Molecular Definition of Pressure

Let Us Study About Pressure

Pressure is determined by the flow of mass from a high pressure region to a low pressure region. Pressure measurements are made on the fluid states--liquids and gases. Air exerts a pressure which we are so accustomed to that we ignore it. The pressure of water on a swimmer is more noticable. You may be aware of pressure measurements in relations to the weather or your car or bicycle tires.

PRESSURE is a force exerted by the substance per unit area on another substance. The pressure of a gas is the force that the gas exerts on the walls of its container. When you blow air into a balloon, the balloon expands because the pressure of air molecules is greater on the inside of the balloon than the outside. Pressure is a property which determines the direction in which mass flows. If the balloon is released, the air moves from a region of high pressure to a region of low pressure.

Atmospheric pressure varies with height just as water pressure varies with depth. As a swimmer dives deeper, the water pressure increases. As a mountain climber ascends to higher altitudes, the atmospheric pressure decreases. His body is compressed by a smaller amount of air above it. The atmospheric pressure at 20,000 feet is only one-half of that at sea level because about half of the entire atmosphere is below this elevation.

Atmospheric pressure at sea level can be expressed in terms of 14.7 pounds per square inch. The pressure in car or bicycle tires is also measured in pounds per square inches. A car should have 26-30 lb/sq.in. and bicycle tires 40-60/sq.in.

Molecular Definition of Pressure

From the kinetic theory of gases, a gas is composed of a large number of molecules that are very small relative to the distance between molecules. The molecules of a gas are in constant, random motion and frequently collide with each other and with the walls of any container. The molecules possess the physical properties of mass, momentum, and energy. The momentum of a single molecule is the product of its mass and velocity, while the kinetic energy is one half the mass times the square of the velocity. As the gas molecules collide with the walls of a container, as shown on the left of the figure, the molecules impart momentum to the walls, producing a force perpendicular to the wall. The sum of the forces of all the molecules striking the wall divided by the area of the wall is defined to be the pressure. The pressure of a gas is then a measure of the average linear momentum of the moving molecules of a gas. The pressure acts perpendicular (normal) to the wall; the tangential (shear) component of the force is related to the viscosity of the gas.

The scientific units of pressure can be determined from its definition:

A force applied to surface with an area A will have will result in a pressure P as defined above. Force has the units "mass length/time^2" and area has the units length^2. Inserting this into the equation above results in the units of pressure as

A pascal is the SI unit of pressure.

Consider gas molecules in a rectangular box. Every time a molecule collides with a wall of the box the collision results in a force on the box. These forces combine and result in the pressure of the gas.