The atmosphere exerts a PRESSURE on us by virtue of the weight of the atmospheric gases above. In spite of the fact that this weight is calculated from a column of gas straight up from our heads, the pressure the atmosphere exerts on us is in the direction normal (perpendicular) to our body's surface, so, in fact, acts in all directions, not just down.
Units of Pressure
Pressure is force per unit area and therefore has units
of Newtons per square meter or kg
/ (m .s2). This unit is also called the PASCAL
after Blaise Pascal (perhaps you remember Pascal's Triangle?). Since the
acceleration due to gravity and the mass of the atmosphere are values that
depend on our earth and its' weather, it may seem silly to also have a
unit of pressure based on the pressure exerted by our atmosphere (the so
called BAROMETRIC PRESSURE), but we do, and this unit of pressure is called the
atmosphere. The atmosphere is taken to br the average earth's barometric pressure and
has an accepted value of 1.013 x 105 Pa. {Note
the letters Pa stand for the Pascal
and kPa stands for 103 Pascal,
etc.} The barometric pressure drops significantly upon the approach of
a big storm, so barometric mapping provides an important part of weather
prediction.
Metereologists report the barometric
pressure with the weather forcasts you have seen on TV, but not usually
in SI UNITS. They usually use the unit Bar
(or millibar) or "Inches
of Mercury". The Bar is a unit related
to the atmosphere, but 'rationalized' in the
SI system to exactly 105 Pascals.
Note that the units differ by only a few percent (How many?)
Inches of Hg as a unit of pressure??? That unit sounds like a length and perhaps a method of measurement. How does this define a pressure? This unit of pressure is derived from the construction of the BAROMETER, historically the first measuring tool capable of measuring barometric pressure.
In a Barometer, a dense, nonvolatile fluid is used to fill a long tube that is closed at one end. This tube is emersed in a bath of the fluid and withdrawn with the closed end up. This creates a primitive vacuum. As we raise the tube gradually from the submersed position, the fluid fills the tube, but is found to rise in the tube only up to a certain height after which it no longer depends on the height of emtpy tube above it. This height is the height at which the force of the atmosphere pushing up on the column just balances the weight of the column of liquid. If the fluid is Hg, the height of the column that is supported by the average atmospheric pressure when measured at the surface of the Earth is 760 mm, about 30 inches. Thus both mm of Hg and Inches of Hg can be used as pressure units. Evangelista Torricelli was the first known human to construct a barometer, and in his honor the unit 'mm of Hg' is called the TORR. If the fluid in the barometer is not mercury or the gravitational field is not the same as on the surface of the Earth, the height of the column for a given pressure will be different. The height of the column is governed by the relation:
p = d g h
where d is the density of the fluid, h is the height of the column, p is the pressure, and g is the acceleration due to gravity.
Unit Conversion
All pressure units can be converted to one another by
application of a multiplicative factor. The unit of ATMOSPHERE (atm) is
common in chemistry and has a value of 760 torr. Other units are tabulated
below
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1 atm = 29.92 Inches of Hg |
A MANOMETER (U-TUBE) is a variation of the Barometer in which the pressure of two gases may be compared. Here the difference in the pressures of the gases on the arms of the manometer is equal to the pressure 'exerted' by the column of fluid, ph = d g h.
Gas Laws
Gases are compressible fluids, unlike liquids. If you
squeeze a gas, it gets smaller. Experimentally, to good degree of accuracy
shows that if you double the pressure exerted
upon a fixed amount of gas, the volume
will halve if the temperature remains constant.
The behavior of the state of a gas can be described mathematically by relationships between the observed properties of that gas. Since two properties remain constant, the two that are changing in relation to each other are pressure and volume.
Robert Boyle is attributed with the quantification of the relationship between pressure and volume of a gas. The equation for this is:
p V = (constant)
or
V = (constant) / p
where p is pressure, V is volume and the (constant) depends on the quantity of the gas sample and its' temperature. This relation seems somehow mechanical in nature.
Heating and cooling of a gas is much more complicated to understand. The temperature of a gas somehow tells us how much energy the molecules in the sample have. If we heat something up, we expect it to expand, because hot molecules will move around and take up more room than cold ones. With a heater and a closed sample (fixed amount) of gas, we can measure the change in Volume of a gas as we vary the temperature at fixed external pressure. Such a measurement would give us the following data:
Jacques Charles is attributed with the discovery that the Volume vs Temperature relationship for a gas at constant pressure is linear or
V = (constant) T
Here, the (constant) depends on the amount of gas and its pressure. Note that this line has an intercept with the x axis. To the left of this intercept a gas would have a negative volume, which is impossible. Every time we make this experiment happen, every gas we use gives the same value of temperature for this intercept, -273oC. The behavior of gases tells us something very unique: Temperature has an ABSOLUTE ZERO. Wow. A doubling of temperature only makes sense if you know what the zero of the scale is. If one wishes to double the volume at constant pressure of a gas initially at 10oC, one heats it to 293oC (not 20oC)
Amount by Count
Gases are made up of molecules. It is not easy to believe
in something you cannot see. It is even harder to count something that
you cannot see. Amedeo Avogadro believed in molecules. He wrote: Volumi
eguali di gas nelle stesse condizioni di temperatura
e di pressione contengono lo stesso numero
di molecole. I bet you can translate that
without a dictionary. This concept allows us to characterize the STOICHIOMETRY
of chemical reactions by measuring the amount of gas evolved by collection
over a fluid:
We can weigh a sample and if we know the MOLECULAR WEIGHT, we can count how many molecules compose the the total mass of the sample. But what if we don't know how much each molecule weighs? Avogadro says:
V = (constant) n
where V is the volume, n is the number of moles of sample, and (constant) is a constant that depends of the temperature and pressure of the gas.