Pressure is the amount of force applied across a specific surface area. For gases, this force is exerted continuously outward on the internal walls of the container, which is measured as gas pressure. To understand the origin of this macroscopic force, we must examine the behavior of the individual particles that make up the gas. This article explores the microscopic actions that create this measurable phenomenon.
Understanding Gases Through Kinetic Theory
The behavior of gases is best explained by the Kinetic Molecular Theory, a model that describes gas as a collection of countless, extremely small particles (atoms or molecules). These particles are in perpetual, rapid, and entirely random motion, moving along straight paths.
A defining feature of the gas state is the significant distance separating the individual particles. The volume occupied by the particles themselves is negligible compared to the total volume of the container. This empty space allows gases to be easily compressed or expanded, distinguishing them from liquids or solids.
The particles move in straight lines until they collide with another particle or the wall of the container. Crucially, these collisions are considered perfectly elastic, meaning that no kinetic energy is lost during the impact, only transferred. The energy of the system remains constant, allowing the motion to continue indefinitely.
How Particle Collisions Create Pressure
The mechanism by which a gas generates pressure is rooted in the constant motion described by the Kinetic Molecular Theory. Pressure is generated specifically by the particles striking the internal surfaces of the container.
When an individual gas particle impacts the wall, it reverses its direction of travel. This change in direction represents a change in the particle’s momentum. According to the laws of physics, a change in momentum over a short period of time generates a force.
Gas particles are moving at high speeds, often hundreds of meters per second, and strike the walls with considerable frequency. Although the force exerted by a single particle collision is infinitesimally small, the sheer number of particles in any macroscopic sample is enormous.
Pressure is the summation of all these individual, tiny forces distributed across the entire surface area of the container. Billions upon billions of particles hitting the walls every second create a continuous bombardment that results in a steady, measurable outward push. This constant barrage ensures that the force exerted on the container walls is uniform and sustained, creating a homogeneous force field against the container.
Variables That Change Gas Pressure
The magnitude of gas pressure can be altered by changing several macroscopic properties of the system.
Temperature
An increase in temperature directly correlates to an increase in the average speed of the gas particles. Faster-moving particles strike the container walls with greater force and hit them more frequently. Both of these effects combine to produce a direct and proportional rise in the measurable pressure.
Volume
Changing the volume of the container also dramatically affects pressure, assuming the temperature and amount of gas remain constant. If the volume is decreased, the same number of particles are confined to a much smaller space. This reduction in space means the particles have less distance to travel before encountering a wall. Consequently, the frequency of collisions with the internal surfaces increases significantly, leading to an inverse relationship between the container volume and the pressure exerted.
Amount of Gas
Increasing the amount of gas, or the number of moles, within a fixed volume also raises the pressure. Adding more particles means there are more entities available to participate in the collision process. This increases the total number of impacts per second against the container walls, resulting in a higher overall pressure.