Glencoe Physics

Chapter 13

States of Matter

Objectives:13.1• Describe how fluids create pressure and

relate how Pascal’s Principle to some everyday occurences.

• Apply Archimedes’ and Bernoulli’s Principles

• Explain how forces within liquids cause surface tension and capillary action, and relate the kinetic model to evaporation and condensation

Objectives:13.2

• Compare solids, liquids, gases, and plasmas at a microscopic level, and relate their properties to their structures.

• Explain why solids contract and expand when the temperature changes

• Calculate the expansion of solids and discuss the problems caused by expansion

Fluids

• A fluid is anything that flows (easily moves, changes shape).– Liquids (system is a hydraulic system)– Gases (system is a pneumatic system)

• To move fluids, a pressure difference is needed

Pressure• Motion of fluids can be

understood by understanding the concept of pressure.

• Pressure is created by fluids when particles of fluid (atoms or molecules) impact a surface.

• Pressure is defined as the ratio of a force and the area over which that force is applied

A

FP

Where P = Pressure F = force applied A = area over which force

is applied

**Pressure is directly related to the number and strength of the collisions of the particles with container.

Pressure

• Units of pressure….– pounds/inch2 (PSI)– newtons/meter2

(Pascal) (SI unit)– kilopascals– mmHg– InHg– Torr– atmospheres– bar– millibar

• Standard Sea Level barometric Pressure

• 14.69 psi• 101,325 Pa• 101.325 kPa• 760 mmHg• 29.92 inHg• 760 torr• 1 atm• 1.013 barr• 1013 mb

1000 Pa = 1 kPa = 1000 N/m2

EvangelistaTorricelli

First manTo create a sustained vacuum, and discovered principle of the barometer

Measurement of pressure

• Barometers– Aneroid

Why is “Fair Weather” or “Dry Weather” associated with higher atmospheric pressure?

Measurement of Pressure

• Barometers– Mercury

Measurement of Pressure

• Pressure gauge• Tire gauge• Altimeter• Digital instrument

Magdeburg Hemispheres

• The Magdeburg hemispheres were a pair of large copper hemispheres precisely cast so that their rims fit tightly together.

• The Magdeburg hemispheres were designed by German scientist Otto von Guericke in 1650 to disprove Aristotle's famous supposition that 'Nature abhors a vacuum'. To get the air out of the sphere, von Guericke also designed the world's first vacuum pump, which consisted of a piston and cylinder with one-way flap valves.

Problem…..The atmospheric pressure at sea level is 101,325 Pa. What is the force, at sea level, that air exerts on the top of a typical desk, 152 cm long X 76 cm wide? Report in SI unit.

P=F/A

PA = F

F = 101, 325 n/m2 X (1.52 m X .76m)F = 1.2 X 105 N

F = PA

Problem…

Area of Contact

A car has a mass of 900. kg. If each tire on the car has a contact area of 80. cm2, what internal air pressure (in Pa) will each tire have? In atm? In PSI?

P = F/A

P= ((900.kg X 9.8m/s)/4) / .0080 m2P = (mg/4)/A

P = 275625 Pa

P = 2.8 X 105 Pa 2.8 ATM 41 psi

Fluid Pressure• Fluid pressure can be “created” in at least

two ways– For an enclosed fluid, the container may provide

the force (physical confinement) or there may be an external force applied

– For an open fluid, the “weight” of the fluid creates the pressure

Pascal’s Principle

• Blaise Pascal (1623-1662), a French physician, noted that the shape of the container did not affect the pressure of the fluid that it contained, at any particular depth.

Pascal’s Vases

Pressure in Closed System

Pascal’s Principle

• “Any change to the pressure of a confined fluid is transmitted undiminished throughout the fluid”

2

2

1

1

A

F

A

FP

Fluid is at same pressure, everywhere……

F1/A1 = F2/A2

F1A2/A1 = F2

Basic hydraulic cylinder

Problem…

• Dentist’s chairs are examples of hydraulic lift systems. If a chair weighs 1600 N and rests on a piston with a cross-sectional area of 1440 cm2, what force must be applied to the smaller piston with a cross-sectional area of 72 cm2 to lift the chair?