 Okay, we've seen that we can convert other types of energy into heat and we can convert heat into other types of energy. But how efficiently can you do it? Can you get all the energy to convert or only some of it? Well, it turns out that if you're converting other forms of energy into heat, you can be 100% efficient. So let's say you've got 100 joules of energy, you can convert all of it into heat. So you have 100% efficiency, which is actually what happens when someone jumps in the water. All the energy ends up as heat, some of it ends up as splashing temporarily, the water splashes back down and the water will end up as heat, some of it's noise temporarily, but the noise will end up as heat. In fact, this is how pretty much everything usually finishes up. Any other form of energy usually ends up as heat. But the other way is much more difficult. Let's imagine you had 100 joules of heat energy and you stick it into an internal combustion engine. You can never get 100 joules of mechanical energy out. You get some other energy out, mechanical, electrical, whatever, but a lot of it peels off and ends up as heat. This is parameterized by an efficiency. So if you have the energy in some other form, it's going to be equal to the efficiency, which is written with the Greek letter eta, just like an N with a very long front bit, times the energy of heat coming in. So for example at a power station, let's say it's burning coal, you take chemical energy, which is another form of energy turned into heat, you can do that with 100% efficiency, but then you try and take the heat and turn it back into electricity, and only a fraction eta of it will actually succeed and the rest of it will be wasted as heat. Now for an internal combustion engine, the efficiency is typically only about 25%. So when you burn a litre of petrol to drive somewhere, one litre of petrol will turn into actual motion, whereas another three litres of petrol are just heating up your engine. That's why your engine gets so hot that you need fans to cool it down so that the air heat can spread around the environment. Of course even the energy that goes into motion will end up as heat. As you're driving, you'll be stirring up the air around it, you'll be bending your tyres and all that will end up as heat as well. So actually 100% will end up as heat eventually, but 25% of it is useful and makes the car go. For a big power station, it's more like 50%. Power stations, a good one, can be around 50% maybe, usually a bit worse than that, but occasionally slightly better, and so about half the energy is wasted turning into heat. That's why power stations need things like these cooling towers to dissipate the other 50% of the energy that's going out in the form of heat. It's a real waste and it's led to the idea of combined heat and power. The idea here is instead of having a whopping great power station out in the middle of nowhere where half the energy gets turned into electricity and half is wasted on heating up the air and the sea and the water around it, maybe you have lots of small power stations actually built in people's houses or neighbourhoods. And so all the heat that's wasted could be pumped down a vent or something into people's houses and keep them warm. And if you're in a cold country like England or Sweden or Canada where you need a lot of heat, that can make it much more efficient. They'll be twice as good as the conventional power stations. Probably not so useful in a hot country like, say, Queensland on a hot summer's day when you want electricity to run the air conditioning. Ah well. So that's efficiency, but it's a fundamental thing that engineers have to worry about. It does seem a bit odd that this physical principle is not reversible, that you can go this way, but you can't go that way. And this, it turns out, is a very fundamental law of physics to do with entropy. This is actually what gives time its direction. Some things, like a person jumping into the water can happen, but the reverse, like this, can't happen. You can't turn the heat of the water back into throwing someone out of the water. Likewise, pens don't spontaneously leap off the tables, like this. You can't turn the heat in the table into a pen jumping into the air. That's a strange word if it was. Every time you're going for a swim, suddenly random people get flung out of the water. I mean, every object you lay on the floor, occasionally you would leap up. But the other way round does work. Very strange and very fundamental.