Home (Härtel)In the literature and textbooks quite a few examples of water models can be found with continuosly runnig pumps to be used as analogy to the flow of free electrons within an electric circuit.
In comparison with the electric circuit, however, all these technically realized water models suffer in one important aspect: the ratio between the kinetic energy of the flowing water and the size of the driving forces or pressure difference. In the electric case this ratio is huge. The kinetic energy of the free electrons is practically zero, the driving force - the EMF - is absolutely dominant. Water in a closed system, however, when continuously driven by a pump, inevitably gains kinetic energy and the existing pressure differences are less dominant. Such models therefore risk to stimulate ideas in line with far spread misconceptions about the electric circuit as a system were the energy is directly linked to the flow of current.
The open (or semi-closed) syringe model has the advantage that the importance of pressure differences can be stressed and - if helpful - can even be directly experienced by students (see figure below)