 So what we're going to do now is we're going to take a look at the second law, specifically what we'll be doing is we'll look at the concept of heat engines. And we'll talk a little bit about the direction of which energy will flow and what heat engines do. So what we just saw with the example about the baseball is that work can be easily converted into other forms of energy that are in our first law. So in our first law we had other forms of energy being Q for heat transfer, delta U internal energy, delta potential energy and delta kinetic energy. So work can easily be converted into these other forms of energy, but the big challenge that mechanical engineers have is taking these other forms of energy and converting it into work, especially thermal or internal energy. So heat engines, what are they? Well, it's kind of what it sounds like. We take heat and we create work. So what do heat engines do? They receive heat from a high temperature source. What else do they do? They convert part of this heat into work. Another thing that they do, they reject some of that heat. The portion of heat that they're not able to convert to work, they need to reject. And where does that heat go? It goes to what we call a sink. So we have a source of heat and we have a sink for the rejected heat. And the last thing that a heat engine does is it operates on a cycle. So those are things that describe a heat engine. Let's take a look at that schematically. So what we said is that we have a source and it is going to be what we call TH for T-Hot. Now heat is flowing out of that source and we will refer to that as being Q into our heat engine which could also be Q from the hot supply. So this is flowing into a heat engine. We haven't defined yet what it is. It could be any number of different cycles that we will study in mechanical engineering thermodynamics. So it's flowing and we said that it also needs to reject heat. So what it can't convert, actually I should write what it's converting here. So we have work net out. So that is the work being produced by the heat engine. And then finally what it's doing, it has to dump the remaining energy or heat to our sink. And we'll refer to that heat as being Q out or Q low. And it is going to a sink and the temperature of that sink is at TL. So that's a schematic, a very generic schematic that we can use to describe what happens in a heat engine. Now on the right hand side what I'm going to do is I'm going to sketch out a cycle that we looked at in our first lecture. And that's one where we have a boiler. So in a boiler we are adding heat, so we could say that that is Q in. And we have some working fluid, actually before it goes to the boiler what I should do is I should draw my pump. And the pump is doing work on the fluid because we're increasing its pressure, causing it to flow up and into the boiler. I will show the direction of fluid flow with the arrows. The working fluid leaves the boiler and it flows into a turbine and then that is work out and then we need to reject. So we reject thermal energy out and then the working fluid comes back up and into our pump. So that is a heat engine. In this particular case it's the rank and cycle that we're looking at. But it is a heat engine. It takes in energy or thermal energy and it rejects it and in the process it does work. And it is a cycle. So we can say work net out equals work out minus work in. And it is also equal to Q in minus Q out. So those are two ways that you can determine the net work out. We can also write the thermal efficiency for this cycle. Whenever we look at any type of heat engine we are always interested in the thermal efficiency. So this is an important thing that you will be using over and over again in the course. The symbol we use for thermal efficiency is eta with subscript th. And that is equal to the net work out of our system divided by total heat in. So we can write it out as work net out divided by capital Q in. And we can also write the thermal efficiency. If we use the definition that we just showed being work net out as Q in minus Q out we can rewrite this in the following manner. And then in terms of our heat engine it would be 1 minus Q low divided by QH. Now thermal efficiency is going to be bounded between 0 and 1. You will never have a thermal efficiency over 1. Actually I'll never go over the Carnot efficiency which is something that we'll talk about later. But that is the definition of thermal efficiency for a heat engine. Now one last thing that I want to say about heat engines is the Kelvin Planck statement. So what the Kelvin Planck statement says is that it's impossible for a device that operates on a cycle to receive heat from a single reservoir. So from a single source and produce a net amount of work. What that means is that it needs to have a sink whereby it can reject the residual heat or the waste heat too. So that is the Kelvin Planck statement and it applies to heat engines.