physics of technology phys 1800

Fluids and Pressure

Introduction Section 0 Lecture 1 Slide 1

Lecture 20 Slide 1

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

Physics of TechnologyPHYS 1800

Lecture 20

Fluids and Pressure

Fluids and Pressure


Lecture 20 Slide 2


Fall 2004


Spring 2009

PHYSICS OF TECHNOLOGY Spring 2009 Assignment Sheet

*Homework Handout

PHYSICS OF TECHNOLOGY - PHYS 1800 ASSIGNMENT SHEET

Spring 2009 Date Day Lecture Chapter Homework Due Feb 16 17 18 19 20

M Tu W H F*

Presidents Day Angular Momentum (Virtual Monday) Review Test 2 Static Fluids, Pressure

No Class 8 5-8 5-8 9

-

Feb 23 25 27

M W F*

Flotation Fluids in Motion Temperature and Heat

9 9 10

6

Mar 2 4 6

M W F*

First Law of Thermodynamics Heat flow and Greenhouse Effect Climate Change

10 10 -

7

Mar 9-13 M-F Spring Break No Classes Mar 16 18 20

M W F*

Heat Engines Power and Refrigeration Electric Charge

11 11 12

8

Mar 23 25 26 27

M W H F*

Electric Fields and Electric Potential Review Test 3 Electric Circuits

12 13 9-12 13

-

Mar 30 Apr 1 3

M W F

Magnetic Force Review Electromagnets Motors and Generators

14 9-12 14

9

Apr 6 8 10

M W F*

Making Waves Sound Waves E-M Waves, Light and Color

15 15 16

10

Apr 13 15 17

M W F*

Mirrors and Reflections Refraction and Lenses Telescopes and Microscopes

17 17 17

11

Apr 20 22 24

M W F

Review Seeing Atoms The really BIG & the really small

1-17 18 (not on test) 21 (not on test)

No test week 12

May 1 F Final Exam: 09:30-11:20am * = Homework Handout

Fluids and Pressure


Lecture 20 Slide 3


Fall 2004


Spring 2009


Lecture 20

Fluids and Pressure

Introduction

Fluids and Pressure


Lecture 20 Slide 4


Fall 2004


Spring 2009

Dennison’s Laws of Fluids

• When push comes to shove, fluids are just like other stuff.

• Pascal’s Principle: Pressure extends uniformly in all directions in a fluid.

• Boyle’s Law: Work on a fluid equals PΔV

• Bernoulli’s Principle: Conservation of energy for fluids

Fluids and Pressure


Lecture 20 Slide 5


Fall 2004


Spring 2009


Lecture 20

Fluids and Pressure

Hydraulics: A Simple Machine with Fluids

Fluids and Pressure


Lecture 20 Slide 6


Fall 2004


Spring 2009

How does a hydraulic jack

work?

• A force applied to a piston with a small area can produce a large increase in pressure in the fluid because of the small area of the piston.

• This increase in pressure is transmitted through the fluid to the piston with the larger area (Pascal’s Principle).

• The force exerted on the larger piston is proportional to the area of the piston: F = PA.

• Applying the same pressure to the larger area of the second piston results in a larger force on the second piston.

• But this comes at a price. Conservation of energy says work in must equal work out that is ΔW=F Δd = (P A) Δd = P ΔV , so Δd1 > Δd2 • Another way to think of this is conservation of stuff: Vin = Vout

Fluids and Pressure


Lecture 20 Slide 7


Fall 2004


Spring 2009

A force of 10 N is applied to a circular piston with an area

of 2 cm2 in a hydraulic jack. The

output piston for the jack has an area of 100 cm2. What is

the pressure in the fluid?

a) 0.002 Pab) 5 Pac) 10 Pad) 50 kPa

F1 = 10 NA1 = 2 cm2 = 0.0002 m2

P = F1 / A1 = 10 N / 0.0002 m2 = 50,000 N/m2 = 50 kPa

Fluids and Pressure


Lecture 20 Slide 8


Fall 2004


Spring 2009

What is the force exerted on the

output piston by the fluid?

a) 50 Nb) 500 Nc) 5,000 Nd) 50,000 N

P = 50 kPaA2 = 100 cm2 = 0.01 m2

F1 = PA1 = (50,000 N/m2)(0.01 m2) = 500 NThe mechanical advantage is 500 N / 10 N = 50.

Fluids and Pressure


Lecture 20 Slide 9


Fall 2004


Spring 2009

Hydraulic Devices

Fluids and Pressure


Lecture 20 Slide 10


Fall 2004


Spring 2009

Hydraulic Brakes

Fluids and Pressure


Lecture 20 Slide 11


Fall 2004


Spring 2009


Lecture 20

Fluids and Pressure

Barometers and Atmospheric Pressure

Fluids and Pressure


Lecture 20 Slide 12


Fall 2004


Spring 2009

Atmospheric Pressure and the Behavior of Gases

• Living on the surface of the earth, we are at the bottom of a sea of air.

• This sea of air is thinner at higher altitudes.• It is also thinner during certain weather conditions.• We describe this property by atmospheric pressure:

the pressure of the layer of air that surrounds the earth.– At sea level, the atmospheric pressure is 100 kPa, or

14.7 pounds per square inch, but it decreases with altitude.

Fluids and Pressure


Lecture 20 Slide 13


Fall 2004


Spring 2009

• Torricelli invented the barometer, a device for measuring atmospheric pressure, in an attempt to explain why water pumps could pump water to a height of only 32 feet.

• He filled a tube with mercury and inverted it into an open container of mercury.

• Mercury worked well because it is much denser than water.– Density is the mass of an object divided by

its volume.• Air pressure acting on the mercury in the

dish supported a column of mercury, of height proportional to the atmospheric pressure.

The Barometer

Fluids and Pressure


Lecture 20 Slide 14


Fall 2004


Spring 2009

• Otto von Guericke performed a famous experiment to demonstrate the effects of air pressure.

• He designed two bronze hemispheres that could be smoothly joined together at their rims.

• He pumped the air out of the sphere formed from the two hemispheres.

• Two eight-horse teams were unable to pull the hemispheres apart.

Making Physics Pay Big Bucks

Fluids and Pressure


Lecture 20 Slide 15


Fall 2004


Spring 2009

• In other experiments on variations in atmospheric pressure, Pascal sent his brother-in-law to the top of a mountain with a barometer and a partially inflated balloon.

• The balloon expanded as the climbers gained elevation.• This was evidence of a

decrease in the external atmospheric pressure.

Fluids and Pressure


Lecture 20 Slide 16


Fall 2004


Spring 2009

Boyle’s Law

• Variations in the volume and density of a gas that accompanies changes in pressure were studied by Boyle and Mariotte.

• The density of a column of air decreases as altitude increases because air expands as pressure decreases.

Fluids and Pressure


Lecture 20 Slide 17


Fall 2004


Spring 2009

Application of Avagadro’s NUmber

Fluids and Pressure


Lecture 20 Slide 18


Fall 2004


Spring 2009

Application of Avagadro’s NUmber

Fluids and Pressure


Lecture 20 Slide 19


Fall 2004


Spring 2009

Boyle’s Law

• Boyle discovered that the volume of a gas is inversely proportional to the pressure.

• Boyle’s Law: PV = constant• If the pressure increases, the

volume decreases.• The density of a column of air

decreases as altitude increases because air expands as pressure decreases.

• P1V1 = P2V2

Fluids and Pressure


Lecture 20 Slide 20


Fall 2004


Spring 2009

A fixed quantity of gas is held in a cylinder capped at one end by a movable piston. The pressure of the gas is

initially 1 atmosphere (101 kPa) and the volume is initially 0.3 m3. What is the final volume of the gas if the pressure is increased to 3 atmospheres at constant temperature?

a) 0.1 m3

b) 0.3 m3

c) 1 m3

d) 3 m3

P1 = 1 atm P2 = 3 atm V1 = 0.3 m3 V2 = ?

V2 = P1V1 / P2 = (1 atm)(0.3 m3) / 3 atm = 0.1 m3

Fluids and Pressure


Lecture 20 Slide 21


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Spring 2009

Range of Pressures

Fluids and Pressure


Lecture 20 Slide 22


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Spring 2009

Barometers

Fluids and Pressure


Lecture 20 Slide 23


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Spring 2009

Pressure Gauges

Fluids and Pressure


Lecture 20 Slide 24


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Spring 2009

Capacitance Manometer

Fluids and Pressure


Lecture 20 Slide 25


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Convection Pressure Gauges

Fluids and Pressure


Lecture 20 Slide 26


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Fluids and Pressure


Lecture 20 Slide 27


Fall 2004


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Lecture 20

Fluids and Pressure

Archimedes's Principle:Buoyant Forces

Fluids and Pressure


Lecture 20 Slide 28


Fall 2004


Spring 2009

Archimedes’ Principle• The average density of an object compared to a fluid determines

whether the object will sink or float in that liquid.• The upward force that pushes objects back toward the surface in

liquids is called the buoyant force.• Archimedes’ Principle: The buoyant force acting on an object

fully or partially submerged in a fluid is equal to the weight of the fluid displaced by the object.

Fluids and Pressure


Lecture 20 Slide 29


Fall 2004


Spring 2009

Fluids and Pressure


Lecture 20 Slide 30


Fall 2004


Spring 2009

Archimedes’ Principle

• For example, consider a block submerged in water, suspended from a string.– The pressure of the water pushes on the block from all sides.– Because the pressure increases with depth, the pressure at the bottom of the block

is greater than at the top.– There is a larger force (F = PA) pushing up at the bottom than there is pushing

down at the top.– The difference between these two forces is the buoyant force.

The buoyant force is proportional to both the height and the cross-sectional area of the block, and thus to its volume.

The volume of the fluid displaced is directly related to the weight of the fluid displaced.

Weight mg VdgVolume Ah

Excess Pressure P WA

dgAh

adgh

Fluids and Pressure


Lecture 20 Slide 31


Fall 2004


Spring 2009

Physics of Technology

Next Lab/Demo: Rotational MotionFluidsThursday 1:30-2:45 ESLC 46

Ch 8 and 9

Next Class: Wednesday 10:30-11:20BUS 318 roomReview Ch 9

physics of technology phys 1800

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