Pressure is often defined as force per unit area. In physics, the term is usually applied to fluids (gases or liquids). If a fluid is exposed to suitable forces, pressure is produced in it. The greater the force, the greater the pressure. Pressure is measured in pounds per square inch in the inch-pound system customarily used in the United States. In the metric system, the standard unit of pressure is the pascal; the nonstandard unit kilogram per square centimeter is also used.
Atmospheric pressure is produced by the weight of the air from the top of the atmosphere as it presses down upon the layers of air below it. At sea level, the average atmospheric pressure is 14.7 pounds per square inch (101.3 kilopascals). This decreases with altitude because of less air pressing from above.
If a fluid is at rest, pressure is transmitted equally to all its parts and, at any one point, is the same in all directions. The fluid acts this way because the molecules in it move freely. The molecules are far apart in a gas and comparatively close together in a liquid.
The French scientist Blaise Pascal discovered the fact that pressure in a fluid is transmitted equally to all distances and in all directions. He formulated Pascal’s law to describe the effects of pressure within a fluid (see Pascal’s law ).
The greater the pressure in a gas, the smaller its volume. This decrease in volume occurs because the molecules are pushed closer together. Under ordinary conditions, the volume of a gas decreases by half when the pressure doubles. The law that describes how the volume of a gas changes when the pressure changes is called Boyle’s law, after Robert Boyle, the Irish scientist who first published it. The volume of liquids and solids also decreases when pressure increases, but by much smaller amounts than for gases.
The ability of a gas to compress and expand has many practical uses. For example, air tires, air cushions, and air brakes are based on this elasticity of air.
Pressure changes the boiling point of water. The boiling point is that temperature at which the pressure of the steam is equal to the atmospheric pressure. At sea level, the two pressures are equal at 212 °F (100 °C). As height increases, the pressure decreases, and the boiling point becomes lower. This makes cooking at high altitudes difficult, because the cooking of food depends upon the temperature to which the food is heated, not on whether the surrounding water is boiling. See Boiling point .
Wind is the movement of air from a point of high pressure to a point of low pressure. Pressure changes precede storms. Barometers detect storms by measuring such changes.
