 Now we can examine PV diagrams as they're used in physics. So basically a PV diagram is a plot for ideal gases where I've got P for pressure on the vertical axis and V for volume on the horizontal axis. Now if I talk just about pressure, that means that any arrow or process on a PV diagram, as we're generally talking about a transition of a gas from one state to another. Any arrow that goes upwards, whether it be straight upwards to the left or right or curved, if it ends up upwards, that means I've got a pressure increase. Similarly, any arrow that points downwards at all in any direction is going to be a pressure decrease. I can talk about the same thing with volume. Any arrow pointing towards the right is a volume increase. Any arrow which points towards the left is a volume decrease. Now notice pressure and volume can both be changing, so you might have to look at the up-down and the left-right at the same time. Now we can also look at temperature. It may not be immediately obvious how temperature is shown on a PV diagram. After all, I've got just pressure and volume here. But remember that for an ideal gas, pressure, volume, and temperature are related to each other. And any of these processes that I've got shown here with purple arrows end up in a higher temperature. So if I keep the pressure constant but the volume increases, if I keep the volume constant but the pressure increases, or if both pressure and volume increase, that means I've got a higher temperature gas sample. And similarly, all these orange arrows result in a lower temperature. So in general, any process that ends up further in this corner is going to be a higher temperature, and any process that ends up closer to this side is a lower temperature. These are often shown by isotherms. So if I were to plot constant temperature but adjust the pressure and volume, the pressure and volume are going to be inversely proportional, giving us these sorts of curves, where this curve represents a higher possible temperature and this curve represents a lower possible temperature. So that's your basic introduction to PV diagrams.