Force
Force
is defined using Newton’s second law of motion which states that the net force Fnet
acting on a body of mass m, is proportional to the time rate of change of its
momentum. If the mass is constant, the net force is proportional to the product
of the mass of the body and its acceleration. Thus,
where
a = dv/dt is the acceleration of the body and K is a proportionality constant
to be determined by the units used. In SI system, K = 1 and force has the units
of newton, N, or kg.m/s2.
Pressure
The
pressure, P, of a fluid on a surface is defined as the normal force exerted by
the fluid per unit area of the surface, i.e.,
Pressure
has the units of Pa (N/m2) in SI units. Absolute pressure
refers to the absolute value of the force per unit area exerted on the containing
wall by a fluid. Gauge pressure is the difference between the absolute pressure
and the local atmospheric pressure. Vacuum represents the amount by
which the atmospheric pressure exceeds the absolute pressure. This can be shown
schematically:
From
these definitions we can see that:
· Absolute pressure cannot
be negative.
· Vacuum cannot be
greater than the local atmospheric pressure.
A
system is said to be in mechanical equilibrium with its surroundings
when there is no pressure difference between them, i.e.,
Psys
= Psurr Condition
of mechanical equilibrium
Isobaric
Process: This is a process that takes place at constant
pressure. It is important to note that if the initial and final states are
at the same pressure, this does not necessarily imply an isobaric process.
Example
1
Which
of the following processes would you consider to be isobaric for analysis purposes?
a)
A valve is opened in a tank of compressed air. (Non-isobaric process)
b)
Heat is added to boiling water on the stove. (Isobaric Process)
c)
Air is compressed in a compressor. (Non-isobaric process)
d)
A tank of compressed air leaks air through a tiny pinhole leak. (Non-isobaric
but if short time periods are involved, it may be considered isobaric)
e)
The air in the cylinder with a frictionless piston held by a constant weight on
it as heated. (Isobaric Process)
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