Plasma Ball
Electricity
Materials:
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Plasma Ball
Fluorescent light bulb
Neon light tube
Rubber mat or insulating stool
Key Points:
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Similar concept to Van de Graaff generator.
A transformer collects (negative) charges on the small center sphere.
Like charges repel. Opposite charges attract.
The charges in the center want to get away from each other. They jump off the
sphere to get farther away from the other charges.
The gas inside the Plasma Ball glows where the electricity passes through the air.
The charges are trying to go where there is no built up charge.
The sphere builds up charge really fast
because it’s small. The earth is REALLY
big so you can pour charges into it
forever and they’ll never build up.
Charges always take the shortest,
easiest path.
If your friend lives next door, you
wouldn’t walk around the block just to
go to their house. It would be easier to
just walk straight there.
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Now if someone puts up a really tall fence between your two houses, then
climbing over it might be a SHORTER path, but it’s probably EASIER to just
walk around it. This is important: "shortest path" doesn’t always mean distance.
"Easier path" is more accurate.
If the charges are given an easy enough path to the earth, they will ALL go that
way (instead of spreading out all over the Plasma Ball and traveling through the
air). When you touch your hand to the surface, the electricity flows from the
Plasma Ball through your body, down to the ground (earth).
Warnings:
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Keep anyone with a heart condition or pacemaker away from all our electricity
demos.
Touching something metal (like the edge of the table) while touching the plasma
ball can shock you. It’s not dangerous, but it can be startling.
Leaving your hand on the sphere in one place for a long time generates heat.
When chaining people together, it can get hot pretty fast for the person holding
the Plasma Ball.
Things to talk about:
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Discuss how the Plasma Ball is like the Van de Graaff generator (charges
collecting on inner sphere)
Like charges repel. Opposite charges attract. This is at the heart of all our
electricity demos.
Explain that the colored arcs are caused by the charges moving through the
gas (this works the same way as lightning in the air).
Tell them that charges want to get to ground (the earth). See above for a good
explanation.
Talk about "easiest path" by choosing a good example (or use the one we
included). Try not to get bogged down with a story that doesn’t illustrate your
point simply.
After talking about these things, put your hand on the Plasma Ball as an example.
This and the next several points are just illustrating these same concepts in
different ways.
Explain that electricity flows through people pretty easily (compared to air,
plastic, wood, etc). The amount of power from the Plasma Ball is very small so
it’s safe. But electric outlets are much more powerful, so that’s why they’re
dangerous.
You can show that electricity is really flowing through you by holding the end of
the neon tube or fluorescent light and touching the other end to the Plasma Ball.
Use your other hand to trace the path of the charges. Explain that the charges
start in the Plasma Ball, go into your arm, then down your body, to the
floor. (Note that putting the bulb near the surface will light it up. This is because
of the changing electric fields due to moving charges. It’s probably best not to
worry about explaining this one.)
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Try putting your hand in the middle of the bulb. Only half of the bulb lights
because the easiest path for the charge is still to go out through your hand (then
down your arm, etc..)
To show just how good conductors people are, try
"daisy chaining" several van members together:
Have the first person touch the Plasma Ball and
hold hands with the rest of the chain. The last two
people should hold the light bulb between them to
show that the electricity is still flowing. When the
first person is touching the sphere, the bulb will
light up at the other end of the chain! Make a big
deal about how the electricity is flowing from the
Plasma Ball, all the way through the chain,
through the light bulb, and down the last person
to the floor.
It’s important to insulate the whole chain from the
ground except for the last person, or else the charge will flow out before it gets to
the light.
The light will be pretty dim after all that resistance, so the room needs to be
pretty dark.
To get the light started, you might need the last person to touch it near the middle
and then draw their hand back. This lowers the starting resistance
Click here for the Adobe PDF version of this demo (text only), for printing purposes.