Pressure in Fluids

Pressure Notes

Pressure in Fluids Review of fluids:  Liquid and gas state  Does not have its own shape  takes the shape of the container it is in  Flows

 Think of a basketball o When you blow it up, does it fill unevenly or evenly? o Does it get harder to pump as more and more air fills the tire? o Fluids exert equal pressure along all sides of the container (Same goes for a bicycle tire, or any inflated container)

Why Fluids Exert Pressure  Particles in constant motion in all directions at random  Collisions occur  Think of a sun ray in your living room o See the dust particles? o Constant motion o Collisions o Mimics the movement of air  It is these collisions that cause pressure o What do you think would happen if there were more particles present in the same enclosed container  higher pressure because of more collisions of particles  Particles of fluids move from an area of high pressure to lower pressure o What happens when the pressures are equal?

The Relationship between Pressure, Force, and Area  Pressure Definition o The amount of force acting on a given area o Equation: . . Where:  Pressure shows the concentrated force on a given area o The smaller the SA to which a pressure is applied, the greater the pressure is o i.e. Push Pin o Apply pressure w/ thumb on the broad end and the force acts on the much smaller SA of the pin end, pressure at pin end is much greater than the pressure you applied with your thumb The SI Unit for Pressure  Force = Newtons (N)  Area = m2 2    P = N/m = Pascal (Pa)

o Pa is named after the scientist Blaise Pascal o 1000 Pa = 1 kPa o Pressures of some common things . Atmospheric pressure = app. 101 kPa . Pointe of a ballerina toe up to 500 kPa  Demo Calc o A rock weighs 5000N and rests on an area of 0.5m2  F = PA o Exerts P of 10kPa but over A of 0.4m2  i.e. 2 situations  Which exerts more P on the ground? o Break dancer F = 800N balancing on palm of hand w/ A = 0.02m2, how much P? o Same break dancer laying on the ground covers area 0.75m2. Now what is the pressure

Factors that Effect Fluid Pressure  Depth of the fluid – fluids exert more pressure at greater depths o What happens to an empty can at great depths? o How do your ears feel when you dive in a pool? o Deeper in a fluid, all of the fluid above it results in more weight pressing down  Density o The denser the fluid, the more pressure it exerts o i.e. Water exerts more pressure than air o Particles are closer together = increased collisions = increased pressure

Pascal’s Law  Pascal studied the behaviours of enclosed liquids  This law states: an enclosed fluid transmits pressure equally in all directions o P in = P out  I.e. Hydraulic (water) and pneumatic (air) devices  Piston = a disk that moves inside a cyclinder o Hydraulic lift . Output piston has a much larger area than the input piston o Pneumatic ex: dentist drill, jack hammer, paint sprayer, air brake on a truck  Pneumatic devices advantages o Compressed air is cheap o Do not create sparks in the system (why mine devices are pneumatic) o Free of electrical hazards (why dentist drills are pneumatic)  If there is a leak in the system, then the system will fail

Summary:  All fluids exert pressure because their particles are constantly moving and bumping into each other and the walls of their container  Pressure is defined as the amount of F acting on a given A; it measures how concentrated a force is  SI units are N/m2 or Pascal (Pa)