This table compares the basic analogous variables in electrical and hydraulic technologies. Conflicts exist in the algebraic and schematic notations for the various quantities.
The best place to start a discussion on electric-hydraulic analogies is with the definitions and units of the basic variables, which are given in Table 1. The analogous pairs are voltage and pressure; electrical charge and fluid volume; current and fluid flow; electrical resistance and hydraulic resistance; electrical conductivity and hydraulic conductivity; and power in the two energy transmission media.
Electrons, protons, electrical charge, and fluid molecules
All known material in the universe is comprised of atoms, and each atom has a nucleus with a positive electrical charge. The nucleus is surrounded by orbiting electrons with their negative charge. Under normal circumstances the number of positively charged protons in the nucleus equals the number of orbiting electrons. The number of protons defines the basic material element.
Some elements have electrons that are loosely held by the parent proton, and some have very tightly held electrons. The elements with the most loosely held electrons are metals, and they are the conductors of electricity. The materials with the tightly bound electrons are non-metals. Because they do not conduct electricity, they are insulators.
Even insulators are able to give up their electrons. An example is when you walk across a carpet on a low-humidity day and then touch a metal object — say, a door knob. Just before your finger touches the door knob, you feel a small electrical shock and may see a spark. The reason is that the friction of your insulated shoes gliding across the insulated carpet causes electrons to be forcibly stripped from the carpet and onto your clothing and body. When you reach for the metal door knob, the isolated charges cause the air to ionize, and there is a sudden rush of charges as they attempt to join the metal, and the result is an arc and the shock.
This process is referred to as electrostatics and will not be pursued further, except to state three important observations. First, electrostatic force explains why after combing your hair on a dry day, you can attract small pieces of paper with the comb. Charge separation accounts for the force. Second, this phenomenon is often referred to by lay people as “magnetism.” It is not. It’s an electrostatic phenomenon caused by charge separation. Magnetism is caused by current — that is, electrical charges in motion. Electrostatics is not very important to those practicing electrohydraulics, but magnetism is. Third, to cause the arc between your finger and the door knob and to pick up little pieces of paper with your comb, there must be some force at work in the process. Indeed, there is!
Continuing with the electron-proton model of material, molecules are formed when atoms of different material join together to make entirely new materials that have properties that bear little or no resemblance to the properties of the constituent atoms. There are, literally, unknown numbers of ways in which atoms can combine to form new materials. Of all those ways, only one category of material is important at the moment, namely, hydraulic fluid. Hydraulic fluid is comprised of molecules, and those molecules are to the hydraulic system what electrons are to the electrical system. The fluid molecules do not need an electrical charge to be useful in the hydraulic system, but an electrical charge is vital to all matters electrical.
Voltage, pressure, force, and particle motion
Let’s begin with the hydraulic system by looking at the motivating factor in moving a molecule of fluid around the hydraulic circuit. Figure 1 shows two pressure-containing envelopes, one at a high pressure and the other at a lower pressure. The two envelopes are connected by a fluid conduit, which conducts fluid, but with some resistance. Therefore, a pressure reduction (gradient) exists, as we move from the high-pressure side to the low-pressure side. If a single molecule and the pressures acting on it are isolated, a lower pressure will exist on one side of the molecule than on the other.