Fluids and DynamicsUnit 3

Science 8Tannant

Chapter 9

There are Both Natural and Constructed Fluid Systems

9.1 Fluids Under Pressure

Fluids under pressure are found in our bodies.

• Blood Circulation• Air Breathing• Kidneys Filter Urine from the blood

Fluids Under Pressure are used in Devices every day.

• Compressed Gases– Tires– Pneumatic devices (chairs, jacks, dentist drills)

• Incompressible Liquids– Hydraulic devices (vehicle brakes, lifts, forklift,

plumbing!!)

Atmospheric PressureThe Earth’s atmosphere extends more than 160 km above the Earth.

Each layer puts pressure on the layers below (Gravity Works!)

Air pressure changes with Altitude

• Air is less dense at higher altitudes because the air there is less air compressed.

Pressure Differences

• Fluids move naturally from an area of HIGH pressure to LOW pressure.

High \LOW

Suction on the straw creates low

pressure in your mouth.

The higher pressure of the atmosphere around

the box creates unbalanced forces

The juice is pushed up the straw, and into your

mouth.

Pressure differences create unbalanced forces…

And unbalanced forces cause motion.

• So…the key to using devices with pressure systems is to create a pressure difference.

Liquid Pressure

• The pressure of all fluids increases with depth.

• (The more particles you have on top of you, the more pressure there is!)

• If you puncture a closed container near the top and the bottom, more fluid will be pushed out of the bottom because of higher pressure.

Atmospheric Pressure at Sea Level

• The atmospheric pressure at Sea Level is 101.3 kPa.

• This is called 1 atmosphere (1 atm)

• For every 10 m you descend in water, the pressure increases 1 atm.

• 1 atm = 100 000 N pressing on 1 m2

Buoyancy

• The tendency of objects in fluids to rise or sink because of density differences with their surroundings.

Buoyancy Force = an upward force

Sink = Gravity > BuoyancyRise = Gravity < Buoyancy

BuoyancyThe ice floats because the

force of gravity is equal to the buoyant force of the ice (due

to its lower density than water)

Will this object sink, rise, or float? How do you know?

Rising and Sinking Air pushed out, water

taken in = sinkAir in = float

Why do your ears “pop”?

9.2 Constructed Fluid Systems

Fluids at Rest

• Blaise Pascal (French physician) studied fluid systems in the 1600’s.

• Observed: when pressure is applied at one point to a fluid in an enclosed system, that pressure is transmitted equally through the entire system.

• Squeezing an enclosed fluid creates Static Pressure–(Static = not moving)–Pressure increases

• No motion yet the container is closed so forces within the container are balanced.

• To get motion, you must open the container and unbalance the forces.

Fluids in Motion

• Dynamic Pressure = a fluid in motion.

• Daniel Bernoulli (Swiss Scientist) observed in the 1700’s that as the speed of a fluid increases, the pressure exerted perpendicular to the motion decreases.

Hydraulic Systems

• Hydraulics = the study of pressure in liquids.

• Hydraulic Systems = devices that create pressure that moves through an incompressible liquid (oil or water).

Hydraulic Fluid Transportation

• Fluid transportation systems require:– Pipes to transport the fluid

– Pumps to provide the force being applied to the fluid (although some systems use gravity)

– Valves to direct the fluid or to ensure the fluid only travels one direction.

Fig. 9.14 Static pressure of water in the pipes causes the water

particles to push against the inside walls of the pipe.

Valves in the taps prevent the water

from escaping.

Hydraulic Multiplication

An incompressible fluid increases and transmits a force from one point to

another.

Pressure stays the same in the device

P = F/AF = 5000N

A = 0.5 m^2P = 10 kPa

P = F/AF = 50 000NA = 5 m^2P = 10 kPa

P of small piston = P of larger piston

Problems in Hydraulic Systems

• Pipes must avoid twists/turns to decrease friction within the system

• Pipes must be connected carefully to prevent leakage (and loss of pressure)

• Corrosion, sludge build-up, and mineral deposits cause increased friction or may block the system.

Pneumatic Systems

• Pneumatics = the use of gas (usually air, sometimes nitrogen or a noble gas like Argon) in an enclosed system under pressure.

• Pneumatic Systems = an enclosed (and compressible) gas that transmits a force.

Pneumatic Systems

• Pneumatic systems need a compressor—a device that compresses air (builds up air pressure).

• As the pressure is released, the air particles move apart suddenly, causing a strong force.

Problems in Pneumatic Systems

• If the movement of air is blocked (clogged hose on a vacuum cleaner)

• If the device develops a leak that allows the air to escape.

• If the pressure is allowed to get too high for the device (the device can explode)

9.3 Natural Fluid Systems

Fluid Systems in HumansWater

Balance in Cells

Water balance

between cells

Blood Circulatio

n

Waste Removal

Respiration