What is pressure? What causes pressure?

Imagine placing a book on a table. The weight of the book applies a force directly down onto the surface of the table. This force, however, is spread over the area of the table that is in contact with the book.

In chemistry, we often discuss pressure in the context of gases and liquids, but the idea is the same. Pressure is the result of force distributed over an area and is a fundamental concept in science. Mathematically, we express pressure as the ratio of force to area, with the equation \( P = \frac{F}{A} \), where \( P \) represents pressure, \( F \) is the force applied, and \( A \) is the area over which the force is spread. To visualize this, consider blowing up a balloon. The air molecules inside exert force on all parts of the balloon's inner surface, but because this force is distributed over the entire inner surface area, the pressure is kept at a level that the balloon can withstand.

Pressure arises from a variety of sources in our daily experiences and in scientific phenomena. One primary cause of pressure is the collision of particles. In a balloon, for instance, the gas particles are constantly moving and colliding with the walls of the balloon, creating pressure. Interestingly, even in a vacuum, where there are few particles, a moving object can exert pressure upon striking a surface, due to its force distributed over the area of impact.

Moreover, pressure is not merely confined to gaseous or liquid environments. Solid objects exert pressure too, like the example of a person standing on the ground where the weight creates pressure against the contact surface. Here, the person’s weight (force due to gravity) divided by the foot's area in contact with the ground results in pressure.

When assessing pressure, it's essential to understand the two main factors that affect its magnitude: the force applied and the area over which it's distributed. Larger forces will produce higher pressures, given that the area over which they are applied remains constant. Conversely, if you exert the same force across a smaller area, you will also increase the pressure. This is why a sharp knife cuts more easily than a blunt one; it applies the same force over a much smaller area, hence exerting higher pressure on the material it's cutting.

In real-life scenarios, such as weather systems, the temperature can also affect pressure. Warmer temperatures increase the energy of particles, leading to more frequent and forceful collisions, thereby increasing pressure.

Tip:

• To understand pressure buildup in a closed system, consider a sealed soda can. As temperature rises, pressure inside the can increases as the molecules move faster and collide more with the can's walls. This analogy helps illustrate the direct relation between temperature and pressure in a given volume.