8-50 Qualitek mR Owner’s Guide
Pascal’s Law
In 1653, Blaise Pascal came up with the idea that in a fluid at rest, the pressure on
any surface exerts a force perpendicular to the surface and independent of the
direction or orientation of the surface. Any added pressure applied to the fluid is
transmitted equally to every point in the fluid. Pascal used his idea to invent the
hydraulic press. Pascal’s principle is often used in devices that multiply an applied
force and transmit it to a point of application. Examples include the hydraulic jack,
and the pneumatic cylinder.
Gas Laws
The actions of gases under varying conditions of temperature, pressure, and
volume can be described and predicted by a set of equations, or gas Laws. These
Laws were determined by measurements of actual gases and are valid for all
substances in a gaseous state.
Measurements on gases were first published by Robert Boyle in 1660. He figured
out that if an enclosed amount of gas is compressed until it is half its original
volume while the temperature is kept constant, the pressure will be doubled.
Quantitatively, Boyle’s Law is: PV = Constant, where the value of the constant
depends on the temperature and the amount of gas present.
Relationships between the temperature of the gas and its volume while keeping the
pressure unchanged were studied by Jacques Charles. He saw a steady increase in
the volume as the temperature went up, finding that for every degree Celsius rise in
temperature, the gas volume increase by 1/273 of its volume at zero degrees C.
Charles’s Law and Kelvin Temperature
Charles’s observations led to the absolute (Kelvin) temperature scale, since the
gas, according to the equation, would have zero volume at -273 degrees C. The
absolute temperature scale was defined by Kelvin so that absolute temperature
equals negative 273 degrees C and each absolute degree is the same size as a
Celsius degree. The modern value for absolute zero is -273.15 degrees C. This
temperature scale allows Charles’s Law to be written V/T = Constant, where V is
the volume of the gas, T is the temperature on the absolute scale, and the constant
depends on the pressure and the amount of gas present.
In 1802, Joseph Gay-Lussac played around with the relationship between pressure
and temperature and came up with an equation a lot like Charles’s Law: P/T =
Constant.
Generalized Gas Law
We can combine Boyle’s, Charles’s, and Gay-Lussac’s Laws to express this
generalized gas Law: PV/T = Constant, where the value of the constant depends
on the amount of gas present and t is the absolute (or Kelvin) temperature.
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