Energy transfer by work
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The document discusses the concepts of energy transfer within a closed system, focusing on heat and work as forms of energy transfer. It explains that heat occurs due to temperature differences, while work is the energy expended through force acting over a distance, and both are path-dependent functions. Additionally, different types of work, including mechanical and electrical work, are described along with their mathematical representations and significance in thermodynamic processes.
Energy transfer by work
- 2. Energy Transfer Energy can cross the boundaries of a closed system in the form of heat or work. If the energy transfer across the boundaries of a closed system is due to a temperature difference, it is heat; otherwise, it is work. Energy transferred across a system boundary that can be thought of as the energy expended to lift a weight is called work. Q W System Q W System
- 3. Both are recognized at the boundaries of a system as they cross the boundaries. They are both boundary phenomena. Systems possess energy, but not heat or work. Both are associated with a process, not a state. Unlike properties, heat or work has no meaning at a state. Both are path functions (i.e., their magnitudes depends on the path followed during a process as well as the end states. The similarities between heat and work are as follows :
- 4. Work Work is defined as the energy transfer associated with force acting through a distance. Symbols of Work: W or 1W2 The amount of work done by or on a system during process from state 1 to state 2. Units: kJ or BTU w Work per unit mass. Units: kJ/kg or BTU/lbm Power: The work done by unit time. kJ/sec = kW
- 5. Path Function 5 Path functions have inexact differentials designated by the symbol , eg. Q and W. 2 1 12 not WWW Point functions have exact differentials designated by the symbol , eg. V 2 1 12 ΔV-VVdV Properties are point functions, but heat and work are path functions.
- 6. Types of work Mechanical Work • Moving boundary work • Shaft work • Spring work Electric Work An electric wire crossing the system boundaries (We=V I t) V = voltage I = current t = time
- 7. Electrical Work The rate of electrical work done by electrons crossing a system boundary is called electrical power and is given by the product of the voltage drop in volts and the current in amps. (W)eW V I& The amount of electrical work done in a time period is found by integrating the rate of electrical work over the time period. 2 1 (kJ)eW V I dt
- 8. Mechanical Work Work is energy expended by a force acting through a distance. Thermodynamic work is defined as energy in transition across the system boundary and is done by a system if the sole effect external to the boundaries could have been the raising of a weight. Mathematically, the differential of work is expressed as W F ds Fds cos here is the angle between the force vector and the displacement vector. As with the heat transfer, the Greek symbol means that work is a path-dependent function and has an inexact differential. If the angle between the force and the displacement is zero, the work done between two states is 2 1 2 1 12 FdsWW
- 9. Net Work Done By System The net work done by a system may be in two forms other work and boundary work. First, work may cross a system boundary in the form of a rotating shaft work, electrical work or other the work forms listed above. We will call these work forms “other” work, that is, work not associated with a moving boundary. In thermodynamics electrical energy is normally considered to be work energy rather than heat energy; however, the placement of the system boundary dictates whether to include electrical energy as work or heat. Second, the system may do work on its surroundings because of moving boundaries due to expansion or compression processes that a fluid may experience in a piston-cylinder device. The net work done by a closed system is defined by botherinoutnet WWWW
- 10. Here, Wout and Win are the magnitudes of the other work forms crossing the boundary. Wb is the work due to the moving boundary as would occur when a gas contained in a piston cylinder device expands and does work to move the piston. The boundary work will be positive or negative depending upon the process. bothernetnet WWW