The general concept of force is often treated as if it acts on an object in an idealized way. (This is actually common for most things in science, and particularly physics, as we create idealized models to highlight the phenomena we way to pay specific attention to and ignore as many other phenomena as we reasonably can.) In this idealized approach, if we say a force is acting on an object, we draw an arrow indicating the direction of the force, and act as if the force is all taking place at that point. In reality, though, things are never quite that simple. If I push on a lever with my hand, the force is actually distributed across my hand, and is pushing against the lever distributed across that area of the lever. To make things even more complicated in this situation, the force is almost certainly not distributed evenly. This is where pressure comes into play. Physicists apply the concept of pressure to recognize that a force is distributed over a surface area. Though we can talk about pressure in a variety of contexts, one of the earliest forms in which the concept came into discussion within science was in considering and analyzing gases. Well before the science of thermodynamics was formalized in the 1800’s, it was recognized that gases when heated applied a force or pressure onto the object that contained them. Heated gas was used for levitation of hot air balloons starting in Europe in the 1700’s, and the Chinese and other civilizations had made similar discoveries well before that. The 1800’s also saw the advent of the steam engine (as depicted in the associated image), which uses the pressure built up within a boiler to generate mechanical motion, such as that needed to move a riverboat, train, or factory loom. This pressure received its physical explanation with the kinetic theory of gases, in which scientists realized that if a gas contained a wide variety of particles (molecules), then the pressure detected could be represented physically by the average motion of those particles. This approach explains why pressure is closely related to the concepts of heat and temperature, which are also defined as motion of particles using the kinetic theory. One particular case of interest in thermodynamics is an isobaric process, which is a thermodynamic reaction where the pressure remains constant.