♪♪
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.
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lab activities, and note-taking guides,
check out the Physics in Motion toolkit.