Work in Fluid Systems

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Work in Fluid Systems

• Notes 2.2

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• Work is done when a force moves an object
  through a distancewe calculate it as pressure
  difference. Fluids flow from regions of high
  pressure to regions of low pressure.

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• 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 bodys circulatory system.

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• B. Open Fluid Systems allows fluids to flow
  through only one timethey 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.

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• 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

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• III. Work Done in Fluid Systems
• A. Simple closed fluid system
• 1. Cylinder and piston
• 2. piston is movable boundary of the system

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• B. ?V A x d
• C. F P x A
• D. W P x ?V
• 1. P is always positive
• 2. ?V can be positive or negative
• a. If ?V, the fluid volume, increases,
  volume and work both are positivethe fluid
  does work
• b. If the fluid volume decreases, volume
  and work both are
• negativework is done on the fluid

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• E. Open fluid system
• 1. W - ?P x V
• 2. If pressure increases, ?P is positive and W
  is negativework is done on the fluid
• 3. If pressure decreases, ?P is negative and W
  is positivethe fluid does work

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Examples

• A. Burning fuel-air mixture in a cylinder of an
  internal-combustion enginepositive work done by
  the expanding gas
• B. A heart pumping bloodthe blood does
  negative work
• C. A refrigerator compressor using a piston and
  cylinder to increase the temperature and pressure
  of the working fluidthe fluid does negative work

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Examples

• D. Burning fuel expanding through a nozzle to
  lift a rocketpositive work is done by the fuel
• E. Pushing down on the input cylinder of a
  hydraulic jackthe fluid does negative workand
  the output cylinder lifting a heavy loadpositive
  work is done by the fluid