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Today, we'll explore the materials electrical engineers work with. We'll look at high-conductors, insulators, and how low-conductivity conductors can be used to generate light and heat.

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CC Kids:
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.