Lecture 8sf

# Lecture 8sf - How do gases differ from solids and liquids...

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How do gases differ from solids and liquids? Gases have much lower densities than solids and liquids. Density of water: 1.00 3 cm g at 25 o C Density of nitrogen: 0.00114 3 cm g at 25 o C, 1 atm The volume of solids and liquids depends slightly on temperature. Most solids, liquids expand with higher temperature. The volume of solids and liquids is almost independent of pressure. They are essentially incompressible. The volume of gases depends on both temperature and pressure. Gases are compressible . Higher pressure lowers the volume of gases Under lower pressure, a gas occupies a larger volume. Gases expand to fill up a container. On a molecular level, why do gases have low density and why are they much more compressible than solids or liquids? In gases, molecules are very far apart. This accounts for both low density and their ability to be compressed (go to lower volume under higher pressure). What is pressure and how do we measure it? Pressure = Newton (N) is the SI unit of force N = kg 2 sec m in terms of base units m 2 is the SI unit of area SI units of pressure are 2 m N or Pascal (Pa)

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Although ) Pascals ( m N 2 are the SI unit of pressure, we usually express pressure indirectly in terms of the pressure under a certain height of liquid. For convenience, atmospheric pressure is usually expressed in terms of the height of a mercury column. Such a measuring device is called a mercury barometer. The liquid is mercury Air pressure at sea level will support a column of mercury 760 mm high.
A mercury height of 760 mm will balance out air pressure on an average day. By definition: 760 mm Hg = 1.00 atm Since the mercury barometer was invented by Evangelista Torricelli in 1643, the unit “mm Hg” is also known as a Torr. 1.00 atm = 760 mm Hg = 760 Torr

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How is the pressure underneath a liquid calculated? Assume the liquid has height h and that the cross sectional area of the column of liquid is A. Volume of liquid = Ah Weight of liquid = mg, where m = mass of the liquid g = acceleration of gravity On the earth, g = 9.80 2 sec meter Mass = density x volume = dV = dAh Weight of liquid = mg = (dAh)g or dAgh Pressure = area force = area liquid of weight = A dAgh = dgh Note that the A cancels out and the pressure is independent of the area of the tube. The pressure depends on the density of the liquid and the height of the liquid. For mercury: d = 13.6 3 cm g or 13.6 x 10 3 3 m kg At sea level, air pressure supports a height of mercury of 760 mm, which is 0.76 meter. P = dgh = 13.6 x 10 3 3 m kg x 9.80 2 sec meter x 0.76 m = 1.013 x 10 5 2 m N = 1.013 x 10 5 Pa or 101.3 kPa As shown above, the pressure under a 760 mm column of mercury is 1.013 x 10 5 Pa. Thus the SI unit of Pascal is a very small unit 1.00 atm = 101,300 Pa, or 101.3 kPa.
Why do we use a height of mercury rather than a height of water to measure pressure? Water density:

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Lecture 8sf - How do gases differ from solids and liquids...

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