2.3 Expansion work 1). The general expression for work the area of the piston, A Externa Extermal pressure, pex pressure, pey the external pressure, p Area the force on the outer face F=Pexa When the system expands through a Distance moved. dz distance dz against Pex, the work done: dw=- Pex a dz where Adz is the change in volume dv, Pressure, p in the course of the expansion. 版权所有:华东理工大学物理化学教研室
版权所有：华东理工大学物理化学教研室 16 When the system expands through a distance dz against pex, the work done: dw = - pex A dz where Adz is the change in volume, dV, in the course of the expansion. 1). The general expression for work the area of the piston, A the external pressure, pex the force on the outer face, F =pex A. 2.3 Expansion work
2.3 Expansion work 1). The general expression for work The work done when the system External External expands by dy against a pressure Pex is Iw=-Fdz=-pex d Distance moved. dz The total work done. w: Pressure, p 版权所有:华东理工大学物理化学教研室
版权所有：华东理工大学物理化学教研室 17 The work done when the system expands by dV against a pressure pex is 1). The general expression for work dw = −Fdz = − pexdV = − f i exd V V w p V The total work done, w: 2.3 Expansion work
2.3 Expansion work In general. the work done on a system can be expressed in the form dw=-Fdz, where F is a 'generalized force and dz is a 'generalized displacement Varieties of work Type of w dw Comments Units Expansion Perd P ex Pa d Surface expansion d NI do Extension f Electric pdq d C 版权所有:华东理工大学物理化学教研室
版权所有：华东理工大学物理化学教研室 18 In general, the work done on a system can be expressed in the form dw = -Fdz, where F is a 'generalized force' and dz is a 'generalized displacement'. Type of w dw Comments Units Expansion -pexdV pex dV Pa m3 Surface expansion d d Nm-1 m2 Extension f dl f dl N m Electrical dq dq V C Varieties of work 2.3 Expansion work
2.3 Expansion work 2). Free expansion Free expansion: the expansion against zero opposing force ex 0) dw=0 for each stage of the expansion =0 No work is done when a system expands freely Expansion of this kind occurs when a system expands into a vacuum. 版权所有:华东理工大学物理化学教研室
版权所有：华东理工大学物理化学教研室 19 No work is done when a system expands freely. Expansion of this kind occurs when a system expands into a vacuum. 2). Free expansion pex = 0 dw = 0 for each stage of the expansion w = 0 Free expansion: the expansion against zero opposing force. 2.3 Expansion work
2.3 Expansion work 3). Expansion against constant pressure For an expansion of the external pressure constant throughout of the expansion Area=pa△V w=-Pex dv V. Volume. V P ps(-V1)=-p△ The magnitude of w, is equal to the Pe area beneath the horizontal line at p=Pex lying between the initial and final volumes 版权所有:华东理工大学物理化学教研室
版权所有：华东理工大学物理化学教研室 20 The magnitude of w, is equal to the area beneath the horizontal line at p = pex lying between the initial and final volumes. = − f i ex d V V w p V 3). Expansion against constant pressure = −pex (Vf −Vi ) = −pexV For an expansion of the external pressure constant throughout of the expansion 2.3 Expansion work