What is Pressure ?

Ch 1, Lesson E, Page 2 - What is Pressure ?

• Pressure is the force that a fluid exerts on a solid surface divided by the area of the surface.
• Fluids exert forces on solid surfaces because their molecules are constantly colliding with the solid surface.
• One interesting aspect of pressure is that it always acts in a direction perpendicular or normal to the solid surface.
• If the surface is curved, then pressure acts in a direction perpendicular to a plane that is tangent to the curved surface.
• The units for pressure are the units of force per length squared.
• In the SI system, the standard unit is a N/m2 which is known as a Pascal.
• A Pascal is a rather small amount of pressure, so kPa and MPa turn out to be more common units in practice.
• Two unit conversions that you might like to memorize are given here.
• 1 atm is 101.325 kPA and 1 bar is 100 KPa.  These will come in handy more than once or twice in this course.
• The standard unit for pressure in the AE System is the lbf/in2.
• The most important conversion to remember is that 1 atm = 14.696 lbf/in2.
• In general, lbf/in2, is abbreviated as a “psi”.
• But, the symbol “psi” is not adequate.
• You must use “psia” when referring to an absolute pressure and “psig” when referring to a gage pressure.
• In fact, you must distinguish between absolute and gage pressure with ANY system of units !
• So, what is the difference between absolute and gage pressure anyway ?
• Flip the page and let’s see.