1. Chapter 12 Gases Gases, Liquids and Soils

2. 12.1 Liquids and Gases Similarities: Both do not have an absolute shape Both are “fluids” because it can flow from one place to another. Differences: When put into a container, a fluid will not equally fill a container, like a gas will do. In this chapter, we will discuss both gases and liquids

3. Atmospheric Pressure If you hold your hand out in front of you, atmospheric pressure is pushing both up and down on your hand with an equal force. Also acts on the sides of your hands equally. The pressure of a gas (or any fluid) acts equally in all directions. Pressure is always at right angles to any surface it acts on.

4. Physics Demonstrations https://www.youtube.com/watch?v=eLcyhT4Oly8

5. Pressure Equilibrium When you get a flat tire, the pressure is not equal to zero, rather it is equal to the atmospheric pressure. When you patch a tire, you inflate the tire to 241 kPa. The true value of the pressure inside your tires is the atmospheric pressure plus the 241 kPa. The increased pressure inside the tires wants to push air out (equilibrium) so it pushes outside on the walls of the tire, supporting the weight of your car. Gauge pressure: P gauge = P – P atmospheric Atmospheric pressure is 101 KPa

6. Force and Pressure Remember… Pressure gauge = Force/Area Example: You push down on a ball with a force of 22 N and the ball flattens out with a circular diameter of 2.0 cm. What is a gauge pressure of the ball?

7. Ideal Gas Law An ideal gas is when the particles have no effect on one another. Main concepts: 1. When the number of particles increase, so does the pressure 2. When the volume is decreased, there are more collisions, so the pressure increases. 3. The higher the temperature, the more collisions of the particles, leading to an increase in pressure. Pressure= k { (number of gas particles X temperature of gas)/(volume of gas) } Boltzmann Constant: K=1.38 X 10 23 SI units: J/K

8. Alternate Equation PV=NkT P= pressure V=Volume N= particles of gas T= temperature Rearrange to find unknown values

9. Ideal Gas and Moles Sometimes it is more convenient to talk about the amount of gas in weight, rather than particles (makes more sense to us) To do this, we use moles (mol) Use Avogadro's number : 6.02 X 10 23 molecules/mol N=Number of particles=number of moles X Avogadro’s number Ideal Gas Law (In terms of Mole): PV=NRT P= Pressure V=volume N=number of moles R= Universal gas constant (8.31 J/mol K) T= Temperature

10. 12.2 Fluids at Rest Density=mass/volume Units: kg/m3 Rearrange this formula to find the mass or volume of an object, using the know densities The more dense an object, the greater the object weighs Different gases and liquids have very different densities Table in your book pg. 425

11. The pressure of a fluid increases with depth As you go deeper in the ocean, there is a larger amount of water pressing down on you, leading to an increase in pressure. Pressure at a certain depth (h) is equal to the atmospheric pressure, plus the density times the acceleration due to gravity times the depth P h = P atmospheric + (density X acceleration X depth)

12. Titanic Example: The Titanic was found in 1985 on the bottom of the ocean at a depth of 4,000m. What is the pressure at this depth? Density of sea water= 1025 kg/m3 P=1.01 X 10 5 + (1025 X 9.81 X 4000m) P= 4.0 X 10 7 Pa This is roughly 6000 lb/inch 2

13. Dependence of Pressure on Depth If you are comparing two points, you can use a different equation to find pressure P 2 = P 1 + (density X gravity X height) Ex. A cubical box that is 20 cm on a side is completely immersed in fluid. At the top of the box the pressure is 105.0 kPa. At the bottom of the box the pressure is 106.8 kPa. What is the density of the fliud?

14. Barometers Measure Atmospheric Pressure The difference in height is directly proportional to the atmospheric pressure that pushes down on the fluid. 1 atmosphere= pressure atmospheric = 760 mmHg Atmospheric pressure is due to the weight of air above our heads pushing downward. At high elevations there is less pressure pushing down, so the air pressure is lower. Changing of atmospheric pressure quickly leads to “popping” ears Low air pressure causes soda to expand…WHY?

15. Pascal’s Principle An external pressure applied to an enclosed fluid is transmitted unchanged to every point within the fluid. Hydraulic Lift: if you displace a volume of liquid, it causes an increase in the volume on the other side. (A1)(D1)=(A2)(D2) Can rearrange this equation to solve for unknown values

16. Buoyant force The upward force that makes objects float Objects float when the buoyant force is equal to the weight of the object Can use displacement to find the density of an object

17. Homework Due Tuesday: Section 12.1 Pg. 423 13, 14, 15, 16, 17, 18 Pg. 446 62-68 even Section 12.2 Pg. 434 32, 34, 35, 36