Lecture214Week8

Lecture214Week8 - Liquids and Gases The unit of volume is...

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05/04/09 Physics 214 Fall 2008 1 Liquids and Gases As we know liquids and gases act very differently than solids. Liquids and gases have mass but their constituent atoms are not bound so that each part of the liquid or gas can move. The atoms of a liquid are more tightly bound so a liquid can be kept in an open container whereas gas usually requires a closed container. Liquids, like solids are not very compressible, that is, it is difficult to change the volume. A volume of gas can have it’s volume changed fairly easily. Both have the property of being able to flow, for example water and gas lines in a house. The unit of volume is the meter cubed , m 3 , which is a very large volume. Very often we use cm 3 = cc. Other everyday units are gallons, quarts, pints
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05/04/09 Physics 214 Fall 2008 2 Pressure A volume of liquid or gas has mass and F = ma is still a good law except a force at a point on the surface of water only moves the water near that point So generally we work with liquids and gases in containers and exert forces over a surface. We define pressure as P = F/A that is the force divided by the area over which the force acts. Any change in pressure is transmitted uniformly throughout the liquid. Units are N/m 2 and 1 N/m 2 = 1Pascal
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05/04/09 Physics 214 Fall 2008 3 Hydraulic Jack If the liquid levels on each side are the same height then the pressure just below each piston is the same and F 1 /A 1 = F 2 /A 2 so F 2 = F 1 A 2 /A 1 If we make A 2 /A 1 = 100 then an F 1 force of 50lbs can lift 5000lbs . Of course if we push F 1 down a distance h 1 then the F 2 Side will only rise h 1 A 1 /A 2 because h 1 A 1 = h 2 A 2 , that is the displaced volumes must be the same. Work done = F 1 h 1 = F 2 h 2
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05/04/09 Physics 214 Fall 2008 4 Atmospheric pressure If I stack a pile of bricks each brick has to support all the bricks above it. So as we go down the stack the force increases as does the pressure. In the example shown the force on the face of the bottom brick would be 6mg and the pressure = 6mg/A. At the earths surface we are supporting a column of air which exerts a force and because it is a gas it exerts an equal pressure In all directions. g F
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05/04/09 Physics 214 Fall 2008 5 Density If one takes two objects of exactly the same volume made of different materials they have different weights. So we define a useful quantity Density ρ = mass/unit volume, kg/m 3 or grams/cc so the mass of an object is ρV and the weight ρVg If an object is put into a container of liquid it will float if ρ object is less than ρ liquid It will sink if ρ object is greater than ρ liquid
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05/04/09 Physics 214 Fall 2008 6 Density and pressure If I take a container of liquid the pressure at any depth is the weight of the water above that depth divided by the area. It is very useful to define Mass/unit volume ρ = kg/m 3 .
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Lecture214Week8 - Liquids and Gases The unit of volume is...

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