade students is
all it takes to have some hair raising experiments.
They each stood on an insulated
platform and put their hand on the high
voltage terminal so that they could be charged to as much as 250,000
volts. The current is tiny so it is safe. When they asked, "Will I be shocked?"
My reply was usually that they may shocked in the sense they would be surprised but wouldn't feel an
electric shock with this experiment unless they wanted to.
As the charge accumulated each hair
was repelled from every other just
like the foils in an electroscope like this one that Ben Franklin would
have recognized.
Here the electroscope is in operation.
A negatively charged balloon causes the foils to deflect as as electrons are repelled from the top bead even without contact.
In the last part of the video the electroscope has been charged so the
foils are separated and come together when a finger touches the bead
giving them a path to ground.
Here are some of the students illustrating the same effect We gave each of them a mirror so they
could see what they looked like
when they were charged. The only sensation they reported was
the slightly prickly sensation as
the hairs on arms and hands were repelled and moved. If they and a friend wanted to feel a
little electricity we would turn
off the generator,
wait a moment for some of the charge to bleed off, and then have them
touch
their friend's hand. They would each feel a small shock as
a spark jumped between them. They wondered what my impersonation
of an electroscope would look like. We put a stack of aluminum foil pans
on the terminal and they were
repelled and tossed into the air as the charge built up. Note the expressions.
This is a video of what that
experiment was like. We also had a set of Franklin's Bells. Here the bell closest to the Van de
Graaff is charged without any
contact with the terminal. Corona discharge from the sharp edges
of the aluminum foil to the
terminal is all it takes. The bell attracts the blue bob which
swings toward it. As soon as it makes contact the bell
and bob have the same charge so
the bob is repelled and swings toward the other bell. When it touches it shares its charge
with that bell and now both of
them have the same charge so the bob is repelled from there. The bob having lost much of it's
charge it can be attracted to the
first bell again and the process repeats. Even after the Van de Graaff is
turned off the process continues. Its terminal is still at high voltage
until the charge leaks away. Here is another demonstration of
corona discharge. The high voltage and sharp points
cause a discharge that ionizes the
air nearby. The reaction as that air is repelled
pushes back on the points causing
the pinwheel to spin. The kids had a chance to see what
electricity feels like too. They ask "Will it hurt?" My usual reply is "You won't be
damaged ... but it will feel like you
are being damaged!" Here the expressions are mostly those
of anticipation though the last
one is clearly after the discharge. Here is a video of some of them
taking on the Van de Graaff mano-a-mano
in a manner of speaking. I like the variety of reactions
especially the ones who seem to be
communing with it and others who are truly shocked by the
experience. Yes, balloons charged with static
electricity will stick to objects
that have an opposite charge.
Another device you may consider
unlikely. It is a motor made from some pop
bottles and aluminum foil and powered
by electricity traveling through the air. Construction is simple, bottle (1) on
one end is charged by corona
discharge from a piece of aluminum foil attached to it and pointed
toward the Van de Graaff. The bottle (3) on the other end
discharges current that reaches it
through a similar bit of foil pointed away. Between them is another bottle (2)
that is free to turn and has
vertical stripes of foil on it. The stripes are charged by corona
from yet another foil electrode
attached to bottle 1. The stripe takes on the same charge
and so is repelled and moves away,
another takes its place, and the process repeats. Charge is removed from the stripes at
the other end by another piece of
foil attached to bottle 3. To be clear, none of the aluminum
foil electrodes touch anything other
than the air and yet there is enough power for the motor to spin nicely. Here the kids were proving that
static (which means not moving)
electricity moves very well indeed. They were standing in a line, linked
by small florescent bulbs. The one closest to the Van de Graaff
was holding a metal bowl that
served as the connection to it. When the Van de Graaff was started
and the bowl was in contact with the
terminal the electricity flowed through the chain and didn't have the
chance to build up very much. The bulbs lit all the way down the
line and they felt the current but
it was bearable. The student holding the bowl was in
charge (pun intended) of the
situation though. If he moved it away from the terminal
charge built up until it was
large enough to jump the gap. At that point the accumulated charge
passed through the line all at
once, the bulbs all flashed and everyone felt more of a shock. It takes approximately 25,000 volts
to jump 1 inch (2.54 centimeters) so
you can estimate the voltage they are feeling. We turned off the lights for part of
this so you could see the bulbs lighting.
They proved their bravery, had fun,
and learned some science and
history.