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Pressure,
Volume and Temperature
The three continuum quantities, pressure , volume,
and temperature define the equilibrium state of a material system. They
are manifestations of the behavior of the atoms that make up the system and
represent averages of quantities associated with this behavior. In general,
the continuum description will be all that is required for understanding the
behavior of human-powered vehicles.
Pressure
In a fluid, the atoms or molecules of
the material can move with respect to each other and will collide with other
atoms in the material or the wall of its container. To a good approximation
this motion can be described by Newton's laws. When an elastic collision occurs, energy and
momentum are conserved. Pressure is a measure of the average force per unit
area acting on the wall as a result of the molecules' change in momentum during
collisions with the wall. Momentum is a vector quantity, and it is only the
component of momentum normal to the wall that contributes to the pressure.
Pressure is the same on all the walls of the container whatever their orientation,
and is said to be an isotropic quantity. The SI units of pressure are
Pascals (Newtons/m2). If pressure is increased in one location it
will come to a new equilibrium value in all parts of the container. Pressure
change is transmitted between these regions at the speed of sound.
If
the collision partner with the fluid molecules is small, more atoms may collide
with one side than another during a small time period and the particle
will move in the direction of the net applied force during that time. Pollen
grains in a liquid are an example of this behavior and the random motion of
these grains is known as Brownian Motion. For a large surface in a fluid, the
average number of atoms striking one side will be the same as the number striking
the opposite side and no macroscopic displacement will occur.
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