Chapter Four Energy Transfers.pdf

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Mechanical Engineering
Chris Ford

Chapter Four: Energy Transfers Monday, 11 November 2013 6:09 PM • Understand definitions of work and heat Definition of Work • Work: Force (F) acting though displacement (s) in the direction of the force; • In thermodynamic systems, work is done by a system if the sole effect on the surroundings could raise a weight • Work out of a system (done by; gas expanding against a piston) is considered positiveand into a system (done on; piston compressing a gas) is considered negative • Work is a form of energy being transferred across the system boundary, this can be flow of electricity Units for Work • Unit for work is joule (J) or kilojoule (kJ) • Power: rate of doing work, • 1 J/s = 1 Watt (W) Work done at the moving boundary of a simplecompressible system • • Therefore, work is considered the integral of PdV, or the area under the curve of a Pressure- Volume graph • As this depends on the path that is undertaken to get from state 1 to state 2, work is said to be a path function or an inexact differential Definition of Heat • Heat (Q): form of energy transferred
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