ENG_sgwhk03 - -3rd lecture- Chapter 2 First law of...

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1 Chapter 2 First law of thermodynamics ( 热力学第一定律 ) -3 rd lecture- ) Thermodynamics investigates the rule of energy transfer and conversion ) The first law of thermodynamics is the concrete form of energy conservation and conversion principium applying to thermodynamic system ) The change of equilibrium can take place only when the interaction occurs between a closed system and its surroundings. The interactions can be ascribed to mechanic interaction (mechanic field) and heat interaction (temperature field)
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2 1.Work Definition 1 The energy transfer between system and surroundings in work form should happen though the boundary the work done from one part to other part inside the system is not included Work is the transfer of energy from a system in a manner that is equivalent to raising a weight in the surroundings. The symbol W stands for the work done on the system Four important points 2.1 Work and Heat ( 功与热 )
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3 2 The kinetic energy and potential energy due to the movement of whole system are not included 3 The symbol of W W 0 The work done by surroundings to the system (System gains energy in work form) W 0 The work done by system to its surrounding System loses energy in work form)
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4 4 W is not a state function, and can not be expressed as total differential. The δ W stands for the infinitesimal work done in an exiguity change process, the d W is not valid . The work done by system to surroundings though volume change Expansion work ( 体积功 ): Classifica- tion of work Other work done by system except the expansion work, using W to stand for it, such as mechanic work, electric work, surface work, etc. Non expansion work ( 非体积功 ):
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5 a). The general expression for work d wF z δ = − The work required to move an object a distance d z against an opposing force of magnitude F is The negative sign presents that when the system moves an object against an opposing force, the internal energy of the system doing the work will decrease Expansion work ( 体积功 )
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6 When the system expands through a distance d z against p ex , the work done: δ w = - p ex A d z where A d z is the change in volume, d V , in the course of the expansion . b) The general expression for expansion work the area of the piston, A the external pressure, p ex the force on the outer face, F = p ex A.
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7 The work done when the system expands by d V against a pressure p ex is c) The expansion work ex δ dd wF z p V = −= f i d V V wp V =− The total work done, w : When W ex <0, V f > V i , expansion process , the system does work to its surroundings; When W ex >0, V f < V i , compression process , the surroundings does work to system.
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8 Non expansion work ( 非体积功 ) 1 ° surface work ( 表面功 ) 2f’ f e l xx + d x Surface tension of liquid γ δ W = f’.dx = γ .2l.dx = γ .dA 2 ° Reversible electrical work ( 可逆电功 ) δ W = E.dQ We distinguish expansion work δ W exp and other work δ W’ , δ W = δ W exp + δ W’
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