control volume

 control volume

control volume is defined as a
volume which encloses the matter and the device inside   a control
surface.
  Every thing external to the
control volume is the surroundings with 
the  separation given by the
control surface.
The  surface may be open or closed to mass flows
and it may have flows from energy   in
terms of heat transfer and work across
it.  The  boundaries may be
moveable or stationary.
 In the case of a control
surface that is closed to the mass flow, so that  no 
mass can enter or escape the control volume, it is called a control mass  containing  same 
amount of matter at all times.

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