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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.