PROPERTIES OF MATTER 12.2

Chapter Twelve: Properties of Matter  12.1 Properties of Solids  12.2 Properties of Fluids  12.3 Buoyancy

Chapter 12.2 Learning Goals  Explain how pressure is created in fluids.

 Discuss differences between the density of solids and fluids.

 Apply Bernoulli’s principle to explain how energy is conserved in fluids.

12.2 Properties of Fluids  A

fluid

is defined as any matter that flows when force is applied.  Liquids like water or silver are kinds of fluid.

12.2 Pressure  A force applied to a fluid creates

pressure

.

 Pressure acts in all directions, not just the direction of the applied force

.

12.2 Forces in fluids  Forces in fluids are more complicated than forces in solids because fluids can change shape.

12.2 Units of pressure  The units of pressure are force divided by area.

 One psi is one pound per square inch.

12.2 Units of pressure  The S.I. unit of force is the pascal.

 One pascal (unit of force) is one newton of force per square meter of area (N/m 2 ).

12.2 Pressure  If your car tires are inflated to 35 pounds per square inch (35 psi), then a force of 35 pounds acts on every square inch of area inside the tire.

What might happen if you over-inflate a tire?

12.2 Pressure  On the microscopic level, pressure comes from collisions between atoms.

 Every surface can experience a force from the constant impact of trillions of atoms.  This force is what we measure as pressure.

12.2 Pressure  In a car engine high pressure is created by an exploding gasoline-air mixture.

12.2 Energy conservation and Bernoulli’s Principle 

Streamlines

are imaginary lines drawn to show the flow of fluid.



Bernoulli’s principle

tells us that the energy of any sample of fluid moving along a streamline is constant.

12.2 Bernoulli’s Principle  Bernoulli’s principle says the three variables of height, pressure, and speed are related by energy conservation.

12.2 Three Variables and Bernoulli’s Principle  If one variable increases along a streamline,

at least one of the other two must decrease.

 For example, if speed goes up, pressure goes down.

12.2 The air foil   One of the most important  pressure is what creates the lift force that supports the plane in the air.

lower than the pressure beneath the wings.

12.2 Hydraulics and Pascal’s Principle  Hydraulic lifts and other hydraulic devices use pressure to multiply forces and do work.  The word

hydraulic

refers to anything that is operated by a fluid under pressure.  Hydraulic devices operate on the basis of Pascal’s principle, named after Blaise Pascal.

12.2 Hydraulics and Pascal’s Principle 

Pascal’s principle

states that the pressure applied to an incompressible fluid in a closed container is transmitted equally in all parts of the fluid.  An

incompressible fluid

increased.

does not decrease in volume when pressure is

12.2 Hydraulics and Pascal’s Principle  A small force exerted over a large distance is traded for a large force over a small distance.

12.2 Pressure 

Pressure

is force divided by area .

12.2 Force  You can calculate the

force

exerted if you know the pressure and area.

Solving Problems  On a hydraulic lift, 5 N of force is applied over an area of 0.125 m 2 .  What is the output force if the area of the larger cylinder is 5.0 m 2 ?

Solving Problems 1.

 Looking for: …output force 2.

 Given …input force = 5 N; input area = .125 m 2 ; output area = 5 m 2 3.

 Relationships: Pressure = Force Force = P x A Area

Solving Problems 4.

    Solution Solve for pressure using input force.

Pressure = 5 N = 40 N/m 2 .125m

2 Use Pascal’s law principle and use equivalent pressure to solve for output force.

Force = 40 N x 5 m 2 = 200 N m 2

12.2 Viscosity 

Viscosity

is the property of fluids that causes friction.

 Viscosity is determined in large part by the shape and size of the particles in a liquid.

12.2 Viscosity and temperature  As the temperature of a liquid increases, the viscosity of a liquid decreases.

 Increasing the kinetic energy of the substance allows the particles to slide past one another more easily.

Investigation 12C Density of Fluids 

Key Question: What is the maximum load a boat can hold before sinking?

How is the maximum load affected by the density of the water in which the boat floats?