Fluids Chapter 13 & 14 Liquids & Gases

Transcription

1 Fluids Chapter 13 & 14 Liquids & Gases

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3 Liquids like solids are difficult to compress. Both liquids and gases can flow, so both are called fluids. The pressure you feel is due to the weight of water (or air) above you The pressure a liquid exerts depends on depth. Also depends on density. Liquid pressure = weight density X depth + pressure of atm. Except for small changes produced by temp., the density of a liquids is practically the same at all depths.

4 Pressure does not depend on the amount of liquid Volume is not the key- depth is Pressure is depth dependent, not volume dependent.

5 Figure 13.5 All vases are filled to equal depths, so the water pressure is the same at the bottom, regardless of its shape or volume

6 Once believed that you had to move water at a slow drop in gradient to move it. Now know water can flow uphill in a closed system if there is pressure behind it. Pipes in the ground can follow the natural slope of the land Garden hose full of water with one end dropped will spill water, but it will flow up hill

7 Liquid pressure is exerted equally in all directions The pressure isn t only downward When liquid presses against a surface, there is a net force that is perpendicular to the surface. While pressure does not have a specific direction, force does. The force exerted by a fluid on a smooth surface is always at right angles to the surface

8 Figure 13.2 Raising the water increases the pressure in all parts of the system

9 Pressure Pressure is defined as Pressure = ( Force ) ( Area ) Metric unit of pressure is Pascal. 1 Pascal = 1 Newtons per square meter Atmospheric pressure is about 100,000 Pascals 3-Dec-13 Physics 1 (Garcia) SJSU

10 Pressure in Liquids Pressure in a liquid depends on depth. As with bricks, weight of what s above determines pressure. Low Low Medium High Medium High 3-Dec-13 Physics 1 (Garcia) SJSU

11 As you move up & down in the atm & in water, the pressure changes Pressure is greatest the farther down you go Altitude is the most important factor in surface air pressure.

12 The strength of the stream of water coming out of the holes in the jug depends on the water pressure at each level. At which hole is the pressure greatest?

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14 Sample Problem Gold brick is roughly: 12 kilograms [Mass] 120 Newtons [Weight] (0.2 m)x(0.1 m) = (0.02 m 2 ) [Area] Pressure on surface is (120)/(0.02) = 6000 Pascals Note: Atmospheric pressure is about 100,000 Pascals so much more than pressure due to the gold brick. 3-Dec-13 Physics 1 (Garcia) SJSU

15 Check Yourself In which case is the pressure greatest? Although the weight of both blocks is the same, the upright block exerts greater pressure against the table A A B B 3-Dec-13 Physics 1 (Garcia) SJSU

16 Check Yourself In which case is the pressure greatest? A B A B 3-Dec-13 Physics 1 (Garcia) SJSU

17 Demo: Bed of Nails One may safely lay or sit on a bed of nails, as long as there are enough nails so that the pressure, measured as force per nail, is small. Weight of 150 pounds is distributed over 300 nails. Force per nail is ½ lb. Need 5 lb per nail to pierce skin. 3-Dec-13 Physics 1 (Garcia) SJSU

18 Demo: Pressure & Weight A can full of water has holes in the sides through which water comes out. What happens when you drop the can? The can is now in freefall and weightless. Water stops flowing as the can falls since the pressure was due to the water s weight. 3-Dec-13 Physics 1 (Garcia) SJSU

19 Pressure P = force/area The force of all the collisions Force-Measured in Newtons, N Area- m2 Pascal- SI unit of pressure = pascal, Pa, 1 N/m2

20 Fluid pressure in the Environment The atm. Pushes downward due to gravity causing atm pressure = Pa

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23 Buoyant Force Why do things sink or float?

24 If the weight of the submerged object is greater than the buoyant force= sink If the weight is less than buoyant= float When the eight is = to buoyant force = remain at same level (won t sink or rise)

25 Rising motion of balloons are caused by unbalanced forces. The air beneath exerts more force than the air above. When it reaches an altitude where the forces are equal, it stops rising.

26 Buoyancy Since pressure depends on depth, a submerged object has more force due to pressure below it than above it. Net effect is to have a net upward force, which we call buoyancy. Pressure Weight Buoyancy Pressure Pressure If weight exceeds buoyancy force then object sinks, otherwise it floats. 3-Dec-13 Physics 1 (Garcia) SJSU

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28 Buoyancy & Depth For a fully submerged object the buoyancy force does not depend on depth, even though pressure depends on depth Buoyancy 2 Buoyancy 5 Buoyancy 6 3-Dec-13 Physics 1 (Garcia) SJSU

29 Figure To control how high they float in the water Crocs swallow stones Fish use air bladders

30 Figure 13.10

31 Figure 13.18

32 Two solid blocks of identical size are submerged in water. One block is lead and the other is aluminum. Upon which is the buoyant force greater? The buoyant force is the same on each, for both blocks displace the same volume of water. For submerged objects, only the volume of water displaced, not the object s weight, determines the buoyant force.

33 Only in the special case of floating does the buoyant force acting on an object equal the object s weight. principle of flotation: A floating object displaces a weight of fluid equal to its own weight.

34 Figure The weight of a floating object equals the weight of the water displaced by the submerged part.

35 Check Yourself A floating object displaces a weight of fluid equal to its own weight. Place block of wood in the water. Scale reading goes up, down, or stays the same? 50 N 40 N Water 10 N Block? 10 N Water 3-Dec-13 Physics 1 (Garcia) SJSU

36 Check Yourself Use similar block of petrified wood. Scale reading goes up, down, or stays the same? 50 N 40 N Water 30 N Block? 10 N Water 3-Dec-13 Physics 1 (Garcia) SJSU

37 Figure The same ship empty and loaded. How does the weight of its load compare with the weight of extra water displaced?

38 The tip of a floating iceberg above the ocean s surface is approximately 10% of the whole iceberg. That s because ice is 0.9 times the density of water, so 90% of it submerges in water. Similarly, a mountain floats on the Earth s semiliquid mantle with only its tip showing. That s because Earth s continental crust is about 0.85 times the density of the mantle it floats upon; thus, about 85% of a mountain extends beneath the Earth s surface. So, like floating icebergs, mountains are appreciably deeper than they are high. Gravity is less at the top of a mountain because of the greater distance to Earth s center. Floating Mountains when mountains erode, they are lighter, and they are pushed up from below to float to nearly their original heights. So, when a kilometer of mountain erodes away, some 85% of a kilometer of mountain thrusts up from below. That s why it takes so long for mountains to weather away.

39 Archimedes Principle Weight of liquid displaced by floating or submerged object equals the buoyant force on the object. 3-Dec-13 Physics 1 (Garcia) SJSU

40 Density & Floating By Archimede s principle, a solid object will float if the density of the object is less than the density of the liquid. 3-Dec-13 Physics 1 (Garcia) SJSU Billiard ball (4.0 g/cm 3 ) floating in a cup of mercury (13.6 g/cm 3 )

41 Demo: Diet or Regular? What can you say about the density of diet cola as compared with regular cola? 3-Dec-13 Physics 1 (Garcia) SJSU

42 Demo: Take a Dip A 1 kg mass (1000 g) submerged in water displaces 150 cm 3. Water has density of 1 g/cm 3 so displaced water has mass of 150 grams. Buoyant force equals weight of 150 grams g 1000 g Beaker of Water 850 g 150 cc displaced 3-Dec-13 Physics 1 (Garcia) SJSU

43 Demo: Floating in the Tub Buoyant force depends only on the volume of fluid displaced, not on the volume of fluid in which the object is immersed. Rubber duck floats the same in a little water or in a full beaker of water 3-Dec-13 Physics 1 (Garcia) SJSU

44 Check Yourself Did the designers of this water bridge have to account for the weight of ships or just the water? 3-Dec-13 Physics 1 (Garcia) SJSU

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50 Floating & Liquid Density The greater the density of a liquid, the greater the buoyant force on objects floating or immersed in the liquid. Floating in Great Salt Lake, Utah is easy because the lake water is dense due to high concentration of salt. 3-Dec-13 Physics 1 (Garcia) SJSU

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54 Pressure exerted against the left piston will be transmitted throughout the liquid and against the bottom of the right piston. The pressure that the left piston exerts against the water will be exactly equal to the pressure the water exerts against the right piston Pascal s principle: A change in pressure at any point in an enclosed fluid at rest is transmitted undiminished to all points in the fluid.

55 Pascal s Principle When force applied to a confined fluid, an increase in pressure is transmitted equally to all parts of the fluid A liquid completely filling a bottle exerts pressure in all directions

56 Figure the piston on the right has 50 times the area of the piston on the left (let s say that the left piston has a crosssectional area of 100 square centimeters and that the right piston has a cross-sectional area of 5000 square centimeters). The additional pressure is exerted against every square centimeter of the larger piston. Since there is 50 times the area, 50 times as much force is exerted on the larger piston. Thus, the larger piston will support a 500-kg load fifty times the load on the smaller piston!

57 Pascal s Principle

58 Figure Increased air pressure produced by an air compressor is transmitted through the air to the surface of oil in an underground reservoir. The oil, in turn, transmits the pressure to a piston, which lifts the automobile. The relatively low pressure that exerts the lifting force against the piston is about the same as the air pressure in automobile tires Pascal s principle in a service station

59 Figure 13.24

60 Surface Tension contractive tendency of the surface of liquids When the bent wire is lowered into the water and then raised, the spring will stretch because of surface tension.

61 Figure Raindrops, drops of oil, and falling drops of molten metal are all spherical because their surfaces tend to contract and force each drop into the shape having the least surface area. This is a sphere Small blobs of water are drawn by surface tension into sphere-like shapes.

62 Surface tension is caused by molecular attractions. Beneath the surface, each molecule is attracted in every direction by neighboring molecules, resulting in no tendency to be pulled in any specific direction. A molecule on the surface of a liquid, however, is pulled only by neighbors on each side and downward from below; there is no pull upward. The water surface sags like a piece of plastic wrap, which allows certain insects, such as water striders, to run across the surface of a pond. These molecular attractions thus tend to pull the molecule from the surface into the liquid, and this tendency minimizes the surface area.

63 Figure The large bubble is elongated due to blowing, but it will quickly settle to a spherical shape due to surface tension.

64 Figure The attraction between unlike substances such as water and glass is called adhesion. The attraction between like substances, molecular stickiness, is called cohesion. When a glass tube is dipped into water, the adhesion between the glass and water causes a thin film of water to be drawn up over the inner and outer surfaces of the tube

65 a thin film of water to be drawn up over the inner and outer surfaces of the tube (Figure 13.31a).It causes this film to contract (Figure 13.31b). The film on the outer surface contracts enough to make a rounded edge. The film on the inner surface contracts more and raises water with it until the adhesive force is balanced by the weight of the water lifted (Figure 13.31c Surface tension causes

66 What happens to a can that is leaking water, when it is dropped? Will it continue to lose water? How would moving up or down in an elevator affect it?

67 Capillary Action If your hair is long, and you let it hang into the sink or bathtub, water will seep up to your scalp. This is how oil soaks upward in a lamp wick and water soaks into a bath towel when one end hangs in water. Capillary action is essential for plant growth. It brings water to the roots of plants and carries sap and nourishment to high branches

68 12. Which will hold the most?

69 Unnumbered Figure A piece of iron placed on a block of wood makes it float lower in the water. If the iron were instead suspended beneath the wood, would it float as low, lower, or higher? Defend your answer

70 Unnumbered Figure A balloon is weighted so that it is barely able to float in water. If it is pushed beneath the surface, will it return to the surface, stay at the depth to which it is pushed, or sink? Explain. (Hint: Does the balloon s density change?)

71 Unnumbered Figure When the wooden block is placed in the beaker, what happens to the scale reading? Answer the same question for an iron block.

72 Unnumbered Figure The weight of the container of water, as shown in a, is equal to the weight of the stand and the suspended solid iron ball. When the suspended ball is lowered into the water, as shown in b, the balance is upset. Will the additional weight needed on the right side to restore balance be greater than, equal to, or less than the weight of the solid iron ball?

73 Unnumbered Figure 13.8A

74 Unnumbered Figure 13.8B

75 Unnumbered Figure In the hydraulic arrangement shown, the larger piston has an area that is fifty times that of the smaller piston. The strong man hopes to exert enough force on the large piston to raise the 10 kg that rest on the small piston. Do you think he will be successful? Defend your answer.

76 Unnumbered Figure 13.10

77 The pressure at the bottom of a submerged object is greater than the pressure at the top. The result is a net force in the upward direction. What is this upward force called?

78 Force Pumps- Causes a fluid to move from one place to another by increasing the pressure on a pump (A medicine dropper) Heimlich maneuver- increases lung pressure & forces object out of windpipe

79 Hydraulic devise Multiplies a force by applying the force to a small surface area. The increase in pressure is then transmitted to another part of a confined fluid, which pushes on a larger surface area

80 Hydraulic pressure

81 Force applied to one piston increases the pressure in the fluid Pressure from a small piston acts over a larger area to produce a greater force in a hydraulic car lift. Used by seastars

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84 Gases & Plasmas

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86 The atmosphere. Air is more compressed at sea level than at higher altitudes. atmospheric pressure is caused by the weight of the air.

87 The reason we don t feel this weight crushing against our bodies is that the pressure inside our bodies equals that of the surrounding air. There is no net force for us to sense.

88 The pressure of the atmosphere is not uniform. Besides altitude variations, there are variations in atmospheric pressure at any one locality due to moving weather fronts and storms. average atmospheric pressure at sea level is kilopascals (101.3 kpa). 1

89 Barometer A common instrument used for measuring the pressure of the atmosphere If the atmospheric pressure increases, then the atmosphere pushes down harder on the mercury and the column of mercury is pushed higher than 76 centimeters. The mercury is literally pushed up into the tube of a barometer by the weight of the atmosphere. Could water be used to make a barometer? The answer is yes, but the glass tube would have to be much longer 13.6 times as long

90 there is a 10.3-meter limit on the height that water can be lifted with vacuum pumps.

91 aneroid barometer Since the atmospheric pressure decreases with increasing measures atmospheric pressure A metal box that is partially exhausted of air and has a slightly flexible lid that bends in or out with changes in atmospheric pressure. altitude, a barometer can be used to determine elevation. An aneroid barometer calibrated for altitude is called an altimeter (altitude meter). Some altimeters are sensitive enough to indicate a change in elevation of less than a meter.

92 hard vacuum can reach Pa. In space A vacuum pump simply provides a region of lower pressure into which fast-moving gas molecules randomly move

93 Boyle s Law Boyle s law applies to ideal gases. An ideal gas is one in which the disturbing effects of the forces between molecules and the finite size of the individual molecules can be neglected. Air and other gases under normal pressures approach idealgas conditions. the product of pressure and volume for a given mass of gas is a constant as long as the temperature doesn t change pressure and volume are inversely proportional;

94 Archimedes principle holds for air just as it does for water: An object surrounded by air is buoyed up by a force equal to the weight of the air displaced. any object less dense than air will rise in air (Left) At ground level, the balloon is partially inflated. (Right) The same balloon is fully inflated at high altitudes where surrounding pressure is less

95 Bernoulli s Principle fluids in motion fluid dynamics. True for liquids and gases The pressure exerted by a moving stream of fluid is less than the pressure of the surrounding fluid Blow on paper? When the speed of a fluid increases, internal pressure in the fluid decreases. pressure at the walls of the pipes decreases when the speed of the water increases.

96 Bernoulli s principle is consistent with the conservation of energy.for a steady flow of an ideal fluid free of internal friction, there are three kinds of energy: kinetic energy due to motion, work associated with pressure forces, and gravitational potential energy due to elevation In a steady fluid flow where no energy is added or taken away, whatever work is done by one part of the fluid on another part makes its appearance as kinetic and potential energy. Then the sum of the three energy terms remains constant.

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98 Bernoulli s Demo Blow over a piece of paper

99 Air pressure above the roof is less than air pressure beneath the roof. Consider wind blowing across a peaked roof. The wind speeds up as it flows over the top, as the crowding of streamlines in Figure indicates. Pressure along the streamlines is reduced where they are closer together. Even a small pressure difference over a large roof area can produce a large upward lifting force Unless the building is well vented, greater pressure inside and beneath the roof can push it off..

100 Curving may be increased by threads or fuzz, which help to drag a thin layer of air with the ball and to produce further crowding of streamlines on one side. a) The streamlines are the same on either side of a nonspinning baseball. (b) A spinning ball produces a crowding of streamlines. The resulting lift (red arrow) causes the ball to curve, as shown by the blue arrow.

101 The vertical vector represents the net upward force (lift) that results from more air pressure below the wing than above the wing. The horizontal vector represents air drag.

102 Bernoulli s principle Pressure is greater in the stationary fluid (air) than in the moving fluid (water stream). The atmosphere pushes the ball into the region of reduced pressure.

103 Bernoulli s principle explains why trucks and boats passing closely on are drawn to each other passing ships run the risk of a sideways collision. Water flowing between the ships travels faster than water flowing past the outer sides. Streamlines are closer together between the ships than outside. Hence, water pressure acting against the hulls is reduced between the ships. the greater pressure against the outer sides of the ships forces them together.

104 On a windy day, waves in a lake or the ocean are higher than their average height. How does Bernoulli s principle contribute to the increased height?

105 plasma a fourth phase of matter the least common phase -but it is the most prevalent phase of matter in the universe as a whole. The Sun and other stars are largely plasma. A plasma is an electrified gas. The atoms that make it up are ionized, stripped of one or more electrons, with a corresponding number of free electrons

106 plasma Plasmas on Earth are created in laboratories by heating gases to very high temperatures, making them so hot that electrons are boiled off the atoms. Plasmas may also be created at lower temperatures by bombarding atoms with high-energy particles or radiation. Ex. Fluorescent tubes Neon signs & street lights Plasma TV screens Auroras Exhaust of jet engine MHD power- Magnetohydrodynamic interaction between plasma & magnetic field

107 Plasma= future? Fusion power Fusion power may not only make electrical energy abundant, but it may also provide the energy and means to recycle and even synthesize elements.

108 Air flowing over the top of the wing travels farther than the air on the bottom in the same amount of time.

109 This means the speed of the air above the wing is much faster than the speed below. The difference in speed leads to a greater pressure over the top of the wing and a lower pressure below the wing. This pressure difference is what causes the lift.

110 Steamlined shape of plane reduces drag Thrust - force which pushes the plane forward provided by plane s propeller/jet To land a plane increases its area and drag by using flaps to change the shape of the wing

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113 Fluid Pressure P = Force /area measured in Pascals

114 Pascal s Principle Explains hydraulic systems Pressure is exerted equally throughout a closed container Hydraulic systems multiply force (over a greater distance)

115 Bernoulli s Principle Pressure exerted on a moving steam of fluid is less than the pressure of the surrounding fluid Explains Flight

116 Archimedes Principle (Sinks) Buoyant force on an object is equal to the weight of the fluid displaced If an object floats- the volume of displaced water = volume of the portion of the object that is submerged.