Work in Fluid
Systems
Notes 2.2
* Work is done when a force moves an object
through a distance—we calculate it as
pressure difference. Fluids flow from regions
of high pressure to regions of low pressure.
I. Open and Closed Fluid
Systems
A. Closed fluid systems retain
and recirculate a working fluid.
1. the pressure is constant
and the volume changes
2. Ex.: hydraulic jack,
hydraulic brake system, and the
body’s circulatory system.
B. Open Fluid Systems allows fluids to flow
through only one time—they are not retained
and recirculated.
1. a fixed volume moves through a pressure
difference
2. Ex: city water systems, irrigation
systems, and a fire truck water system.
II. Hydraulic Actuator
A. closed system (p.
97)
B. used to control the
movement of airplane
landing gears,
bulldozer shovels and
forklifts
C. several
subsystems
III. Work Done in Fluid Systems
A. Simple closed fluid system
1. Cylinder and piston
2. piston is movable boundary of the
system
B. ΔV = A x d
C. F = P x A
D. W = P x ΔV
1. P is always positive
2. ΔV can be positive or negative
a. If ΔV, the fluid volume,
increases, volume and work both
are positive—the fluid does work
b. If the fluid volume decreases,
volume and work both are
negative—work is done on the fluid
E. Open fluid system
1. W = - ΔP x V
2. If pressure increases, ΔP is
positive and W is negative—work is
done on the fluid
3. If pressure decreases, ΔP is
negative and W is positive—the fluid
does work
Examples
A. Burning fuel-air mixture in a cylinder of an
internal-combustion engine—positive work done
by the expanding gas
B. A heart pumping blood—the blood does
negative work
C. A refrigerator compressor using a piston and
cylinder to increase the temperature and
pressure of the working fluid—the fluid does
negative work
Examples
D. Burning fuel expanding through a nozzle
to lift a rocket—positive work is done by
the fuel
E. Pushing down on the input cylinder of a
hydraulic jack—the fluid does negative
work—and the output cylinder lifting a
heavy load—positive work is done by the
fluid