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work in fluid systems

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Chapter 2.2
Principles of Technology I
Edinburg North High School
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Describe how open and closed systems are different.
Explain the relationship between work and pressure
in a fluid system, as given by the equation,
W = P  V.
Explain the relationship between work and pressure
in a fluid system, as given by the equation,
W = -P  V.
Explain what is meant by positive work and
negative work in fluid systems
Solve work problems for fluid systems in English
and SI units.
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As with mechanical
systems, work is done in
when a force moves and
object through a
distance.
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But it is easier to calculate
work in a fluid system
using pressure (prime
mover in fluid systems).
Pressure difference can
cause a fluid to move.
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A closed fluid system retains and recirculates a
working fluid.
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Example: Brake system in a car
Fluid flows through an open fluid system only
one time.
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Examples: city water systems, and irrigation
systems, and a fire truck water system.
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When heated, gas in
cylinder expands.
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F  P A
W  F d
 P  A d
 P  A d 
 P  V
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Volume changes:
V = Vf - Vi
Since cross-sectional area
does not change,
V = A  d
In this system, pressure
does not change because
the weight supported by
the piston does not
change.
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W  P  V
V can be positive or
negative (P is always
positive).
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W is positive, work is
done on the load.
W is negative, work is
done on the fluid.
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Open fluid systems
Steady-flow
 If density of fluid is constant,
volume moved is V = m/
 Equation for steady=flow
process when density is
constant.
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W  P V
Work is positive, the fluid
does work.
 Work is negative, work is
done on the fluid.
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