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Pascal’s Law — In 1653, Blaise Pascal came up with the idea that in a uid at rest,
the pressure on any surface exerts a force perpendicular to the surface and indepen-
dent of the direction or orientation of the surface. Any added pressure applied to the
uid is transmitted equally to every point in the uid. 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 Law — 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 the gaseous state.
Measurements on gases were rst published by Robert Boyle in 1660. He gured 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 doubles. Quantitatively, Boyle’s Law is:
PV = Constant
Where the value of the constant depends on the temperature and the amount of gas
present.
Jacques Charles studied relationships between the temperature and volume of gases,
while maintaining a constant pressure. He saw a steady increase in volume as tem-
perature increased, nding that for every degree Celsius rise in temperature, the gas
volume increased by 1/273 of its volume at zero degrees C.
Charles’s Law and Kelvin Temperature — Charles’s observations led to the abso-
lute (Kelvin) temperature scale. Since the gas, according to the equation, would have
zero volume at –273 degrees C. Kelvin dened the absolute temperature scale so that
absolute zero 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 al-
lows 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.
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