work in fluid systems notes 2.2. * work is done when a force moves an object through a distance—we...

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 SystemsA. 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