[SOUND] [SOUND] Energy is the rearrangement of chemical or nuclear bonds into a more stable state. What I did right here with this flash cotton is take a small spark, and take a substance that's extremely unstable. It's actually cotton soaked in nitroglycerin. It's so unstable that the tiniest bit of spark instantaneously combusts it, burns it into carbon dioxide and a tiny bit of water paper. There's no ash left, and no smoke in the room. So let's start with a slightly simpler chemical reaction. This is the one you'll see when you see rockets blast off into space. They actually take hydrogen, hydrogen gas, and oxygen, and they turn this into water vapor. We have to make this a balanced chemical reaction. So we actually need two molecules of this, and we end up with two waters. This is a real classic example of taking chemical bonds and making them into something more stable. If I take just a hydrogen, and I take two of these, so I've got the H2 molecule, and I've got the O2 molecule, it's a double bond, and we have another hydrogen molecule here as well. If I disassemble these, and that's where I have to put that little bit of spark, I have to break up these molecules. But this collection of 4 hydrogens and 2 oxygens will end up, after that spark, after that combination, into a water molecule, which is bound like this, and another water molecule. We still have 4 hydrogens and 2 oxygens, but this level, the water, is much more stable. I can't put water in a tiny little flash device, and spark it, and have it turn into something else. But I certainly can take hydrogen and oxygen gas, ignite a fuse, and [SOUND] rearrangement of chemical bonds into a more stable state. All of energy can really be related to that simple definition. Those are the chemical bonds. What about nuclear bonds? I didn't say bombs, although that is an example of having nuclear bonds be recombined. So let's think about that hydrogen. When I wrote it as chemical, I wrote it like this, or you had two hydrogen atoms. But remember, in an individual atom, it has a proton, and it's got some electrons going around it. They don't really go in circles. There really is a probability distribution around there. But generally, we've got electrons orbiting a nucleus. And this is hydrogen, one proton, one electron. But hydrogen has a variety of isotopes. These are things that look and chemically act just like hydrogen because they have the same number of electrons and protons, but they're a little heavier. Instead of water, you could have heavy water. That's deuterium, and it's nucleus, instead of one proton, is a proton and a neutron. There's another isotope of hydrogen as well, called tritium, that has one proton, still hydrogen, and two neutrons. And it turns out that this reaction is the easiest one to actually fuse, to make a nuclear reaction from. So if I call this D, for deuterium, and T, for tritium, I can combine this. And here, I need a lot more than that little spark for my flash powder. But I could combine this, and I could make helium, the normal isotope of it, helium 4, and an extra neutron. So this bonding that's inside the nucleus rearranges into helium, which is 2 protons and 2 neutrons, plus an extra free neutron that goes zipping off. This reaction, just like with the chemistry with the water, rearranges the nuclear bonds into a more stable state. The helium is tremendously more stable than the deuterium or tritium nuclei. So just like with the hydrogen and oxygen, you get a large flash. This type of reaction, the fusion reaction, gives you a really large flash. It's called the Sun. The Sun is not doing, exactly, deuterium to tritium. But it is, in the end, making helium out of hydrogen, because the Sun operates on nuclear fusion, rearranging bonds into more stable states, into helium. This means that all of the energy sources that use the Sun, in essence, are still the rearrangements of those nuclear bonds. So the Sun is a nuclear furnace, and it rearranges nuclear bonds into more stable states, into helium. And that means all of our energy that derives from the Sun actually derives, in essence, from rearrangements of nuclear bonds, or when we burn the ex products of the sun, the fossil fuels, we're rearranging chemical bonds. That's really all you need to know about the definition of energy. [MUSIC]