Fluid: a substance that has the capacity to flow : a substance that has the capacity to flow and assume the form of the container it has and assume the form of the container it has

been placed in.been placed in.

Ch. 3: What is a Fluid? Ch. 3: What is a Fluid?

Particle Particle ModelModelLiquid: particles close together, weak forces of

attraction; slide passed each; take shape of container.Gas: particles far apart, no forces of attraction. Gases move in every direction; take up all available space.

• Are 100% adaptable• All liquids and many gases are fluids. Ex: water, milk, blood, saliva, air, helium, ozone.• Arrangement of particles makes a substance a fluid.• 2 types: compressible fluids and incompressible fluids.

Compressible FluidsCompressible Fluids• Is a fluid whose volume can change in response

to change in pressure. Ex: gas is compressibleIncompressible FluidsIncompressible Fluids

• Is a fluid whose volume cannot be varied. Ex: liquids What is What is pressure?pressure?• Pressure is the result of a force applied in a

perpendicular fashion to a surface.• Involves both force and surface area.What is a What is a

force?force?• Force is an action that modifies the movement of an object (accelerate, slow down, stop) or causes the shape of an object to change or attraction.

What is surface What is surface area?area?• Surface dimension of

object.• To measure the

pressure being exerted, we need to know the area of that surface.

Force and PressureForce and Pressure• The stronger the force the greater the pressure.• If force increases, pressure increases.• If force decreases, pressure decreases.Surface Area & Surface Area & PressurePressure• The greater the surface

area exposed to a force, the less pressure.

• If the surface area exposed to a force increases, the pressure decreases.

• If the surface area exposed to a force decreases, the pressure increases.

Calculating Calculating PressurePressureP = P = FF

AA PP is pressure, measured in is pressure, measured in

Pascals (Pa) Pascals (Pa) F F is force, measured in is force, measured in

Newtons (N) Newtons (N) AA is the exposed surface is the exposed surface

area, measured in marea, measured in m22

Pressure Exerted by FluidsPressure Exerted by Fluids• Particles in a fluid are in constant motion, pressure is

exerted equally in all directions.• Incompressible fluid

– Force exerted comes from the mass of the fluid situated above the object.

– The more fluid above the object, the greater the force and pressure by the fluid.

– The denser the fluid the greater the pressure.– Instruments to measure: depth gauge, U-shaped

manometer, tonometer.• Compressible fluid:

– Pressure depends upon:• # of collisions with fluid particles; the more

collisions the greater the pressure.• Instruments to measure: u-shaped

manometer, pressure gauge

Pressure Exerted by Compressible FluidsPressure Exerted by Compressible Fluids• Factors affecting # of collisions:1. # of particles; the more particles the more they collide2. Temperature: increase temp., increases particle speed,

increases # of collisions.3. Volume of fluid: is variable; o at the same temperature and particle # (vice versa):• if volume increases, pressure decreases (less

collisions).• If volume decreases, pressure increases (more

collisions).*Pressure of a compressible fluid is inversely

proportional to its volume.• Atmospheric pressure: the pressure exerted by the air

that makes up our atmosphere. It changes at different altitudes. At sea level it is 101.93 kPao Instrument to measure is a barometer.

Principles of Variations in the Pressure of Principles of Variations in the Pressure of FluidsFluids

• 1st principle: fluids move from an area of high pressure towards an area of low pressure. Ex: gas canister, exhalation and inhalation

• 2nd principle/Pascal’s principle: pressure applied to the surface of a fluid inside a closed container gets uniformly distributed to every part of the fluid. Ex: pushing the plunger of a closed syringe, brakes (brake fluid) in a car, heart pumping blood 16 kPa (120 mm Hg) to 10 kPa (75 mm Hg).

• 3rd principle: A transfer of pressure in a fluid can increase the force involved. Ex: hydraulic system