 The second law of thermodynamics is a bit of a bear to get your head wrapped around all at once. It can be represented by looking at examples of the second law, and one of the ways to do that is to consider the concept of reversibility. And the flawed analogy I like to use to think about effects of reversibility and irreversibility is to consider the conversion of energy from potential energy to kinetic energy. If you consider a ball on the top of a valley, and we said that the ball had five kilojoules of potential energy relative to the bottom of the valley, and we let it roll down the hill, then when it reaches the bottom it has converted all of its potential energy to kinetic energy. So we could say its kinetic energy at that point is five kilojoules, or that its kinetic energy has increased by five kilojoules. Then as it rolls back up the hill it is converting kinetic energy back into potential energy, and theoretically, if everything were perfect, we would end up back where we started with a potential energy of five kilojoules. So if this were perfect, it would become a situation that could happen in either direction. It would be reversible. We're converting five kilojoules of potential energy to five kilojoules of kinetic energy back and losing nothing in the process. Now of course in reality, the presence of friction is going to represent an irreversibility. We are losing some of the energy. We might end up with a kinetic energy at the bottom of the hill of four and a half kilojoules, which means that we might end the process with only four kilojoules of potential energy. So the process of converting from potential to kinetic and back may incur some losses, and those losses represent irreversibilities. So for our purposes, a perfect process is reversible, and the presence of irreversibility represents losses. That leads us to our first definition. A reversible process is a process that can be reversed without leaving any trace on its surroundings. And then an irreversible process is a process that is not reversible. By considering reversible conversions and irreversible conversions, we come up with a hierarchy of energy qualities. And we can say that energy will naturally or spontaneously go from high quality to low quality, but to go from low quality to high quality requires an investment of energy. For example, work is a higher quality energy than heat transfer, which means going from work to heat can happen for quote-free, unquote. We could do that without, theoretically, losing any energy in the process. We have no irreversibilities. But since heat is a lower quality energy than work, we can't go from heat to work perfectly. To go from heat to work would require an investment of energy, which means, for our purposes, we would have to construct a power plant or other sort of heat engine and then convert the heat into work and have losses in the conversion process. We can also consider higher temperature thermal energy, a higher quality energy, than lower temperature thermal energy. That means that heat will be naturally driven from a high temperature to a low temperature. Going from a low temperature to higher temperature wouldn't violate the first law of thermodynamics, but it would violate the second law. Energy has a quality in addition to a magnitude. For convenience, we can also break reversibility into internal and external reversibility. That's whether or not the irreversibilities are happening within our system boundaries or not. And that allows us to simplify our problem as much as possible. The logic goes like this, reversible systems and irreversible analysis allows us to establish a best-case scenario. That may not actually occur in nature, so you may think that it has no value, but by understanding what the best case scenario is, we can relate performance. And by understanding where we are losing energy, we can try to optimize whatever process it is that we're working on. As engineers, your goal isn't necessarily to produce a perfect process, but to try to come up with a cost-benefit analysis for reducing losses. To do so requires understanding where the losses are occurring, which requires that you compare your analysis to the theoretical maximum. For one such example, let's consider expansion and contraction. An ideal compression process occurs very slowly. An ideal expansion process occurs very slowly. And when I say ideal in this context, I mean the most reversible compression and expansion process happens slowly. The presence of expanding or contracting quickly represents losses. The fastest, most lossy, most irreversible type of expansion or compression is unrestrained. So to consider an example, imagine that you had a swimming pool full of water and one of the walls moved for some reason, and you were expending work to move the wall around. If you expanded or compressed slowly, you are giving the water time to rearrange itself, and it will help you as much as it can. If you compare that to rapid expansion or compression, rapidly expanding or rapidly compressing isn't giving the water an opportunity to rearrange itself. If you expanded very quickly, you end up with less water pushing against your wall, which means that you're having to do more of the work to move the wall. Or if you're considering a compression process, compressing quickly means that you are fighting even more water than you would have if you gave it a chance to rearrange itself. Two manifestations of the second law of thermodynamics are the Kevin Planck statement and the Clausius statement. The Kelvin Planck statement says that it is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work, meaning you can't go from heat to work perfectly. The maximum thermal efficiency of a heat engine is less than 100%. That system is impossible. The Clausius statement says that it is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a lower temperature body to a higher temperature body. That means that you cannot move heat from low temperature to high temperature for free. Doing so requires an investment of energy. That system is impossible. Another way to think of that would be to say your refrigerator doesn't work unless you plug it in. The Clausius statement can be considered also as a refrigeration cycle that is being powered by a perfectly efficient heat engine. You would end up with a perfect return on investment, meaning that you are moving the heat for free.