What is pressure really? It's something we all know about, some feel it everyday. Indeed, what does pressure look like physically? Actually, it's a force, however, before the physicists crucify me, I must mention the relatonship to the surface. Perhaps pressure is easier to understand, if one considers the thermodynamic model. What is meant here, are the oscillating molecules, whose oscilation speed shows the connection to the temperature.

Now the molecules are just swinging about. As we said, filling all directions and somtimes also coliding with one another. This happens elastically and without results, neither for the molecules, nor for the energie or gases. It only becomes interesting, when we have a look at the edge of this gas filled space.

Imagine something like a drum-roll. If that, e.g., was an air-cooled cylinder, the one side we would have the molecules and on the other side, molecules made up of an alloy of aluminium and silicium. If they both have the same temperature, then their oscillation speed is the same. What however happens, if the cylinder is being cooled at that moment?

Then the metal-molecules drum considerably more often against the boundary surface, and they get tired doing this, but still drive the air-molecules on to a higher urgency. Outwardly, we notice, that the temperatures approach each other. If the temperature in the cylinder is still too high, through the cooling fan, further multitudes of relatively slowly oscillating air-molecules are brought into play, onto which the metal-molecules go to work.

Perhaps you can now understand why an air-cooled cylinder needs to have fins. Here the contact surface between the air- and metal-molecules is enlarged, so that no air-molecule can slip by without being influenced.

Now, what does all this have to do with pressure? All you have to do, is to shift the contact surface between a solid surface and the air, e.g., the way that a piston does. If the intake air is compressed, the piston gives the molecules a severe push, thus causing them to increase their oscillation rate. This also has an effect on the piston itself, because the higher temperature of the compressed air, according to the above described process, is transferred to it. 09/13