 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 we'd 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 could heat something up, you could 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 it 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 go bing! That's energy 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 the 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 long you'd end up with about the same energy in each. This is called the principle of super equipartition and it's fundamental in large areas of thermodynamics. So that's solids and monatomic gases. How about molecule gases? Well here's a CO2 molecule and what we're doing here is applying an austenitic electric field. This is what happens when light falls on carbon dioxide or some molecule. The light is an electromagnetic wave which means it's got an oscillating electric field. The electric field pulls a positive charge one way and the negative charge the other way and because of the bonding in here the positive negative charges are opposite ends and it sets the whole thing vibrating. So if you have something like carbon dioxide it can have some energy as a carbon dioxide molecules move around it can also have energy as they vibrate like this. Or for that matter if you apply a faster electromagnetic wave to it you can get different oscillation. This time it's resonating another way and the centre is moving backwards and forwards in this direction and so you can also have energy in that form. Something like a water molecule. This has even more exciting things happen when you apply an electric field to it like light because the charges are not all lined up. You have the oxygen, the middle and the hydra on the outside and so in this case if you're applying radiation to it like sunlight falling on some water it can do all sorts of cool things. It can vibrate and rotate. So in general if you have a monatomic thing just single molecules then all that's going to happen is they'll move and so in heat that we just need to get their kinetic energy increased. If you have a solid you're going to have to put some energy in to make the move and also some energy to make the springs stretch and unstretch. If you have a molecule that's free to rotate you'll also have some energy coming in rotation, spinning and vibration of the modes. So these are all forms of energy that you can put into things. How do you heat things up? Well one way is to apply light radiation to them. That's what we've seen here. The alternate electric field pulls the charges around and makes things hotter. That's why you get warm on a summer's day. Another way is to put your object in something hotter. So let's say for example you put a baked potato in a campfire. The atoms in the baked potato are moving slowly so they're not rotating very much. They're not vibrating very much. The springs are not very much stretched. There's low energy in all those forms. But what's going to happen when it's in the fire is the air around it is very hot. So the very hot air molecules are going to be spinning like crazy and bouncing and vibrating and running around like crazy will smash into the molecules in the outer parts of the baked potato. And of course if you're a slow moving atom and a fast moving atom bashes into you it's going to get you moving faster. And so first of all the molecules in the outside will start moving faster and because they're attached with bonds to the ones on the inside it'll pull the inner ones moving faster. And slowly over time all the molecules will go faster and your baked potato will cook. That's very similar to what happens here. When you have a stove you're cooking something with fire underneath. The very fast moving molecules down here will bash into the bottom of the pan and get the molecules in the pan moving really fast. The molecules in the bottom of the pan are attached to the ones in the top of the pan so the whole pan will warm up and that will then heat up whatever's inside it. Another way to apply heat is with electricity. If you have an electric kettle like this one it'll have a heating element somewhere at the bottom which is basically a piece of wire. So a piece of wire and an electric field is applied along it by your mains and that means electrons will start accelerating along it. As they accelerate along they'll bump into atoms and they'll make the atoms jiggle. The electron will then bounce off but then the electric field the voltage will cause it to accelerate again and that will cause another one and cause that to jiggle. So there'll be some sort of resisting element inside with electrons being accelerated through by the voltage and that will cause the atoms in that to jiggle faster and faster and faster and the fast jiggling molecules in the wire will then bounce off the molecules in the water around it and cause them to go faster and before long you've got a nice hot cup of tea. So to summarize heat is the amount of energy you need to put into something to change its temperature. You need to put energy into change something's temperature to make the atoms move faster, to make them vibrate, to make the chemical bonds stretch or unstretch and you can do this in various ways. Usually putting it something in touch with something that's hotter will make it happen, radiation can make it happen and a current flowing through it can make it happen and there are many other ways too.