Make Electricity Go Round and Round - The Thermoelectric Effect

- When I was a child, I used to connect two ends of a wire to a battery so that current would flow through it, and then very quickly join the ends of the wire. My hope was that if he could do it fast enough, he would catch the current and be going around and around that loop of wire for years. I don't think it's possible, but it is really possible to

make

electricity

flow around and around in a loop of wire. All you have to do is heat one end and cool the other like this, and look with my multimeter, you can see that there is actually a small current flowing over and over in this loop of wire.

I say all you have to do is

make

one end hot and one end cold. There's actually something a little unusual about the cables: they're not made of copper like cables normally are. Instead, they are made of two different alloys. One is called Chromel, the other is called Alumel and the alloys in general are nothing special. And alloys are simply a mixture of different metals, in this case mainly nickel, but with a few different ingredients to differentiate them. And surprisingly, when you have two different wires made of two different metals joined together in this way, and you heat one of the joints, while you cool the other, the

electricity

will spin, spin, spin.

It's an incredible

effect

that is incredibly useful, especially considering that it is reversible. So in this case, we are using a temperature difference to create a voltage, but the opposite is also true. If it presents a voltage, it creates a temperature difference. These two phenomena together are known as the

thermoelectric

effect

. And I'll talk about both in this video, but specifically in this case, where a temperature difference creates a voltage that's called the Seebeck effect. What causes the Seebeck effect? Well, let's look inside this little piece of wire. You may know that metals have these freely moving electrons.

Therefore, normally electrons are tightly bound to their atoms, but in the case of metals, some of the electrons are free to move within the mass of the metal. That's what makes metal conductive when electricity moves through the metal, it's because those free electrons flow through it. These freely moving electrons also have some thermal energy, unless the metal is at absolute zero, which is rare. I'm showing that thermal energy as a movement of electrons, because that's all thermal energy is, a movement on an atomic scale. These freely moving electrons are sometimes called an electron sea. But thinking of electrons as a gas is actually a more useful analogy for our purposes.

And you probably know this: when you heat a gas, it expands. In other words, the gas particles move away from each other. And that's because as you heat the gas, they get more thermal energy, the momentum increases, and they move faster. They bump into each other more often and have more energy. So they are separating from each other. And the same thing happens with the gas in quotes of the electrons of our metal. As you heat one end of the rod, those electrons will separate from each other. And as you cool, the other end will contract.

They have less thermal energy. They collide with each other with less energy, less frequently and can get closer to each other. So the overall effect now is a deficit of electrons at the hot end and a surplus of electrons at the cold end. So what it looks like from the outside is a slight positive charge on one end and a slight negative charge on the other end. In a nutshell, that's the Seebeck effect, but more importantly, the Seebeck effect is more pronounced in some metals and less pronounced in others. Let's imagine then that we take a different metal, heat one end and cool the other in exactly the same way. but that charge separation is less pronounced, because the Seebeck effects are not as strong, because the metal's ion lattice has a stronger hold on those electrons.

So further charge separation is not allowed, but in the case, what happens if you take these two different metal cables and join the two ends together? Well, the Seebeck effect is strong in the top wire of our diagram here. Thus, the electrons are pushed strongly to the right, and in the lower wire the Seebeck effect is less strong. So those electrons are being pushed weakly to the right. And they're actually pushing against each other. So the top wire pushes clockwise and the bottom wire pushes counterclockwise. But because the Seebeck effect is stronger on the top wire, the top wire wins, or to put it in less anthropomorphic terms, the net effect of the top wire pushing clockwise and the bottom wire pushing clockwise.

Counterclockwise is a net clockwise push of the top wire. electrons, so you get a clockwise flow of electrons. When you have two different metals joined together, like this, it's called a thermocouple, and a thermocouple is the working principle behind many thermometers, especially those probe thermometers that you put in the food you're cooking, or the one I used. when I was calculating absolute zero, that time. If you look inside the tip of one of those sonar monitors, you'll find two different metals joined together. The voltage you get from the thermocouple is really small. If you want a decent voltage, you need to put a load of thermocouples in series.

And that's really what's happening in here. So here's a bunch of wires going up and down, up and down, linked in series. So all the load on the joints is hot. A large number of crossovers are much cooler. This is called a thermopile and the voltage you can get from it is much higher, as you can see here. A thermopile is an important component of your boiler. You could call it a furnace or a water heater. So your boiler has something called a pilot light. In reality, modern boilers don't always have one, but one of the functions of a pilot light is to ensure that unburned gas does not leak into the house.

That's why the pilot light is always on. It's right next to the burner. So if any gas comes out of the burner, it will ignite. In this way, the pilot light prevents unburned gases from escaping. But what happens if the pilot light goes out? You need some way to detect it and then turn off the gas supply. And that's where the thermopile comes into play. If you look just above the pilot light on your boiler, you will see a thermopile. And that generates enough voltage to power a solenoid that can keep the gas valve open. So if the pilot light goes out, the thermopile cools, the voltage drops, the solenoid turns off, and the gas valve closes.

Instead you could use a single thermocouple that produces a small voltage and use that as the switching mechanism. Then you have a separate power supply for the solenoid and the thermocouple is only used as a switch. The problem then is that every time there is a power outage, the gas valve closes and heat is also lost. In contrast, a thermopile system is completely autonomous. In fact, you've experienced the effect of a thermopile on your boiler if you've ever had to turn it back on after the pilot light went out. And to relight a boiler with a pilot light out, you must press the dial that overrides the solenoid;

When you release it, you give control back to the solenoid. So if you don't leave the dial pressed long enough after turning the boiler back on, the thermopile won't have enough time to heat up. When you release it, the solenoid closes again and you have to try again. I mentioned that opposites of the Seebeck effect also exist. In other words, if you apply voltage to this circuit, one junction will get hot and the other will get cold. That's called the Peltier effect. It's analogous, by the way, to the Sterling engine in my Entropy video, you can use temperature difference to spin a thing, or you can spin a thing to generate a temperature difference.

Anyway, I'll show you an application of the Peltier effect in a minute, but first what causes it? To see why the Peltier effect exists, we must look at metal in a slightly different way than we did when explaining the Seebeck effect. Let's think about the energy levels of electrons. So let's imagine that we can extract all the electrons from a metal and then pour them back into it. They would begin to occupy the lowest energy levels they can, that is, I, the ones closest to the nuclei of the metal ions, and the next atoms that enter will go to the next level above that, and the next level above, and to the next higher level.

You think of them as the orbitals of atoms, but in a metal you reach the point where they are no longer orbitals, but energy bands where all energy levels are shared between atoms. But in any case it is analogous to pouring balls into a beaker. God, it's been a long time since I've poured anything from a glass. At the moment, this analogy is missing something. It lacks thermal energy. At the moment it seems to be at absolute zero. So to add thermal energy, we need to move something there. And it's only the top electrons that can move.

In other words, they are able to jump up and down through higher energy levels, and that is what I have tried to illustrate here. I want to make it clear that when you fill a METAL with electrons, it's not like they are sloshing around at the bottom of the metal with no electrons on top. It's that they're filling the energy levels close to the atoms, and then increasingly outward until you see these energy levels occupying the entire mass of the metal. It's just that we are plotting those energy levels on the vertical axis. It is important to note that different metals have different numbers of electrons and different core strengths. and thus they will fill two different levels.

So here's a different metal, and you can see those free electrons, jumping up and down, are higher overall. We can then join these two metals together to form a joint. And actually, there would be some movement of electrons between the metals, but I won't show it because it confuses what I'm trying to explain. So now what we're going to do to show the Peltier effect is apply a voltage and you can think of the voltage as just pushing the electrons around. In other words, we are going to push the electrons. In this case, we will push them from the left and see what happens.

Well, it's a bit like balls bouncing on two different shelves, on different levels. We will push the balls that bounce on the bottom shelf until they reach the top shelf. Well, let's see what happens to the bouncing balls when I throw them on the ground, they have about 60 centimeters of bounce here. But look at the ones that end up on the table, they only have about 10 centimeters of bounce, if that. And that is because there is a constant interaction between kinetic energy and gravitational potential energy. In other words, when a ball bounces it is constantly converting the energy it has between kinetic energy and gravitational potential energy.

At the bottom of the bounce, it has a lot of kinetic energy, not gravitational potential energy. At the peak of the bounce, all that kinetic energy is converted to gravitational potential energy. And if at that moment you move it to the shelf, well, it will just sit there on the shelf, because it has no kinetic energy. And the same thing happens when you push electrons up that high shelf, in an inverted trade. It is not an interaction between kinetic energy and gravitational potential energy that we would like, as with bouncing balls, it is an interaction between kinetic energy and electrical potential energy.

In other words, the potential energy it has because it is a negatively charged particle near a positively charged nucleus from which it moves away as it rises through the levels. The important thing is that those bouncing electrons that reach the shelf no longer bounce as much. And that bounce up and down, that shake, that kinetic energy is on the atomic scale. So it's thermal energy. In other words, the electrons that reach the shelf have less thermal energy and cool the metal. So when you apply a voltage across the junction in such a way that it pushes electrons from the bottom shelf to the high shelf, you cool the junction.

The opposite happens when you push the electrons off the shelf, they gain kinetic energy because they have to fall further. And that kinetic energy gain is equivalent to a thermal energy gain, so the junction heats up. If you use the Peltier effect in a device, you will have a Peltier element like this one here. If I connect these two cables to a nine volt battery, one side gets very hot. On the other side it is very cold. I pulled this one apart so you can see what's going on inside. It's really hard to make sense of it because, you know, some parts were stuck to the floor, some parts were stuck to the ceiling, but hopefully you can see that there are a lot of thermocouples, all in series with each other.

And actually different semiconductors are used here, as opposed to different metals. With a heat sink and a water bathyou can even use a Peltier element to freeze the water. ElectroBOOM made a video on Peltier elements a while ago that is worth watching. And I know Alpha Phoenix is ​​using Peltier elements in an upcoming video trying to grow huge water monocrystals. So subscribe to Alpha Phoenix early so you don't miss out. It's a great channel anyway, so go and check it out. I'll leave a link to your channel in the description and on the final screen. So I thought I'd share some more Blinkist recommendations for you.

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