Electrons are now presumed to be a self contained atomic particle, not composed of anything any smaller, and have a mass of 1/1836 of that of the proton. If you could get 1836 of those guys glued together, the group would balance a proton on the other end of a teeny tiny teeter totter. But you couldn’t do that because an electron is something called a fermion which means only one can occupy a given space at a time due to a rule called the Pauli exclusion principle and since they all have a negative charge, they each push the others away from themselves. The other 1835 guys would have to float around somewhere else, all of them staying out of the way of each other. At the moment however, we don’t know of any element that has that many electrons floating around the nucleus anyway so the 1/1836 value is really only of academic interest. The electron has a charge of -1.602 x 10-19 coulomb. Notice that the charge of the electron is negative. Meanwhile, the charge of the much more massive proton, is +1.602 x 10-19 coulomb. So, charge-wise, the electron and proton are equivalent but have opposite signs. When you smoosh one of each together, you get an annihilation of both particles, a net charge of zero, and the release of gamma rays and photons. Congratulations, you’ve just learned your first bit about quantum electrodynamics. Not too bad was it?
Electrons are very mobile, light, and energetic particles. Protons on the other hand are pretty heavy, comparatively, less energetic, and tend to stay put. Since the number of protons in the nucleus of an atom of an element determine the type of element, you don’t want protons moving around and being exchanged between adjacent atoms. Otherwise, you would have one element changing into another element unpredictably. You don’t want a dish of carbon changing into part sulfur and part nitrogen at the drop of a hat. You want that elemental carbon to stay as it is until purposely caused to change. On the other hand, since the highly mobile electrons are easily attracted to other protons, those electrons can be pretty easily shared with the protons in the nucleus of nearby other elements. This behavior permits elements to combine into molecules of compound chemicals by sharing electrons yet keeping the protons in the compound’s elements intact.
The chemistry of elements and compounds comes into play in electronics all the time. By changing the characteristics of chemicals involved in materials, we can cause LEDs to emit different colors of light, cause transistors to have different gain and loss characteristics, and cause different types of transistors in integrated circuits to operate at different voltages and speeds of transmission. Chemistry plays vital roles in all areas of electronics, setting the characteristics of conducting and insulating materials as well.
Back to seeing what those electrons are capable of. Always remember that electrons are negatively charged. When they move, they move from an area of a concentrated negative charge to an area of positive (or less negative) charge. This is really important to understand and remember. Life’s experiences sort of direct you in the wrong way here, making you think that electricity flows from the “+” (plus, or red) terminal to the “-” (minus or black) terminal. But no! That’s not what happens. Its the other way around! When those mobile electrons move, it causes a current to flow. Just like water flowing in a stream, the electrons move from a point of higher negative charge to a point of lower negative charge (or more positive) causing a stream of electron current. Current flow is the result. From the minus terminal (usually labeled as black- but not always) to the plus terminal (usually labeled as red, but again, not always; you always have to be careful about this color labeling business).
Remember that the electric charge of a single electron is -1.602 x 10-19 coulomb. If we have 1×1019 electrons flowing from point A to point B, we will have a charge difference of -1.602 coulomb between A and B. Similarly, if we happened to have 6.2422 x 1018 electrons laying around and we got them all to zoom past an arbitrary point in one second, we’d have exactly 1 ampere of current flowing past that point. (6.2422 x 1018 x |-1.60210-19| = 1.0). If, however, we had only 3.1211 x 1018 electrons, and it still took them that one second to pass that arbitrary point, we’d only have half an ampere of current flow. The charge of each electron remains constant but we have only half as many electrons as before so the quantity of current flow is half as much. Current flow then, measured and expressed in amperes, is defined as 1 coulomb of charge flowing past a point in 1 second. To have current, you have to have movement of the electrons and their associated charge.
That’s pretty simple. So, what’s a coulomb? The short answer is that it is the arbitrary name given to a measure of the combined amount of electric charge on 6.2422 x 1018 electrons. The long answer is a bit more involved and puts us down a rabbit hole that leads to a whole different world of stuff called atomic and quantum theory. This is actually pretty cool stuff and all links together pretty nicely but leads us into looking at some pretty scary mathematics.
6.2422 x 1018 electrons sounds like quite a few electrons. Is there some way to get a feeling for how much that really is? Sure, let’s use an amount of iron to see how big of a pile of it we would have to have to accumulate that many electrons. Since iron has an atomic number or weight of 26, that means that a single atom of iron has 26 protons in its nucleus. Since there are 26 protons, there are also 26 electrons. If we divide 6.2422 x 1018 by 26 we have 2.4008 x 1017, still a pretty big number. Since electrons don’t have any mass, it is the mass of the protons that is going to account for the mass in this accumulation of iron. The mass of a proton is about 938 MeV/c2. Oops, that looks like it isn’t going to be as straight forward as it was looking, I was hoping for grams. But no. MeV by the way is million electron volts and c2 is the speed of light squared.
938 million electron volts divided by the speed of light squared. Volts and the speed of light? Well, we’ll get to volts in a little while so maybe we stumbled across something useful. But the speed of light? What’s that got to do with it? Well, you’ve probably heard of Albert Einstein and his famous equation E = mc2. That equation established that mass and energy are directly proportional to each other and related by the speed of light squared. What that means is, a given mass of matter has a lot of energy in it, that mass and energy are equivalent, and that mass and energy can be converted to each other. Those electron volts are really a measure of the amount of energy contained within the proton expressed in joules.
As we’ve just seen, its hard to talk about things in physics, electronics, and chemistry without encountering that equal sign in Einstein’s equation. Mass and energy are equivalent, we can’t have one without the other. The equivalence is so common that it has been simplified to the value of 1 GeV/c2 = 1.783 x 10-27 kg. A really small mass of 1.783 x 10-27 kilograms is equivalent to 1 giga electron volts accelerated to the speed of light squared.
Our proton mass then equates to 0.938 x 1.783 x 10-27 kg, 938 MeV/c2 divided by 1 GeV/c2 or 1.6725 x 10-27 kg per proton. Multiply that by 2.4008 x 1017 protons and our pile of iron should have a mass of 4.0153 x 10-10 grams. A really small amount! A coulomb of electrons is contained within that tiny amount of iron.
Lets stop for a minute and explain some things.
This coulomb stuff is just an arbitrary name given to the measure of electric charge. We’ll get back to that in a minute. First, note that we are going to run into all kinds of things that have these arbitrary names. Names like Ohms, Volt, Ampere, Henry, Farads. All these names have quantities of “stuff” associated with them. The “stuff” is all different, which is why they have different names and they are not equivalent. Although, in some cases, one type of quantity of stuff may be expressed as some other quantity of other stuff, both related by some numeric quantity. We’ll try to point those cases out when we run into them. For now, note that each of these names refer to some person who, back in history, discovered and then described the physical characteristics of a particular property of material, a process, or an effect. Their name has then been ascribed to each of these properties in order to be able to conveniently refer to them. Each of these historical people have extremely interesting stories associated with them. How they first noticed the property of matter that became associated with their name, how they developed techniques to quantify or measure the property, and subsequently, how their discovery led to our further understanding of the nature of the characteristics of the universe. There are lots of these names and it will become important to understand each of the properties of matter that the name refers to so that eventually, you have a complete understanding of the full nature of the sciences associated with electronics. Although there are lots if these names, they will become familiar and their meaning will eventually be comfortable to you. I urge you to explore the stories of each of the people whose names we will be using. Almost every one of the stories is interesting and the remarkable thing is just how long ago their discoveries were made. Think of how observant, then curious and clever they were to have worked out their contributions to knowledge of the universe. The question is, will you be as clever and make such an important a contribution as any of them? Note that as time passed, nearly each of those people studied the discoveries of their predecessors, came to understand the previous discoveries, then used that previous information to formulate their own contributions. Nobody understood it all to begin with. Interestingly, just think about all the information that is available to you, right now, that those folks did not have available to them in their lifetimes!
Ok, let’s get back to the coulomb. Wait a minute, who says the electric charge of an electron is -1.602 x 10-19 coulomb? How did they figure that out? Actually Robert Milliken figured it out in 1909 by performing his famous oil drop experiment. The reason its famous is because it finally determined what the electric charge on a single electron was. How? By determining how many electrons were in a tiny drop of oil which he was able to suspend in mid-air by applying opposite electrostatic potentials to two plates which allowed the oil drop to float in mid-air. By carefully measuring the electrostatic potential, and knowing roughly how many molecules of oil were in the drop and thus how many protons and electrons, it was possible to calculate the amount of charge that had to be applied to all of the electrons to get the molecules of oil to overcome the attraction of the force of gravity. Dividing the amount of charge by the number of electrons results in the charge contributed by a single electron.
How many molecules of oil were in the drop? Well, if we make a whole bunch of drops, count how many of the drops it takes to comprise a given volume, say ten cubic centimeters, then divide 10 cc by that number of drops, we can pretty closely determine the volume of a drop. Since they could determine the density of the oil, the mass of a droplet could be determined. The mass then reveals the number of molecules. Knowing the number of molecules and the chemical formula of the oil tells us how many atoms of each element make up the molecules and then how many protons and electrons are contained in that volume of oil within the droplet. Very clever progressive reasoning. Won him a Nobel prize.
It gets a bit more complicated and involved to fully explain a coulomb because it also involves forces in different directions or vectors, so let’s try to simplify it a bit and leave the full explanation for you to discover some other time. In about 1777, a French physicist named Charles Coulomb developed a device called a torsional balance which is a very sensitive mechanism for measuring the amount of force applied to a fine fiber, spring, or wire. By using the torsional balance, Coulomb determined that electrically charged materials – probably copper or brass spheres (with a static charge applied by rubbing those materials with probably cat fur) repelled each other with forces that varied directly (which means that the effect is mathematically described within the numerator of an equation) by both the amount of charge that was on the material (determined probably by the number of times he rubbed the material with the fur), as well as inversely (the effect is is described in the denominator of an equation) by how far apart two pieces of the materials were. In fact, the repellant forces decreased by 1/4 when the distance doubled and by 1/16 when the distance quadrupled. So, in 1785 he was able to say that the repellant forces varied inversely as the square of the distance between them. That’s about all that could be determined back then. It remained a while (actually 1861 when published by James Maxwell) before it was noted that the electric charge (the value in the numerator) influenced things at the speed of light (determined by a number of investigators moderately accurately between 1676 and 1862) and how well the materials involved could become magnetic (called magnetic permeability and described by mathematics developed by Oliver Heaviside in 1885). Together then, Coulomb’s initial observations were refined by many famous names that appear in physics until a value for the unit electrical charge on an electron was finally determined in 1909.
And there we have it. We have learned how physicists figured out the first property of electricity: current. And what current really is: the quantity of unit electric charges flowing past a point in a given amount of time. One ampere or amp of current consists of 6.2422 1018 electron’s worth of unit charges flowing past a point in one second. The ampere was named for Andre-Marie Ampere, a French professor of physics and chemistry born a year before America declared its independence. Ampere demonstrated the relationship between current flow and magnetics by showing that two parallel wires attracted or repelled each other proportionally depending upon the length of the wire and the current they carried. The ampere is abbreviated “A” or called amp and is usually referred to as “I” in equations. Other values may be expressed as milliamperes, milliamps, or ma (1 thousandth of an ampere), or microamp (1 millionth of an amp). Next we’ll explore the volt and then we will have learned about two of the three elements of the most basic equation of electronics, ohms law. From there, everything else is down hill.