Chapter 2.2 Principles of Technology I Edinburg North High School 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. As with mechanical systems, work is done in when a force moves and object through a distance. 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. A closed fluid system retains and recirculates a working fluid. Example: Brake system in a car Fluid flows through an open fluid system only one time. Examples: city water systems, and irrigation systems, and a fire truck water system. When heated, gas in cylinder expands. F P A W F d P A d P A d P V 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. W P V V can be positive or negative (P is always positive). W is positive, work is done on the load. W is negative, work is done on the fluid. 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. W P V Work is positive, the fluid does work. Work is negative, work is done on the fluid.