1 14 gases, pressure, Boyle’s Law & Bernoulli’s Principle RQ: 4, 13, 15, 17, 20. Exercises: 7, 56. Problems: 1, 2.

2 Gases (almost) independent molecular motion (an “ideal” gas supposes no interaction) expand to fill space “fluid” (but not liquid) exert “isotropic” pressure (same amount in all directions)

3 Archimedes’ Principle for Air Air provides a small buoyancy, noticeable when object has very low density Examples: helium or hot-air balloon. TemperatureDensity (kg/m 3 ) 0°C (32°F) °C (50°F) °C (68°F) °C (86°F)1.16

4 Boyle’s Law for a confined gas, (pressure)x(volume) = constant, i.e., PV = constant. Ex. A balloon squeezed to half its original size has double its internal pressure. Reasoning: in a smaller container, the molecules will hit the walls more often, exerting more force on them.

5 Bernoulli’s Principle When the speed of a fluid* increases, internal pressure in the fluid* decreases. *or gas

6 Why does speed decrease pressure? before the air blows, air molecules exert isotropic pressure, moving in all directions. air motion removes the isotropy, i.e. gives a preferred direction for air molecules at top of tube. The air molecules below are able to move up and out with the stream, leaving a vacuum behind.

7 Examples: Roof, Ball

8 Bernoulli’s Principle: Curve Ball

9 Bernoulli’s Principle: Currents

10 summary gases exert isotropic pressure, but loose this effect when flowing (Bernoulli). gases exert a small buoyant force on all objects confined gases have constant product of pressure and volume

11 Plasma plasma: gas containing ions and free electrons

12 Atmospheric Pressure 101 kpascal = 101,000 pascal pascal = newton per square meter significance of differential pressure Example: 100kg is hung as shown. How large is the piston?

13 Gas Densities (at 0°C and 1 atm pressure) GasDensity (kg/m 3 ) hydrogen0.090 helium0.178 nitrogen1.25 oxygen1.43 air1.29

14 Gas Densities (at 0°C and 1 atm pressure) GasDensity (kg/m 3 ) hydrogen0.090 helium0.178 nitrogen1.25 oxygen1.43 air1.29

15 Air Density TemperatureDensity (kg/m 3 ) 0°C (32°F) °C (50°F) °C (68°F) °C (86°F)1.16