Physics 101: Lecture 23, Pg 1 Physics 101: Lecture 23 Fluids: Gases and Liquids l Today’s lecture will cover Textbook Sections è Density è Pressure

Physics 101: Lecture 23, Pg 2 l Density = Mass/Volume è  = M/V SI unit: [kg/m 3 ] l Densities of some common things (in kg/m 3 ): è Water (at 4 degrees Celsius) 1000 è ice 917(floats on water) è blood 1060(sinks in water) è lead 11,300 è Copper 8890 è Mercury 13,600 è Aluminum 2700 è Wood (yellow pine) 550 è air 1.29 è Helium Density For comparisons of densities we use: Specific gravity = density of substance/density of water

Physics 101: Lecture 23, Pg 3 Pressure l The magnitude of a force acting perpendicular to a surface per unit area is called pressure: P = F / A SI Unit: [N/m 2 ] N/m 2 is also called Pascal [Pa] Pa = 1 bar of pressure Because of pressure any gas or liquid applies a force perpendicular to the surface with which it comes in contact. The air above the surface of the earth creates a pressure at sea level of (=atmospheric pressure) x 10 5 Pa = 1 atmosphere

Physics 101: Lecture 23, Pg 4 Pressure and Depth in a Static Fluid l In a static fluid when knowing the pressure at a certain depth, P 1 at d 1, the pressure at a larger depth, P 2 at d 2, can be calculated from the increase in pressure due to the weight of the fluid that is located between d 1 and d 2 : P 2 A = P 1 A + m g = P 1 A +  V g P 2 = P 1 +  h g (h=d 2 -d 1 ) Here we assumed that the fluid is not compressible, i.e. its density is the same at any depth.

Physics 101: Lecture 23, Pg 5 Pressure and Depth Barometer: a way to measure atmospheric pressure P 2 = P 1 +  g h P atm =  g h  Measure h, determine P atm or Example: Mercury  = 13,600 kg/m 3 P atm = 1.05 x 10 5 Pa  h = m = 757 mm = 29.80” (for 1 atm) Open tube manometer: P 1 = P atm  P 2 -P atm = ρ g h : gauge pressure P2 : absolute pressure h P 2 =P atm P 1 =0

Physics 101: Lecture 23, Pg 6 Concept Question Suppose you have a barometer with mercury and a barometer with water. How does the height h water compare with the height h mercury ? 1. h water is much larger than h mercury 2. h water is a little larger than h mercury 3. h water is a little smaller than h mercury 4. h water is much smaller than h mercury CORRECT water is much less dense than mercury, so the same amount of pressure will move the water farther up the column. h P 2 =P atm P 1 =0

Physics 101: Lecture 23, Pg 7 Concept Question Evacuate the straw by sucking. How high will the water rise? no more than h = P a /  g = 10.3m = 33.8 feet no matter how hard you suck! h P a P=0

Physics 101: Lecture 23, Pg 8 Concept Question Is it possible to stand on the roof of a five story (50 foot) tall house and drink, using a straw, from a glass on the ground? 1. No 2. Yes Even if a person could completely remove all of the air from the straw, the height to which the outside air pressure moves the water up the straw would not be high enough for the person to drink the water. CORRECT

Physics 101: Lecture 23, Pg 9 Summary Density:  = M/V Pressure: P = F/A P 2 = P 1 +  g h