chapter 14 gasses & plasmas weight of air is small but not negligible. for example, weight of...

Chapter 14 Gasses & Plasmas

Weight of air is small but not negligible. For example, weight of air in this room is

comparable to your weight (volume of room about 1000x your volume).

Figure 13.5

Once believed that you had to move water at a slow drop in gradient to move it.

Pressure does not depend on the amount of liquid

Volume is not the key- depth is

The force

exerted by a

fluid on a

smooth surface

is always at right

angles to the

surface

Apr 20, 2023 Physics 1 (Garcia) SJSU

Pressure in Liquids

High

Medium

Low

High

Medium

Low


Pressure

Pressure is defined as

Pressure =

Metric unit of pressure is Pascal.

1 Pascal = 1 Newtons per square meter

Atmospheric pressure is about 100,000 Pascals

( Force )

( Area )


Demo: Bed of Nails

Sample Problem

Note: Atmospheric pressure is about 100,000 Pascals so much more than pressure due to the “gold” brick.


Check Yourself

In which case is the pressure greatest? A B

A B


Check Yourself

In which case is the pressure greatest? A B

AB

Crush the Rail Tanker Car

Buoyancy

Since pressure depends on depth, a submerged object has more force due to pressure below it than above it.

Net effect is to have a net upward force, which we call buoyancy.

Buoyancy

Weight

If weight exceeds buoyancy force then object sinks, otherwise it floats.

Pressure Pressure

Pressure


Buoyancy & Depth

For a fully submerged object the buoyancy force does not depend on depth, even though pressure depends on depth.

1

2

3

45

6

Buoyancy

Buoyancy

Buoyancy

Only in the special case of floating does the buoyant force acting on an object equal the object’s weight.

A floating object

displaces a weight of

fluid equal to its own

weight.

Ice cubes

in water?

Figure 13.15

To control how high they float in the water

Crocs swallow stones

Fish use air bladders

Figure 13.17


Check Yourself

Did the designers of this “water bridge” have to account for the weight of ships or just the water?

Figure 13.20


Check Yourself

?

50 N

10 N Block

10 N Water

40 N Water

A floating

object

displaces a

weight of fluid

equal to its

own weight.

Archimedes’ PrincipleWeight of liquid displaced by floating or submerged

object equals the buoyant force on the object.

Floating & Liquid

Density

Floating in Great Salt Lake, Utah is easy because the lake water is

dense due to high concentration of salt.

Pascal’s Principle

• When force applied to a confined fluid, an increase in pressure is transmitted equally to all parts of the fluid

A liquid completely filling a bottle exerts pressure in all directions

Figure 13.23

Surface

Tension

• adhesion- attracted to unlike

• Cohesion- attracted to like

contractive tendency of

the surface of liquids

Surface tension causes

Buoyancy


Buoyancy in Air

Objects can float in air, just as they float in water, if the objects’ average density is less than the density of air.

Hot Air BalloonHelium-filled Blimp

Zeppelins

Hindenburg was 10 times longer than today’s blimps

Atmosphere

• The thickness is determined by• kinetic energy From sun) -tends to spread the molecules

apart; tend to fly away: if disappeared- molecules moved too slowly our “atmosphere” would be a liquid or solid layer

• gravity, which tends to hold molecules near the Earth. shut off= dissipate and disappear.

• height of the atmosphere• gets thinner and thins out to emptiness in interplanetary

space.

AtmosphereWe live at the

bottom of an ocean of fluid—the fluid is air & “ocean” is the atmosphere.


Demo: Stop the Funnel

Water will not enter an air-tight container.

H

A

Weight

A

Weight

A

ABlock exit hole


Atmospheric Pressure

Demo: Magdeburg Hemispheres

A A

AA

L

A

L

- Atmospheric pressure

- Low pressure

Barometer Any device that measures

atmospheric pressure.

• A simple mercury barometer:

The vertical height of the

mercury column remains

constant even when the tube is

tilted, unless the top of the tube

is less than 76 centimeters

above the level in the dish—in

which case the mercury

completely fills the tube.

Demo: Drinking Straw

L

A AWeight


Prairie Dog VacuumPrairie dogs captured by giant vacuum truck

with a padded bin.

L

A

Force

There is a 10.3-meter limit on the height that water can be lifted with vacuum pumps.

At sea level, however strong your lungs may

be, or whatever device you use to make a

vacuum in the straw, the water cannot be

pushed up by the atmosphere higher than 10.3

m.

Boyle’s Law

Density of a gas increases as it is compressed (volume decreased)

As the density of a gas increases, the pressure in the gas also increases.

P1V1 = P2V2

Compress the gas by pushing in the syringe. Dial gauge shows increase in pressure.

SyringeGauge

Tank

Boyle’s Law

• A flat tire registers zero pressure on the gauge, but a pressure of about one atmosphere exists there. Gauges read “gauge” pressure—pressure greater than atmospheric pressure.

Double volume &

•½ pressure

•Dec speed of partaicles

•Dec. temp

Decrease volumeDensity and pressure are increased

P1V1 = P2V2

applies to ideal gases.

Archimedes’ principle

• holds for air just as it does for water: An object surrounded by air is buoyed up by a force equal to the weight of the air displaced.

Bernoulli’s principle

Blow the RoofIf wind blows hard

enough the low pressure above can create a large enough force to lift the roof off.

New Orlean’s Superdome after hurricane Katrina

L

A

Even a small pressure difference over a large roof area can produce a large upward “lifting” force


Check YourselfWind blowing over the ocean causes waves to build due to Bernoulli’s principle.Where is the pressure lowered?

• Airfoil - shape of wing

• Lift- upward force which results when

the pressure on upper surface is less

• Drag- opposing force to forward

motion

• Thrust - force which pushes the plane forward

provided by plane’s propeller/jet

Airplane Wing

L

A

Wing

LIFT FORCE

Curving may be increased by threads or fuzz, which help to drag a thin layer of air with the ball and to produce further

crowding of streamlines on one side.

Bernoulli’s principle

• How does this relate to fireplaces?

• On a windy day, waves in a lake are higher than normal. Why?

Plasma

• a plasma (different from a gas)

• readily conducts electric current,

• it absorbs certain kinds of radiation that

pass unhindered through a gas,

• it can be shaped, molded, and moved by

electric and magnetic fields.

Plasma PowerMHD power, the magnetohydrodynamic interaction

between a plasma and a magnetic field.

• Low-pollution MHD power is in operation at a few places in the world already.

• operate at high temperatures without moving parts

• Fusion power may not only make electrical energy abundant, but it may also provide the energy and means to recycle and even synthesize elements

http://en.wikipedia.org/wiki/Moving_parts

Pascal’s Principle

•Explains hydraulic systems•Pressure is exerted equally throughout a closed container•Hydraulic systems multiply force (over a greater distance)

Bernoulli’s Principle

Pressure exerted on a moving steam of

fluid is less than the pressure of the

surrounding fluid•Explains Flight

Archimedes’ Principle

(Sinks) Buoyant force on an object is equal to the

weight of the fluid displaced

If an object floats- the volume of displaced water = volume of the portion of

the object that is submerged.

chapter 14 gasses & plasmas weight of air is small but not negligible. for example, weight of...

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