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When you hear
"static electricity,"
you're probably thinking of
sliding across a rug
in your socks
and getting a shock
when you touch metal.
Or clothes that stick
together in a dryer.
Static electricity
is the accumulation
of electric charge
on the surface of,
or within, a material.
The shock happens when
the extra electrons move
and are released or
discharged from an object,
say, from your finger
to the light switch.
The charge doesn't
have to be moving.
It's still there even when
it's resting on an object.
One example of
electrostatic force
that you might not
expect is car painting.
When these robots
paint cars,
they use an
electrostatic sprayer,
which makes the paint
go on very smoothly
and evenly.
How does static electricity
help make that happen?
The special paint sprayer
charges the paint particles
with extra electrons,
giving them
a negative charge.
The surface of the car
is positively charged,
so when the negatively
charged paint droplets
are pushed
through the sprayer,
they are attracted
to the car's
positively charged surface.
As long as the nozzle
is close to the car,
and there isn't
anything else around it
that would
attract the paint,
it works really well.
Less wasted paint
and drips.
Workers wear
lint-free clothing
so they don't
attract the paint
instead of the car.
So let's look at this whole
electrostatic process
more thoroughly.
Charge is a basic
property of matter,
like mass.
Just like all matter
weighs something,
so all matter
has a charge,
which can be
positive, negative,
or neutral.
At the atomic level,
protons have
a positive charge,
and electrons have
a negative one.
The charges on
a proton and electron
are equal in magnitude,
and opposite in sign.
They balance
one another out,
both having a charge
of 1.6 times 10
to the negative
nineteenth coulombs.
When an object's charge
is all balanced out,
that object is
said to be neutral.
The Earth is an example
of a neutral object.
Every charge creates
a field around it,
which we call
an electric field.
Other charges in the field
will feel a pull
if the charges
are opposite,
and a push if
the charges are the same.
Now, there are some rules
about how the charges move.
Metal,
like in cars,
is one of the better
conductors of electricity.
A conductor is
a material electrons
can easily
travel across.
An insulator,
like a car's tires,
is a material that resists
the movement of charge.
Every material lets
charge move to some extent.
We just call them
conductors or insulators
depending on where on
the spectrum they fall.
Even if objects
start out neutral,
there are several ways we
can give them a negative
or positive charge.
Let's try some experiments
to show you how it's done.
I have an uncharged balloon
hanging by an insulating thread,
a glass rod,
and some fur.
Let's look at charging
by friction first.
Let me prove to you this
glass rod is uncharged.
I'll bring it close to
the balloon without touching,
and see, the balloon
is neither attracted
or repelled.
So right now, the rod
has no net charge.
Now, if I rub the fur
on the glass rod,
and then bring the rod
near the balloon...
See what happens?
The balloon is pulled
toward the rod,
so when I hold the rod
close to the balloon...
Ooh, that was a good one.
The balloon is polarized
and attracted to the rod.
Proof that we really
have given the rod
a net charge
just using friction.
There is a list
of materials called
the Triboelectric Series
that ranks materials
on how easily they give up
or receive electrons.
Here's where fur is
on the Triboelectric List,
and here's glass.
Since glass is higher
up on the list,
that means when
fur and glass are
rubbed together,
the glass becomes
positively charged,
and the fur gains
a net negative charge.
So friction is one
way that objects
can accumulate an
electrostatic charge.
A second way to accumulate
charge is called conduction,
where we transfer charge
by direct contact.
This is an electroscope,
a device that shows
how charge accumulates.
It has a metal
sphere on the top
connected to two
metal foil leaves
at the bottom
by a metal rod.
The leaves will either
repel each other,
or attract,
depending on the charge
they are given.
Right now,
it's neutral.
So the leaves
are straight down,
not attracting or
repelling each other.
But when I rub this glass rod
with a piece of fur,
I give it a positive charge,
and the electrons move
from the rod to the fur.
When I touch the rod to
the top of the metal sphere
on the top of
the electroscope,
electrons move from
the electroscope to the rod,
making the foil
leaves at the bottom
both positively charged,
and so they
repel each other.
When the rod makes direct
contact with the sphere,
the sphere gains
positive charge.
So that's conduction.
There's one more way that
objects can acquire a charge
called induction.
Induction is when a neutral
object is charged
by bringing a charged
object close to
but not touching
the object.
I'm gonna rub the rod
with the fur again,
giving the glass
a positive charge,
and the fur will
gain electrons,
giving it a negative charge.
When I hold the rod up to
the electroscope this time,
just bringing it
close enough,
but not touching,
it causes the positive
charges in the electroscope
to be repelled
and to accumulate
in the foil leaves.
Now, I'm gonna ground
the electroscope
by touching it
with my hand
while the rod
is still close.
This stops the leaves from
repelling each other.
Why do you think that is?
It's because my hand
gave the electrons
that are still residing
in the metal sphere
a path away from
the electroscope.
The electroscope now
has a positive charge.
When I remove the rod,
the electroscope retains
its positive charge,
without the charging
object ever touching it.
We humans have figured out
how to use induction as well
to help improve
our environment.
What looks like smoke
coming from a smoke stack
is actually an aerosol,
which are droplets
suspended in the air.
This is serious pollution,
but many industrial
plants today
have gigantic electrostatic
machines called precipitators
that first charge the dirt
particles negatively,
and then capture them
on a positively
charged plate
where they can't
escape into the air.
That's induction.
Precipitators can capture
up to a large percentage
of the particles from
the burning material,
though some
pollution does escape.
We use electrostatic
charges for other things
in our daily lives,
like photocopiers.
Here's how that works.
The copier drum
is positively charged
through the use of
a light source and lens
and images formed
on the charged surface.
The surface containing
the image is covered with
negatively charged
toner powder
which sticks only
to the imaged area.
A piece of paper,
which has been
given a charge,
rolls over this area.
Toner then sticks
to the paper,
which is then heated
to make it stay in place.
No matter how
the objects are charged,
whether by friction,
conduction, or induction,
like other forces,
the electrostatic force
is conservative.
That means, like matter,
it cannot be created
or destroyed.
That seems to be a common
theme in the universe.
In the case
of electricity,
it's called the law of
conservation of charge,
where electric charge cannot
be created or destroyed,
but can be transferred from
one object to another.
That means if I rub
the rod with fur,
and the rod gains positive
two microcoulombs of charge,
the fur must now have
a negative two
microcoulomb of charge.
The charge the rod gains cannot
be created out of nothing,
and the charge the rod has
lost can't just disappear.
It is transferred
to the fur.
Second, charge only
comes in certain amounts,
which we call quantized.
That means something can
only be charged in multiples
of the same value.
In this case, the amount
of charge we find
on a single electron
is 1.6 times 10
to the negative
nineteenth coulombs.
So let's recap what
we've learned about
static electricity.
It's one of two forms
of electricity...
And the Law of Conservation
of Charge says that...
For a more in-depth look
at static electricity,
check out our Closer
Looks on friction,
conduction, and induction.
That's it for this segment
of "Physics in Motion."
We'll see you next time.
(announcer)
For more practice problems,
lab activities,
and note-taking guides,
check out the
"Physics in Motion" toolkit.
