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Balloons 2
Introductory QuestionIntroductory Question
A helium balloon has mass, yet it A helium balloon has mass, yet it doesn’t fall to the floor. Is there a doesn’t fall to the floor. Is there a real force pushing up on the helium real force pushing up on the helium balloon?balloon?
A.A. YesYes
B.B. NoNo
Balloons 4
5 Questions about 5 Questions about BalloonsBalloons
How does air “inflate” a rubber How does air “inflate” a rubber balloon?balloon?
Why doesn’t the atmosphere Why doesn’t the atmosphere collapse?collapse?
Does air around a balloon exert a Does air around a balloon exert a force on it?force on it?
Why does a hot air balloon float in Why does a hot air balloon float in cold air?cold air?
Why does a helium balloon float in Why does a helium balloon float in air?air?
Balloons 5
Question 1Question 1
How does air “inflate” a rubber How does air “inflate” a rubber balloon?balloon? How does air occupy space?How does air occupy space? How does it push on the balloon’s How does it push on the balloon’s
elastic skin?elastic skin?
Balloons 6
Air’s CharacteristicsAir’s Characteristics
Air is a gasAir is a gas It consists of individual atoms and It consists of individual atoms and
moleculesmolecules Its particles are kept separate by Its particles are kept separate by
thermal energythermal energy Its particles bounce around in free fallIts particles bounce around in free fall
Balloons 7
Air and PressureAir and Pressure
Air has pressureAir has pressure Air particles transfer momentum when Air particles transfer momentum when
they bouncethey bounce Each momentum transfer/bounce Each momentum transfer/bounce
involves forcesinvolves forces Air particles exerts forces on container Air particles exerts forces on container
wallswalls Average force is proportional to surface Average force is proportional to surface
areaarea Average force per unit ofAverage force per unit of
area is called “pressure”area is called “pressure”
Balloons 8
Air and DensityAir and Density
Air has densityAir has density Air particles have massAir particles have mass Each volume of air has a massEach volume of air has a mass Average mass per unit ofAverage mass per unit of
volume is called “density”volume is called “density”
Balloons 9
Air Pressure and DensityAir Pressure and Density
Air pressure is proportional to Air pressure is proportional to densitydensity Denser particles hit surface more oftenDenser particles hit surface more often Denser air Denser air more pressure more pressure
Balloons 10
Pressure ImbalancesPressure Imbalances
Balanced pressure exerts no overall Balanced pressure exerts no overall forceforce Forces on balloon’s sides cancelForces on balloon’s sides cancel
Unbalanced pressure exerts overall Unbalanced pressure exerts overall forceforce Forces on balloon’s sides don’t cancelForces on balloon’s sides don’t cancel Forces push balloon toward lower Forces push balloon toward lower
pressurepressure Air pressure also pushes on the air Air pressure also pushes on the air
itselfitself Air itself is pushed toward lower Air itself is pushed toward lower
pressurepressure
Balloons 11
Question 2Question 2
Why doesn’t the atmosphere Why doesn’t the atmosphere collapse?collapse? If air has weight, why doesn’t it fall to If air has weight, why doesn’t it fall to
the ground?the ground? Why is the air denser at lower Why is the air denser at lower
altitudes?altitudes?
Balloons 12
The AtmosphereThe Atmosphere Air near the ground supports the air overheadAir near the ground supports the air overhead
The atmosphere is in stable equilibriumThe atmosphere is in stable equilibrium Air pressure decreases with altitudeAir pressure decreases with altitude Each layer of air experiences an upward forceEach layer of air experiences an upward force This upward force balances the layer’s weightThis upward force balances the layer’s weight
Supporting itself structures the atmosphereSupporting itself structures the atmosphere Air pressure is highest near the groundAir pressure is highest near the ground Air density is highest near the groundAir density is highest near the ground
Balloons 13
Question 3Question 3
Does air around a balloon exert a Does air around a balloon exert a force on it?force on it?
Balloons 14
The Buoyant ForceThe Buoyant Force
Because of the atmosphere’s Because of the atmosphere’s structure,structure, air pressure isair pressure is
stronger near the bottom of a balloon,stronger near the bottom of a balloon, weaker near the top of the balloon,weaker near the top of the balloon,
so air pushes up harder than it pushes so air pushes up harder than it pushes down,down,
and this imbalance yields an upward and this imbalance yields an upward buoyant forcebuoyant force
The atmosphere pushes upward on a The atmosphere pushes upward on a balloon!balloon!
Balloons 15
Archimedes’ PrincipleArchimedes’ Principle
A balloon immersed in a fluid A balloon immersed in a fluid experience an upward buoyant force experience an upward buoyant force equal to the weight of the fluid it equal to the weight of the fluid it displacesdisplaces
Balloons 16
Question 4Question 4
Why does a hot air balloon float in Why does a hot air balloon float in cold air?cold air? Why does a cold air balloon sink in cold Why does a cold air balloon sink in cold
air?air? What is the difference between hot air What is the difference between hot air
and cold air?and cold air?
Balloons 17
Room-Air Balloon in AirRoom-Air Balloon in Air A rubber balloon filled with room airA rubber balloon filled with room air
weighs more than the room air it displacesweighs more than the room air it displaces experiences a downward net force in room airexperiences a downward net force in room air sinks in room airsinks in room air
Its average density > density of room airIts average density > density of room air
Balloons 18
Air and TemperatureAir and Temperature
Air pressure is proportional to Air pressure is proportional to temperaturetemperature Faster particles hit surface more and Faster particles hit surface more and
harderharder Hotter air Hotter air more pressure more pressure
Balloons 19
An Aside About An Aside About TemperatureTemperature
Like most things, air has a Like most things, air has a temperaturetemperature Air particles have thermal kinetic Air particles have thermal kinetic
energyenergy Average thermal kinetic energy is Average thermal kinetic energy is
proportional to absolute temperatureproportional to absolute temperature SI absolute temperature: kelvins or SI absolute temperature: kelvins or
KK 0 K is absolute zero — no thermal 0 K is absolute zero — no thermal
energy leftenergy left Step size: 1 K step same as 1 °C stepStep size: 1 K step same as 1 °C step
Balloons 20
Hot-Air Balloon in AirHot-Air Balloon in Air
A rubber balloon filled with hot airA rubber balloon filled with hot air contains fewer air particles than if it contains fewer air particles than if it
were coldwere cold weighs less than the room air it weighs less than the room air it
displacesdisplaces experiences an upward net force in experiences an upward net force in
room airroom air floats in room airfloats in room air
Its average density < density of Its average density < density of room airroom air
Balloons 21
Question 5Question 5
Why does a helium balloon float in Why does a helium balloon float in air?air? How does helium differ from air?How does helium differ from air? Doesn’t helium have mass and weight?Doesn’t helium have mass and weight?
Balloons 22
Helium vs. AirHelium vs. Air
Replacing air particles with helium Replacing air particles with helium atomsatoms leaves particle density unchangedleaves particle density unchanged
all particles contribute equally to pressureall particles contribute equally to pressure reduces the gas’s densityreduces the gas’s density
helium atoms are less massive than air helium atoms are less massive than air particlesparticles
leaves the gas’s pressure unchangedleaves the gas’s pressure unchanged helium atoms travel faster & hit more oftenhelium atoms travel faster & hit more often
Balloons 23
Helium Balloon in AirHelium Balloon in Air
A rubber balloon filled with heliumA rubber balloon filled with helium has same particle density as airhas same particle density as air weighs less than the air it displacesweighs less than the air it displaces experiences an upward net force in airexperiences an upward net force in air floats in airfloats in air
Its average density < density of Its average density < density of room airroom air
Balloons 24
Introductory Question Introductory Question (revisited)(revisited)
A helium balloon has mass, yet it A helium balloon has mass, yet it doesn’t fall to the floor. Is there a doesn’t fall to the floor. Is there a real force pushing up on the helium real force pushing up on the helium balloon?balloon?
A.A. YesYes
B.B. NoNo
Balloons 25
The Ideal Gas LawThe Ideal Gas Law
This law is a summary relationship This law is a summary relationship for gases:for gases:
pressure = Boltzmann constant x pressure = Boltzmann constant x
particle density xparticle density x
absolute temperatureabsolute temperature
Assumes, however, perfectly Assumes, however, perfectly independent particlesindependent particles
Real particles aren’t perfectly Real particles aren’t perfectly independentindependent
Balloons 26
Summary about BalloonsSummary about Balloons
Balloons float when their average Balloons float when their average densities are less than that of airdensities are less than that of air
Helium balloons float because Helium balloons float because helium atoms are lighter than air helium atoms are lighter than air particlesparticles
Hot-air balloons float because hot air Hot-air balloons float because hot air has lower particle density than cold has lower particle density than cold airair