 Now we've talked about temperature. Temperature is a measure of how fast the atoms and molecules inside something are jiggling about, moving at random. Now I would like to talk about heat. This is a bit of a tricky thing to talk about because in normal English we use heat in many different ways. You can say someone is hot, you can heat something up, you can put on a heat pack. It's often used as a synonym for temperature. But in physics it has a precise definition which is it's the amount of energy you need to add or subtract from something to change its temperature. Okay, so what does that mean? Why do you need to put energy in or take it out of a system to change its temperature? Well, let's think of the gas. This is a simulation we saw before. A gas that's hot has all its atoms moving at random. And that means that all these atoms have kinetic energy. If you want to make the gas hotter, you'll have to put some energy in and that energy will make the atoms move faster. The energy is conserved, you don't get energy from nothing. If you want the atoms to have more energy, you need to put it in somehow. For a monatomic gas like this one where you've just got single atoms, that's all there is to it. You just need to increase the kinetic energy of the particles. If you have a solid however, it's a bit more complicated. Here we've also, if you want to make it hotter, we also have to make the atoms go faster, but that's not enough. Let's say for example you freeze this at some particular moment. There's no motions, but there's still energy here because some of the springs are compressed, some of them are bent, some are stretched. So even if you manage to freeze everything here, the springs would still start moving again when you released it. The springs here of course are the chemical bonds. So in something like this, the energy inside it is two forms. It's partially the kinetic energy of the atoms jiggling around, and partially it's the energy stored in all those chemical bonds that act like springs. Sometimes they're stretched, sometimes they're compressed, and you know if you have a compressed spring it can do work and they're being that energy is stored in it. It turns out that on average the energy in the bonds, the springs, and the kinetic energy of the things moving around is about the same. If you had all the energy in the springs, that wouldn't last long. The springs are pushing things around to stop atoms moving. If you had atoms moving but no energy in the springs, that motion will cause the springs to get compressed and uncompressed, and so before along you'll end up with about the same energy in each. This is called the principle of super equipartition, and it's fundamental in larger areas of thermodynamics.