1. Balloons 1 Balloons

2. Balloons 2 Introductory Question A helium balloon has mass, yet it doesn’t fall to the floor. Is there a real force pushing up on the helium balloon? A helium balloon has mass, yet it doesn’t fall to the floor. Is there a real force pushing up on the helium balloon? A. Yes B. No

3. Balloons 3 Observations about Balloons Balloons are held taut by the gases inside Balloons are held taut by the gases inside Some balloon float in air while others don’t Some balloon float in air while others don’t Hot-air balloons don’t have to be sealed Hot-air balloons don’t have to be sealed Helium balloons “leak” even when sealed Helium balloons “leak” even when sealed

4. Balloons 4 5 Questions about Balloons How does air “inflate” a rubber balloon? How does air “inflate” a rubber balloon? Why doesn’t the atmosphere collapse? Why doesn’t the atmosphere collapse? Does air around a balloon exert a force on it? Does air around a balloon exert a force on it? Why does a hot air balloon float in cold air? Why does a hot air balloon float in cold air? Why does a helium balloon float in air? Why does a helium balloon float in air?

5. Balloons 5 Question 1 How does air “inflate” a rubber balloon? How does air “inflate” a rubber balloon? How does air occupy space? How does air occupy space? How does it push on the balloon’s elastic skin? How does it push on the balloon’s elastic skin?

6. Balloons 6 Air’s Characteristics Air is a gas Air is a gas It consists of individual atoms and molecules It consists of individual atoms and molecules Its particles are kept separate by thermal energy Its particles are kept separate by thermal energy Its particles bounce around in free fall Its particles bounce around in free fall

7. Balloons 7 Air and Pressure Air has pressure Air has pressure Air particles transfer momentum when they bounce Air particles transfer momentum when they bounce Each momentum transfer/bounce involves forces Each momentum transfer/bounce involves forces Air particles exerts forces on container walls Air particles exerts forces on container walls Average force is proportional to surface area Average force is proportional to surface area Average force per unit of area is called “pressure” Average force per unit of area is called “pressure”

8. Balloons 8 Air and Density Air has density Air has density Air particles have mass Air particles have mass Each volume of air has a mass Each volume of air has a mass Average mass per unit of volume is called “density” Average mass per unit of volume is called “density”

9. Balloons 9 Air Pressure and Density Air pressure is proportional to density Air pressure is proportional to density Denser particles hit surface more often Denser particles hit surface more often Denser air  more pressure Denser air  more pressure

10. Balloons 10 Pressure Imbalances Balanced pressure exerts no overall force Balanced pressure exerts no overall force Forces on balloon’s sides cancel Forces on balloon’s sides cancel Unbalanced pressure exerts overall force Unbalanced pressure exerts overall force Forces on balloon’s sides don’t cancel Forces on balloon’s sides don’t cancel Forces push balloon toward lower pressure Forces push balloon toward lower pressure Air pressure also pushes on the air itself Air pressure also pushes on the air itself Air itself is pushed toward lower pressure Air itself is pushed toward lower pressure

11. Balloons 11 Question 2 Why doesn’t the atmosphere collapse? Why doesn’t the atmosphere collapse? If air has weight, why doesn’t it fall to the ground? If air has weight, why doesn’t it fall to the ground? Why is the air denser at lower altitudes? Why is the air denser at lower altitudes?

12. Balloons 12 The Atmosphere Air near the ground supports the air overhead Air near the ground supports the air overhead The atmosphere is in stable equilibrium The atmosphere is in stable equilibrium Air pressure decreases with altitude Air pressure decreases with altitude Each layer of air experiences an upward force Each layer of air experiences an upward force This upward force balances the layer’s weight This upward force balances the layer’s weight Supporting itself structures the atmosphere Supporting itself structures the atmosphere Air pressure is highest near the ground Air pressure is highest near the ground Air density is highest near the ground Air density is highest near the ground

13. Balloons 13 Question 3 Does air around a balloon exert a force on it? Does air around a balloon exert a force on it?

14. Balloons 14 The Buoyant Force Because of the atmosphere’s structure, Because of the atmosphere’s structure, air pressure is air 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 down, so air pushes up harder than it pushes down, and this imbalance yields an upward buoyant force and this imbalance yields an upward buoyant force The atmosphere pushes upward on a balloon! The atmosphere pushes upward on a balloon!

15. Balloons 15 Archimedes’ Principle A balloon immersed in a fluid experience an upward buoyant force equal to the weight of the fluid it displaces A balloon immersed in a fluid experience an upward buoyant force equal to the weight of the fluid it displaces

16. Balloons 16 Question 4 Why does a hot air balloon float in cold air? Why does a hot air balloon float in cold air? Why does a cold air balloon sink in cold air? Why does a cold air balloon sink in cold air? What is the difference between hot air and cold air? What is the difference between hot air and cold air?

17. Balloons 17 Room-Air Balloon in Air A rubber balloon filled with room air A rubber balloon filled with room air weighs more than the room air it displaces weighs more than the room air it displaces experiences a downward net force in room air experiences a downward net force in room air sinks in room air sinks in room air Its average density > density of room air Its average density > density of room air

18. Balloons 18 Air and Temperature Air pressure is proportional to temperature Air pressure is proportional to temperature Faster particles hit surface more and harder Faster particles hit surface more and harder Hotter air  more pressure Hotter air  more pressure

19. Balloons 19 An Aside About Temperature Like most things, air has a temperature Like most things, air has a temperature Air particles have thermal kinetic energy Air particles have thermal kinetic energy Average thermal kinetic energy is proportional to absolute temperature Average thermal kinetic energy is proportional to absolute temperature SI absolute temperature: kelvins or K SI absolute temperature: kelvins or K 0 K is absolute zero — no thermal energy left 0 K is absolute zero — no thermal energy left Step size: 1 K step same as 1 °C step Step size: 1 K step same as 1 °C step

20. Balloons 20 Hot-Air Balloon in Air A rubber balloon filled with hot air A rubber balloon filled with hot air contains fewer air particles than if it were cold contains fewer air particles than if it were cold weighs less than the room air it displaces weighs less than the room air it displaces experiences an upward net force in room air experiences an upward net force in room air floats in room air floats in room air Its average density < density of room air Its average density < density of room air

21. Balloons 21 Question 5 Why does a helium balloon float in air? Why does a helium balloon float in 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?

22. Balloons 22 Helium vs. Air Replacing air particles with helium atoms Replacing air particles with helium atoms leaves particle density unchanged leaves particle density unchanged all particles contribute equally to pressure all particles contribute equally to pressure reduces the gas’s density reduces the gas’s density helium atoms are less massive than air particles helium atoms are less massive than air particles leaves the gas’s pressure unchanged leaves the gas’s pressure unchanged helium atoms travel faster & hit more often helium atoms travel faster & hit more often

23. Balloons 23 Helium Balloon in Air A rubber balloon filled with helium A rubber balloon filled with helium has same particle density as air has same particle density as air weighs less than the air it displaces weighs less than the air it displaces experiences an upward net force in air experiences an upward net force in air floats in air floats in air Its average density < density of room air Its average density < density of room air

24. Balloons 24 Introductory Question (revisited) A helium balloon has mass, yet it doesn’t fall to the floor. Is there a real force pushing up on the helium balloon? A helium balloon has mass, yet it doesn’t fall to the floor. Is there a real force pushing up on the helium balloon? A. Yes B. No

25. Balloons 25 The Ideal Gas Law This law is a summary relationship for gases: This law is a summary relationship for gases: pressure = Boltzmann constant x particle density x particle density x absolute temperature Assumes, however, perfectly independent particles Assumes, however, perfectly independent particles Real particles aren’t perfectly independent Real particles aren’t perfectly independent

26. Balloons 26 Summary about Balloons Balloons float when their average densities are less than that of air Balloons float when their average densities are less than that of air Helium balloons float because helium atoms are lighter than air particles Helium balloons float because helium atoms are lighter than air particles Hot-air balloons float because hot air has lower particle density than cold air Hot-air balloons float because hot air has lower particle density than cold air