Pascal’s Principle
When you squeeze one end of a tube to get toothpaste out the other end, you are watching Pascal’s principle in action. This principle is also the basis for theHeimlich maneuver, in which a sharp pressure increase properly applied to the abdomen is transmitted to the throat, forcefully ejecting food lodged there. The principle was first stated clearly in 1652 by Blaise Pascal (for whom the unit of pressure is named)
A change in the pressure applied to an enclosed incompressible fluid is transmitted undiminished to every portion of the fluid and to the walls of its container
Demonstrating Pascal’s Principle
Consider the case in which the incompressible fluid is a liquid contained in a tall cylinder, as in Fig. 15-7. The cylinder is fitted with a piston on which a container of lead shot rests. The atmosphere, container, and shot put pressure Pe xt on the piston and thus on the liquid. The pressure P at any point P in the liquid is then P = P« xt + pgl1. Let us add a little more lead shot to the container to increase P«xt by an amount l1Pex,’ The quaint ties p, g, and h in Eq. IS-II are imchanged. so the pressure change P is IIp = l1Pex,’ (\5-12) This pressure change is independent of 11, so it must hold for all points within the liquid, as Pascal’s principle states
