Pressure is an expression of force exerted on a surface per unit area. The standard unit of pressure is the pascal (Pa), equivalent to one newton per meter squared (N/m 2 or N · m -2 ). Alternatively, pressure can be measured in dyne s per centimeter squared (dyn/cm 2 or dyn · cm -2 ). To convert from pascals to dynes per centimeter squared, multiply by 10. Conversely, multiply by 0.1.
Consider an enclosed chamber filled with a gas and surrounded by a vacuum. The pressure exerted on the walls of the chamber by the gas depends on three factors: (1) the amount of gas in the chamber, (2) the temperature of the gas, and (3) the volume of the chamber. As the amount of gas increases, assuming the volume of the chamber and the temperature remain constant, the pressure increases. As the temperature increases, assuming the amount of gas and the size of the chamber remain constant, the pressure increases. As the volume of the chamber increases, assuming the amount of gas in the chamber and the temperature remain constant, the pressure decreases. These are idealized examples; in practical scenarios these three factors often interact.
The pressure of the Earth's atmosphere at sea level is approximately 10 5 Pa. A unit of 100 Pa is known as a millibar (mb); atmospheric pressure is generally about 1,000 mb at sea level.