 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, or the energy ends up as heat, some of it ends up as splashing temporarily, but 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 in 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.