When you drive under a high voltage power transmission line, your AM radio makes a buzzing noise that your FM radio does not. The reasons why are related to why an AC solenoid can burn out if it doesn't shift enough, but a DC solenoid will not. You don't need to know how or why the relationship exists if your aim is to simply replace the valve, or even design the valve, but knowing the relationships expands your knowledge continuum.

Figure 1. Research has revealed that the atom is much more complex than the Bohr-Rutherford model illustrated here. However, it will suffice to explain the analogy between electrical current, the flow of electrons, and hydraulic current (flow).

I will rely upon analogies between hydraulics and electronics, so that your present knowledge of things hydraulic can help you understand things electronic. There are many analogous concepts, but there are also many divergent ones that are profound indeed. I will cover some of both.

Atoms and molecules
In the Bohr-Rutherford (B-R) model of an atom, Figure 1, the atom was visualized as a super-miniature solar system made up of electrons with their negative charges, spinning around a much larger, positively charged nucleus, comprised of closely packed protons, Figure 1. Research has revealed that the atom is much more complex than the B-R model, but it will suffice to explain the analogy between electrical current, the flow of electrons, and hydraulic current, or, as it is more commonly called, hydraulic flow.

Unlike the planets in our solar system, which all have different masses and even different elemental concentrations, the "planets" in the atomic solar system, the electrons, are all the same. Additionally, all the protons of the nuclei are the same. In fact, every proton in the entire universe looks exactly the same as every other proton in the universe, and ditto for the electrons.

The electron and proton are extremely tiny bodies, and each has a mass, with the mass of a proton many times greater than the mass of the electron. But what sets these sub-atomic particles apart from other masses is that the electron and proton carry an electrical charge. Electrical charge, simply referred to as charge, is measurable — and the unit of measure is the coulomb. A coulomb is a quantity of electrical charge. The amount of charge in a single electron or proton is about 1.6×10-19 coulombs.

If we just reciprocate that minuscule amount of charge, we conclude that in order to make one coulomb of charge, we need 6.28×1018 electrons. A complete atom contains as many electrons as protons, and each has the same electrical charge, so the atom is said to be electrically neutral. Atoms can lose electrons in the normal course of their interactions, and when this happens, they lose their electrical neutrality and become ions with net positive charge.

Learning from the Bohr-Rutherford model
The B-R atomic model teaches us that the orbiting electrons, like the planets in our solar system, orbit the nucleus at different distances from the nucleus. The orbital paths for the electrons are taken to be elliptical, and they exist at ordered distances from the nucleus. Each orbit can hold only a specific number of electrons.

The B-R model also teaches us that there are two very broad categories of materials that have an impact on our quest for practical electronic knowledge: conductors and insulators. In some materials, primarily metals, the B-R model reveals that the electrons in the outer rings are very loosely bonded to the parent atom, but in insulators, such as sand and glass (silicon), the outer ring of electrons is very tightly bonded to the nucleus, that is, the parent atom. Knowing this important difference means we can now visualize a very important and practical electronic variable, electrical current.

Figure 2. A conductor is a strand of gold, copper, platinum, lead, aluminum, or other material that contains loosely bound electrons in its outer ring.

Electrical current is the movement of charged particles. Any time an electron is in motion, it constitutes a current, another measurable variable. The unit of measure for electrical current is the ampere. It is defined as the passage of one coulomb of charge in one second — the coulomb per second.

If we have a conductor, say copper wire, and we observe that 6.28×1018 electrons have gone past a point in that wire, then we conclude that there is one ampere of current in the wire. We don't actually count the number of electrons passing, instead, we insert an ammeter, an instrument for measuring electrical current, and simply read the value.

So just how do all those electrons get through that solid wire? Some fluid power specialist once quipped that he couldn't see how to get all that power through a solid wire that didn't even have a hole down the middle. That's where we turn to the B-R atomic model. The wire conductor is a strand of gold, copper, platinum, lead, aluminum, or other material that contains loosely bound electrons in its outer ring.

If we could somehow deposit an electron in one end of our wire and deposit it into one of the metal's atoms, that atom would give up an electron and pass it to its neighbor, and that neighbor would pass one on to its neighbor, and in this fashion, at the other end of the wire, one electron would have to jump out. This is how electrons pass through a solid wire. Actually, the wire is not solid at the atomic level, it is filled mostly with empty space. The conductor's atoms act like a bucket brigade and pass the electron from one to the next, Figure 2. In a complete circuit, then, the electron motion is indeed an electrical current.