The electronic–hydraulic analogy (derisively referred to as the drain-pipe theory by Oliver Lodge) is the most widely used analogy for "electron fluid" in a metal conductor. Since electric current is invisible and the processes at play in electronics are often difficult to demonstrate, the various electronic components are represented by hydraulic equivalents. Electricity (as well as heat) was originally understood to be a kind of fluid, and the names of certain electric quantities (such as current) are derived from hydraulic equivalents. As with all analogies, it demands an intuitive and competent understanding of the baseline paradigms (electronics and hydraulics).
There is no unique paradigm for establishing this analogy. Two paradigms can be used to introduce the concept to students:
Other paradigms highlight the similarities between equations governing the flow of fluid and the flow of charge:
Conducting wire: a simple pipe.
Resistor: a constricted pipe.
Node in Kirchhoff's junction rule: A pipe tee filled with flowing water.
Capacitor: a flexible diaphragm sealed inside a pipe.
Inductor: a heavy paddle wheel or turbine placed in the current.
Voltage or current source: A dynamic pump with feedback control.
A simple one-way ball-type check valve, in its "open" state acts as a diode in its conducting state.
A pressure-actuated valve combined with a one-way check valve acts as a transistor.
Like a one-way check valve, a diode blocks current that flows the wrong way. Current that flows the right way goes through almost unchanged.
A simple A/C circuit consisting of an oscillating pump, a "diode" valve, and a "capacitor" tank. Any kind of motor could be used here to drive the pump, as long as it oscillates.
See also Bond graph
Some examples of analogous electrical and hydraulic equations:
|quantity||volume [m]||charge [C]||heat [J]||momentum [Ns]|
|potential||pressure [Pa=J/m=N/m]||potential [V=J/C=W/A]||temperature [K]||velocity [m/s]|
|flux||Volumetric flow rate [m/s]||current [A=C/s]||heat transfer rate [J/s]||force [N]|
|flux density||velocity [m/s]||current density [C/(m·s) = A/m²]||heat flux [W/m]||stress [N/m = Pa]|
|linear model||Poiseuille's law||Ohm's law||Fourier's law||Dashpot|
If the differential equations have the same form, the response will be similar.
If taken too far, the water analogy can create misconceptions. For it to be useful, one must remain aware of the regions where electricity and water behave very differently.
Usefulness requires that the reader or student has a substantial understanding of the model (hydraulic) system's principles. It also requires that the principles can be transferred to the target (electrical) system. Hydraulic systems are deceptively simple: the phenomenon of pump cavitation is a known, complex problem that few people outside of the fluid power or irrigation industries would understand. For those who do, the hydraulic analogy is amusing, as no "cavitation" equivalent exists in electrical engineering. The hydraulic analogy can give a mistaken sense of understanding that will be exposed once a detailed description of electrical circuit theory is required.
One must also consider the difficulties in trying to make an analogy match reality completely. The above "electrical friction" example, where the hydraulic analog is a pipe filled with sponge material, illustrates the problem: the model must be increased in complexity beyond any realistic scenario.
|Wikimedia Commons has media related to Hydraulic analogy.|
አማርኛ የኤሌክትሪክ ፈሳሻዊ ተምሳሌት ▪ Deutsch Elektro-Hydraulische Analogie ▪ فارسی مفاهیم مرتبط در هیدرولیک و الکتریک ▪ Nederlands Hydraulische analogie ▪ Русский Метод электрогидравлических аналогий ▪ Српски / srpski Metod hidrauličke analogije ▪ Українська Аналогія електрогідродинамічна ▪