Chapter 14 Gasses & Plasmas

Apr 20, 2023 Physics 1 (Garcia) SJSU

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).

A pressure of 1 atm

=1.013 25 bar

= 101.325 kilopascal (kPa)

= 1013.25 millibars (mbar)

= 760 torr

= 14.7 pounds/in 2 (psi)

Pressure is depth dependent, not volume dependent.

Gases as well as liquids flow; hence, both are called fluids.

• In a gas, the molecules are far apart and free from the cohesive forces that dominate their motions when in the liquid and solid phases.

Apr 20, 2023 Physics 1 (Garcia) SJSU

The force exerted by a fluid on a smooth surface is always at right angles to the surface

• Liquid pressure is exerted equally in all directions

• The pressure isn’t only downward

• When liquid presses against a surface, there is a net force that is perpendicular to the surface.

• While pressure does not have a specific direction, force does.

Buoyancy • If the weight of the

submerged object is greater

than the buoyant force= sink

• If the weight is less than

buoyant= float

• When the weight is = to

buoyant force = remain at

same level (won’t sink or

rise)

Apr 20, 2023

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

Apr 20, 2023 Physics 1 (Garcia) SJSU

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.

principle of flotation:

A floating object displaces a weight of fluid equal to its own

weight.

Apr 20, 2023

Check Yourself

?

Place block of wood in the water. Scale reading goes up, down, or stays the same?

50 N

10 N Block

10 N Water

40 N Water

A floating

object

displaces a

weight of fluid

equal to its

own weight.

Apr 20, 2023

Archimedes’ PrincipleWeight of liquid displaced by floating or submerged

object equals the buoyant force on the object.

Pascal’s Principle

• When force is 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 13.22 • The additional pressure is exerted against every square centimeter of the larger piston. Since there is 50 times the area, 50 times as much force is exerted on the larger piston. Thus, the larger piston will support a 500-kg load—fifty times the load on the smaller piston!

the piston on the right

has 50 times the area

of the piston on the left

(let’s say that the left

piston has a cross-

sectional area of 100

square centimeters

and that the right

piston has a cross-

sectional area of 5000

square centimeters).

Hydraulic devise

•Multiplies a force by applying the force to a small surface area.

•The increase in pressure is then transmitted to another part of a confined fluid, which pushes on a larger surface area

Surface

Tension

• When the bent wire is lowered into the water

and then raised, the spring will stretch

because of surface tension.

• adhesion- attracted to unlike

• Cohesion- attracted to like

contractive tendency of

the surface of liquids

• These molecular attractions thus tend to pull the molecule from the surface into the liquid, and this tendency minimizes the surface area.

 Surface tension is caused by molecular

attractions.

Beneath the surface, each molecule is

attracted in every direction by

neighboring molecules, resulting in no

tendency to be pulled in any specific

direction.

A molecule on the surface of a liquid,

however, is pulled only by neighbors on

each side and downward from below;

there is no pull upward.

The water surface sags like a piece of

plastic wrap, which allows certain

insects, such as water striders, to run

across the surface of a pond.

Surface tension causes • a thin film of water to be drawn up over the inner and outer surfaces of the tube. It causes this film to contract.

• The film on the outer surface contracts enough to make a rounded edge.

• The film on the inner surface contracts more and raises water with it until the adhesive force is balanced by the weight of the water lifted

BuoyancyThe pressure at the bottom of a submerged object is greater than the pressure at the top. The result is a net force in the upward direction.

What is this upward force called?

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. •There is no outer edge

Apr 20, 2023 Physics 1 (Garcia) SJSU

Apr 20, 2023 Physics 1 (Garcia) SJSU

Atmosphere

Density of air in the atmosphere decreases with increasing altitude.

Most of atmosphere in the first 10 km (about 6 miles) of altitude.

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

Apr 20, 2023 Physics 1 (Garcia) SJSU

Atmospheric Pressure

Atmospheric column of airBase: 1 square meterHeight: 10 kilometers (10,000 m)Volume: 10,000 cubic metersMass: 10,000 kilogramsWeight: 100,000 Newtons

(= 22,000 lb = 11 tons)Pressure: 100,000 Pascals

(= 15 lb per sq. inch)

Apr 20, 2023

Magdeburg Hemispheres

Pair of hemispheres fit together with air-tight seal

Most of the air is pumped out from the interior.

Air pressure holds the two pieces tightly together.

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.

• Whatever the width of the tube, a 76-centimeter column of mercury weighs the same as the air that would fill a super-tall 30-kilometer tube of the same width.

• A water barometer would be —13.6 times as long, or 10.3 meters high—too tall to be practical.

• You may recognize this number as the density of mercury relative to that of water    

Strictly speaking, these two do not suck the soda up the straws.

• They instead reduce pressure in the straws and allow the weight of the atmosphere to press the liquid up into the straws. Could they drink a soda this way on the Moon?

Apr 20, 2023

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.

• “hard vacuum- orbiting in space- satellites can reach 10−13 Pa.

• A vacuum pump simply provides a region of lower pressure into which fast-moving gas molecules randomly move.

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.

Boyle’s Law

• Boyle’s law applies to ideal

gases. An ideal gas is one in

which the disturbing effects of

the forces between molecules

and the finite size of the

individual molecules can be

neglected. Air and other gases

under normal pressures

approach ideal-gas conditions.

    

• the product of pressure and volume for a given mass of gas is a constant as long as the temperature doesn’t change

• pressure and volume are inversely proportional;

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.

• Unlike water, the atmosphere has no definable surface. There is no “top.” Furthermore, unlike water, the atmosphere becomes less dense with altitude

• If less dense at that altitude it will rise till the force inside balloon is equal to pressure outside.

Rising motion of balloons are caused by

unbalanced forces.

• The air beneath exerts more

force than the air above.

• When it reaches an altitude

where the forces are equal, it

stops rising.

•

Apr 20, 2023 Physics 1 (Garcia) SJSU

Plasma An electrified gas containing ions and free electrons. Most of the matter in the universe is in the plasma phase.

How high will a balloon rise?At ground level the balloon is partially

inflated, but at high altitues it is full when the surrounding pressure is less.

• (1) A balloon will rise only so long as it displaces a weight of air greater than its own weight. Air becomes less dense with altitude, so, when the weight of displaced air equals the total weight of the balloon, upward acceleration of the balloon ceases.

• (2) When the buoyant force on the balloon equals its weight, the balloon will cease to rise.

• (3) When the average density of the balloon (including its load) equals the density of the surrounding air, the balloon will cease rising

fluids in motion—fluid dynamics.

• Bernoulli’s principle When the

speed of a fluid increases, internal pressure in the fluid decreases.

• As a consequence of continuous flow, the water will slow down in the wide parts and speed up in the narrow parts.

Apr 20, 2023 Physics 1 (Garcia) SJSU

Bernoulli’s principle

Air pressure above the roof is less than air pressure beneath the roof.

• Consider wind blowing across a peaked

roof. The wind speeds up as it flows over

the top, as the crowding of streamlines in

Figure 14.20 indicates.

• Pressure along the streamlines is

reduced where they are closer together.

• Unless the building is well vented, greater

pressure inside and beneath the roof can

push it off.

• .

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

• a) The streamlines are the same on either side of a nonspinning baseball.

• (b) A spinning ball produces a crowding of streamlines. The resulting “lift” (red arrow) causes the ball to curve, as shown by the blue arrow.

• Air flowing over the top of the wing travels farther than the air on the bottom in the same amount of time.

 Bernoulli’s principle plays an important role for animals living in underground burrows.

• Entrances to their burrows are usually mound shaped, producing variations in wind speed across different entrances. This provides necessary pressure differences of air to enable circulation in the burrow

• How does this relate to fireplaces?

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

Plasma

• An electrified gas containing ions and free electrons.

• Most of the matter in the universe is in the plasma phase.

• The Sun and other stars are largely plasma

• Also fluorescent & neon lights, plasma TV, auroras

• 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

• • In submarines, low speed MHD generators using liquid metals would be nearly silent, eliminating a source of tell-tale mechanism noise.

• In spacecraft and unattended locations, low-speed metallic MHD generators have been proposed as highly reliable generators, linked to solar, nuclear or isotopic heat sources.

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.