Imagine you’re putting together your dream
home.
Maybe you’re after a mansion on the rolling hills
of LA, or a mountain-top ski lodge, or a beautiful
cottage somewhere in the countryside.
Whatever it is, unless you’re willing to
give up a lot of modern conveniences, you’ll
want electrical power in your home.
It took scientists and engineers hundreds of years
of experimenting with electricity to get us to a point
where we can use it for our benefit.
Along the way, electrical engineers had to make choices
about the materials they used to stop, start, and change
the flow of electrical current as it was needed.
And although we may take it for granted,
getting power to your new house will involve
some pretty clever thinking.
[Theme Music]
Electrical power is carried in a material
by the flow of electrons – charged subatomic
particles found in all materials.
Most electrons are bound up in atoms not doing
very much, but certain materials allow electrons
to flow through them, carrying charge.
That flow is what we call an electrical current,
and it’s measured in units called Amperes.
In diagrams, you’ll see that current is shown to be
flowing from positive to negative, which might seem
kind of confusing, since it’s opposite to the direction
the electrons are flowing.
But it becomes much easier to decode if you
think about current as a flow of charge.
As electrons flow toward a positive charge,
their destination becomes more negative, and
their source becomes more positive.
Essentially, charge is flowing in the opposite
direction to the electrons.
For our purposes, though, we just need to
know how easily electrons flow in the material.
When they’re able to flow in a material with a voltage
applied to it – meaning it’s connected to something that
can generate an electric current, like a battery –
we say the material conducts an electrical current.
The degree to which a material can conduct
the flow of electricity is called its conductivity.
Then there’s resistance, which describes how much
a material resists the flow of current through it.
It’s inversely proportional to conductivity,
meaning that when one increases, the other
decreases proportionally, and vice versa.
Resistance is measured in units called ohms,
represented by the capital Greek letter omega, so we
talk about conductivity in units of inverse ohms
– that is, 1 divided by ohms.
For electrical engineers, conductivity is
often a good thing, like for the material
inside an electrical wire.
We want that to carry a flow of electrical
power to our devices.
As for the casing around that wire, if we
don’t want our electrical current escaping
into unwanted materials,
we’d better make sure that the casing’s
material doesn’t have a lot of conductivity.
In electrical engineering, we can broadly
categorize materials into three types based
on their conductivity:
Conductors, which, as you might expect, have
high conductivities.
Insulators, which have extremely low conductivities
And semiconductors, which are somewhere in
between.
For now, we’ll be concentrating on those
first two.
Metals, like silver, copper, gold, or aluminum,
are good conductors.
You’ve probably noticed that a lot of electrical
circuitry is made of metal.
Their conductivities are nice and high at
around ten million inverse ohms per meter.
On the other end of the scale, insulators
barely conduct electricity at all.
Those are materials like plastic, glass, and
rubber.
As an electrical engineer, which type of material
appeals to you usually depends on how much
conductivity you need.
So it’s going to be important to consider
that when figuring out how to get power to
your dream home.
Chances are, we’ll have to transport the
electrical power over long distances – far
from a power plant.
Of course, the electricity grid has to supply
other users too, like businesses, factories
and other homes.
It takes a lot of material to supply electricity to
an entire nation or continent, so we need to make
a sensible decision about the cables we use to
carry power throughout the grid.
Our material has to be highly conductive, to transport
as much of the required electrical current as possible,
and high in tensile strength to ensure that it lasts for
a long time hanging between transmission towers.
And, like so many materials in electrical
engineering, it needs to be ductile so we
can shape it to our needs.
The good news is that there is a material
that fits the bill!
Copper is ductile, and has a high tensile
strength, and best of all, is very conductive.
Job done, right?
Well, the bad news is that copper is too expensive
to make a power grid from.
Cost is one of those pesky engineering considerations
that’s hard to get around!
Instead, transmission lines are typically
made of a cheaper metal: aluminum.
Aluminum has a high conductivity – although
not quite as high as copper – and is lightweight,
so it’s less likely to sag over time.
But despite this, it’s not strong enough
to support the tension power cables are put
under for extended periods.
Well, no worries.
We’re not restricted to using just one material!
And steel has a very high tensile strength.
So if we take strands of conductive aluminum
and arrange them around a core of high strength
steel strands,
the cable can transmit lots of power while
the steel provides extra strength and support.
Which is exactly how power cables are designed.
But just because copper is expensive for power
grids, doesn’t mean it’s not used at all.
With all that power now being supplied to
your house, you probably have some electrical
appliances that you’d like to put in there.
Copper will have an important role in those
appliances!
In smaller quantities, copper is useful in
electronic circuitry, especially since it’s very
easy to shape, weld and solder.
The purer the copper, the better it is for
conducting electricity.
Perhaps one of the most important examples
is in printed circuit boards, or PCBs.
PCBs are boards that allow for complex
arrangements of electrical components to be
connected and arranged on small scales.
And they’re absolutely everywhere!
Any modern electronic items in this dream
home, from televisions to microwaves or even
a digital clock, will contain a PCB.
If you’re watching this on your phone or
your computer, there’s a PCB already hidden
away in the circuitry of your device.
And it’s copper, with its marvelous conducting
properties, that provide the tracks on those
boards, connecting all the tiny components inside
your devices together.
High conductivity isn’t the be all and end
all of electrical engineering though.
Stopping an electrical current from going
where it shouldn’t is just as important as
helping it flow where it should!
Consider the wiring in the walls of your dream
home.
As with the wire we considered earlier, insulators
like plastic or rubber will be vital to stop the currents
from flowing out of the circuit and getting where they
shouldn’t.
But even within that circuit itself, materials
on the lower end of being conductors are often
super important.
You might have noticed that your new place
doesn’t really have much lighting yet.
That’s where low conductivity conductors
can help us!
See, when we apply a current in a low
conductivity conductor,
the electrons, which are carrying the current,
can’t zip past the material’s atoms quite as easily
and carry all the electrical energy through.
Instead, the material’s resistance causes the
electrons to convert some of their electrical energy
to heat, and in some cases, even light.
Which means low conductivity conductors
give us a way to generate heat and light from
an electrical current.
So, resistance isn’t always futile;
sometimes it’s rather useful.
The amount of power lost to a resistor to generate
heat and light is given by the square of the current
multiplied by the total resistance of the material.
Under the circumstances they’re typically
used in, it’s also helpful for low-conductivity
conductors to have some other features,
like a high melting point and mechanical strength.
They still need to be ductile, though, so
we can shape them into a wire.
So it’s also handy if those materials are
resistant to corrosion, have a long lifetime,
and are inexpensive – ideally.
Not asking much, are we?
We do have some options, though.
Tungsten, for example, is a metal that’s
typically extracted from chemical compounds
in ores or tungstic acid.
It’s name comes from the term “Tung sten”,
meaning “Heavy stone” in Swedish.
And that’s a pretty apt name for it!
Tungsten is heavy; by density it’s so similar to gold
that counterfeiters sometimes cover a slab of tungsten
with a bit of gold to make fake solid gold bars!
But it also has the highest tensile strength and
melting point of any metal on the periodic table.
That makes it perfect for drawing into long,
thin wires – better known as filaments.
And here’s the important thing: tungsten
is quite a low conductivity conductor, with twice
the resistivity of a material like aluminum.
So it also does a great job of generating
light.
Tungsten filaments can handle extremely high
temperatures, up to two thousand degrees Celsius!
Which means a filament can give off a lot of
light before hitting a temperature where it melts.
Unfortunately, though, we can’t expose it to the
atmosphere, since oxygen reacts with the tungsten to
form tungsten oxide, which burns out the filament.
So instead, we encase it in glass, and surround the
filament with an inert gas like argon or nitrogen so it can
continue to shine when we put a current through it.
If you feel like you’ve just had a light-bulb
moment you’re completely right.
We’ve literally just described how incandescent
light bulbs work!
Admittedly, more modern light bulbs, like
LEDs, are much more efficient and better
for the environment.
But tungsten’s ability to withstand the
destructive forces of electrical contact are
still pretty useful elsewhere.
For example, tungsten can handle being
bombarded by electrons –
those electrons cause it to emit electromagnetic
radiation in the form of X-rays, which is how X-rays
are generated in hospitals!
Another low-conductivity conductor that shows
up in electrical engineering is carbon,
which is used in things like resistors, electrical
contacts, and battery cell elements.
So we have light, but can we provide some
extra heat?
A dream mountain-top ski lodge, for example,
definitely needs a way to keep you extra nice
and toasty in the winter.
Well, one low-conductivity conductor, nichrome,
is up to the task.
It’s a metal alloy that’s mostly nickel and chromium,
and because it’s on the lower end of conducting,
when we apply a voltage through it, it does a great
job of heating up.
That makes it perfect to function as the heating
element in an electric heater or furnace.
And with that, we’ve got the electrical
essentials covered for your dream home!
Of course, if you want a device like a computer
in your new place, we’ll have to take a look at the
intermediate materials on the spectrum that
ushered in the computer age: semiconductors.
But that’s a story for the next episode.
Today, we looked at the materials electrical
engineers work with.
We looked at how high-conductors help us transport
electrical power and form the basis of circuitry,
how insulators stop flow from going where
it shouldn’t,
and how low-conductivity conductors can
be used to generate light and heat.
Crash Course Engineering is produced in association
with PBS Digital Studios, which also produces Space Time.
Space Time explores the outer reaches of space,
the depths of astrophysics, the possibilities
of sci-fi, and anything else you can think
of beyond Planet Earth.
Check out Space Time and subscribe at the
link below.
Crash Course is a Complexly production and this
episode was filmed in the Doctor Cheryl C. Kinney
Studio with the help of these wonderful people.
And our amazing graphics team is Thought Cafe.
