♪♪
In this segment of
Physics in Motion,
we'll explain the science
behind the wonder
of electricity...
(thunder rumbling)
Current electricity.
This is a Tesla coil,
named after a scientist
who was sometimes
called a mad inventor
but was definitely
a genius, Nikola Tesla.
He searched for a way
to harness electricity
without wires
but was outgunned by
a competing method
from a guy named
Thomas Edison.
But Tesla's work with
antennas led to technology
that is still used in
TVs and radios today.
And, yes, he's the guy
the cars were named after.
We'll see more about
how the Tesla coil works
and what we can learn
about electricity from it.
But let's take
a step back, first.
You might remember
that there are two forms
of electricity.
First, there's
static electricity,
which is the accumulation
of electric charge
on a surface or
within a material.
Static electricity
is associated with
potential energy,
which means it possesses
the ability to do work.
But it's the other
form of electricity
that defines so much
of the way we live,
and that's
current electricity.
Current electricity is
the flow of electric charge.
It's dynamic, so the charges
are always on the move.
It's what turns on our lights,
runs our computers,
and powers
the modern world.
It rarely exists
in nature,
but we do see it
in lightning strikes.
(thunder rumbling)
About 125 years ago,
we started using
electricity to power lights,
and motors,
and all the rest.
Current electricity
is a good name
because it moves
like currents of water
flowing through a channel.
To break that down,
let's go back to basics.
Atoms, we know,
make up elements that
make up all matter.
And atoms are made up
of protons,
which have
a positive charge,
neutrons, which have
no charge,
and electrons,
which whiz around the atom
in an electron cloud
and, of course,
has a negative charge.
The charges create
electric fields around them,
and, as we know,
those fields exert force
on whatever charges
are in the vicinity.
That force impels
the other charges to move,
but because
the protons are bound
within the nucleus
of the atom,
they can't really
go anywhere.
But the electrons,
they're a different story.
If electrons are
part of a material
that is a good conductor,
like metal,
they will move easily.
That's why we make
electric wires out of metal.
Now remember
electric potential?
That's the energy capacity
of the unit of charge.
When there's a point
of higher potential
and one of lower potential,
the electrons
will want to move
to that point of
lower potential,
like water will
fall down a waterfall.
Electric potential
is the same as voltage,
which is the energy capacity
of a unit of charge.
We can think of it
as electric pressure,
which is what pushes
a charge down the wire.
We measure it in
units called volts.
That's how a battery works.
One terminal has high
potential, the positive.
And one has lower
potential, the negative.
So when we connect them
by way of a wire...
Voila!
The charges start to flow,
making a current,
current electricity.
But here's something that
might come as a surprise.
The charge moves down
the wire very slowly.
It takes a few hours
to move a meter.
The electrons,
though they move at
almost the speed of light,
are not moving
in a constant direction.
They move almost randomly,
colliding with atoms
and generally behaving
like bumper cars.
So why does this
light bulb come on
as soon as I hook up
the battery?
Though the current
flows slowly,
the electric field
that drives it does not.
It goes into effect throughout
the whole length of the wire
at almost
the speed of light.
So electrons begin moving
through the entire wire
almost as soon as
the current begins.
The electrons that are closest
to the bulb light it up,
while the charges
that are further behind,
slowly make their way.
As long as
the parade of electrons
continues past
the charge point,
and the electric field
is unbroken,
the light stays lit.
Let's look at the formula
for the relationship
between the electric force
and electric field.
F sub e, electric force,
is equal to charge
times electric field.
This force is what
pushes or pulls charge,
making it flow.
In a constant electric field,
electrons will tend
to flow in the same
net direction,
despite the
pinballing around.
The overall motion
is in the direction
of the electric force.
And here's a weird thing
to remember
when we talk about
electricity.
We describe the flow
in the reverse
of the direction
it actually goes.
When Benjamin Franklin was
investigating electricity
more than 200 years ago,
he thought a positive charge
flowed through the wire
toward a negative charge.
The terminology just stuck,
and we still talk
about it that way today.
But we now know that
it's in reverse.
The electrons,
the negative charges,
are what move.
And the positive charges
are bound in place.
Now even though
electrons are flowing
in the same
basic direction,
it doesn't mean that
they aren't still hitting
atoms along the way
and creating friction
as they do so.
That is called resistance.
Resistance is the
opposition of a material
to the flow of
electric current.
And what does
friction cause?
Heat, right?
So when you feel the
warmth of a wire, or bulb,
or heating element
in your hairdryer,
you have the bumper car
effect of electrons and atoms
to thank for that
or to blame if you
get too close.
Resistance, current, and
voltage are interdependent.
When one changes, at least
one of the others do, too.
Let's look at
how that works.
I have two buckets.
We put holes in the bottom
of these buckets,
but this one has
a larger hole.
Both buckets have the same
amount of water in them,
creating the same
amount of pressure
on the hole
in the bottom.
Let's imagine these buckets
are carrying charges
instead of water.
The level of water
represents the voltage,
the pressure created
by the charge in the system,
represented by V.
The higher the level of water,
the greater the voltage.
The diameter of the hole
in the bottom of the bucket
represents electrical
resistance,
represented by R.
Now when I release the water
from the buckets,
let's see what happens.
The water in the bucket
with the smaller hole
drains more slowly.
It has the
higher resistance.
The other bucket
has a larger hole,
so the water flows
more easily.
The resistance is what?
Right. Lower.
And the flow of water
represents current,
which we show as I.
It's a measure of the
flow rate of electric charge.
The higher the resistance
or smaller the hole,
the lower the current flow,
or water flowing out.
So let's recap.
Voltage is the push
or pressure,
and resistance is
the resistance to flow.
And current is the rate
at which charges flow.
Current is what results
from voltage and resistance.
There's a formula
showing that resistance
is inversely proportional
to current
when voltage
is kept constant.
Resistance equals
voltage divided by current.
You might also have
seen the same equation
arranged like this.
V equals IR,
or voltage equals
current times resistance.
This relationship
is known as Ohm's law,
after the German
physicist, Georg Ohm,
who figured out
that in certain materials,
voltage is directly
proportional to current.
Current, voltage, and
resistance are interconnected,
So when one changes,
at least one of
the others change, too.
Understanding
that relationship
led to our being able
to harness electricity.
But it didn't
happen overnight.
Remember this?
In Tesla's coil,
which Tesla invented
in 1891,
he used very high voltage
so that a spark
flies between the gap
and the two coils.
Tesla hoped to be able
to transmit electricity
directly through the air
over very long distances.
But you can see it was
a little out there.
Tesla made a lot
of huge advances
in our understanding
of how electricity works,
inventing alternating
current,
allowing us transmit
electricity long distances,
and pioneering research
with wireless electricity.
We still use Tesla's
technology today.
For instance, when we charge
our cell phones wirelessly.
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.
