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Chapter 1. Electrons in Motion

The field of electrical theory and electronics is huge, and it can be somewhatdaunting at first. In reality, you don’t need to know all the littletheoretical details to get things up and running. But to give yourefforts a better chance at success, it is a good idea to understand the basicsof what electricity is and how, in general terms, it works. So that’s what we’regoing to look at here.

The main intent of this chapter is twofold. First, I want to dispense with theold “water-flowing-in-a-pipe” analogy that has been used in the past to describe theflow of electrons in a conductor; it’s not very accurate and can lead to someerroneous assumptions. There is, I believe, a better way to visualize what isgoing on, but it does require a basic understanding of what an atom is and howits component parts work to create electric charge and, ultimately, electriccurrent. It might sound rather like something from the realm of physics (and,to be honest, it is, along with chemistry), but once you understand theseconcepts, things like fluorescent lights, neon signs, lightning, arc welders,plasma cutting torches, heating elements, and the electronic components youmight want to use in a project will become easier to understand. The oldwater-flowing-in-a-pipe model doesn’t really scale very well, nor does ittranslate easily to anything other than, well, water flowing through a pipe.

Second, I’d like to build on this atom-based model to introduce some basicconcepts that will come up later as you work on your own projects. By the endof this chapter, you should have a good idea of what the terms voltage, current,and power mean and how to calculate these values. If you need more details on alower level, you’ll find them in Appendix A, including overviews of serialand parallel circuits, and basic AC circuit concepts. Of course,numerous excellent texts are readily available on the subject, and I encourage youto seek them out if you would like to dig deeper into the theory of electronics.

If you are already familiar with the basic concepts of electronics, feelfree to skip this chapter. Just don’t forget to take advantage of Appendix Aand the suggested references in Appendix C if you run into a need for furtherdetails somewhere along the line.


Atoms and Electrons

In common everyday usage, the term electricity is used to refer to the stuffthat one finds inside a computer, in a wall outlet, in the wires strung betweenpoles beside the street, or at the terminals of a battery. But just what isthis stuff, really?

Electricity is the physical manifestation of the movement of electrons, littlespecks of subatomic matter that carry a negative electrical charge. As we know,all matter is composed of atoms. Each atom has a nucleus at its core with anet positive charge. Each atom also has one or more negative electrons bound toit, each one whipping around the positively charged nucleus in a quantumfrenzy.

It is not uncommon to hear of the “orbit” of an electron about the nucleus, butthis isn’t entirely accurate, at least not in the classical sense of the termorbit. An electron doesn’t orbit the nucleus of an atom in the way a planet orbitsa star or a satellite orbits the earth, but it’s a close enough approximationfor our purposes.

In reality, it’s more like layers of clouds wrapped around the nucleus, with theelectrons being somewhere in the layers of the cloud. One way to think of it isas a probability cloud, with a high probability that the electron is somewhere ina particular layer. Due to the quirks of quantum physics, we can’t directlydetermine where an electron is located in space at any given time without breakingthings, but we can infer where it is by indirect measurements. Yes, it’s a bitmind-numbing, so we won’t delve any deeper into it here. If you want to know moreof the details, I would suggest a good modern chemistry or physics textbook, or fora more lightweight introduction, you might want to check out the “Mr. Tompkins”series of books by the late theoretical physicist George Gamow.

The nucleus of most atoms is made up of two basic particles: protons and neutrons,with the exception of the hydrogen atom, which has only a single positive protonas its nucleus. A nucleus may have many protons, depending on what type of atomit happens to be (iron, silicon, oxygen, etc.). Each proton has a positivecharge (called a unit charge). Most atoms also have a collection of neutrons,which have about the same mass as a proton but no charge (you might think ofthem as ballast for the atom’s nucleus). Figure 1-1 shows schematicrepresentations of a hydrogen atom and a copper atom.


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Figure 1-1. Hydrogen and copper atoms

The +1 unit charges of the protons in the nucleus will cancel out the –1 unitcharges of the electrons, and the atom will be electrically neutral, which is thestate that atoms want to be in. If an atom is missing an electron, it will havea net positive charge, and an extra electron will give it a net negative charge.

The electrons of an atom are arranged into what are called orbital shells (theclouds mentioned earlier), with an outermost shell called the valence shell.Conventional theory states that each shell has a unique energy level and eachcan hold a specific number of electrons. The outermost shell typicallydetermines the chemical and conductive properties of an atom, in terms of howeasily it can release or receive an electron. Some elements, such as metals, havewhat is considered to be an “incomplete” valence shell. Incomplete, in this sense,means that the shell contains fewer than the maximum possible number of electrons,and the element is chemically reactive and able to exchange electrons with otheratoms. It is, of course, more complex than that, but a better definition is waybeyond the scope of this book.

For example, notice that the copper atom in Figure 1-1 has 29electrons and one is shown outside of the main group of 28 (which would bearranged in a set of shells around the nucleus, not shown here for clarity).The lone outermost electron is copper’s valence electron. Because the valenceshell of copper is incomplete, this electron isn’t very tightly bound, so copperdoesn’t put up too much of a fuss about passing it around. In other words,copper is a relatively good conductor.

An element such as sulfur, on the other hand, has a complete outershell and does not willingly give up any electrons. Sulfur is rated as one ofthe least conductive elements, so it’s a good insulator. Silver tops the listas the most conductive element, which explains why it’s considered useful inelectronics. Copper is next, followed by gold. Still, other elements are somewhatambivalent about conducting electrons, but will do so under certain conditions.These are called semiconductors, and they are the key to modern electronics.

This should be a sufficient model for our purposes, so we won’t pry any furtherinto the inner secrets of atomic structure. What we’re really interested inhere is what happens when atoms do pass electrons around, and why they woulddo that to begin with.


Electric Charge and Current

Electricity involves two fundamental phenomena: electric chargeand electric current. Electric charge is a basic characteristic of matter andis the result of something having too many electrons (negative charge), ortoo few electrons (positive charge) with regard to what it would otherwiseneed to be electrically neutral. An atom with a negative or positive chargeis sometimes called an ion.

A basic characteristic of electric charges is that charges of the same kindrepel one another, and opposite charges attract. This is why electrons andprotons are bound together in an atom, although under most conditions theycan’t directly combine with each other because of some other fundamentalcharacteristics of atomic particles (the exceptional cases are a certaintype of radioactive decay and inside a stellar supernova). The importantthing to remember is that a negative charge will repel electrons, and apositive charge will attract them.

Electric charge, in and of itself, is interesting but not particularly usefulfrom an electronics perspective. For our purposes, really interesting thingsbegin to happen only when charges are moving. The movement of electronsthrough a circuit of some kind is calledelectric current, or current flow, and it is also what happens when the staticcharge you build up walking across a carpet on a cold, dry day is transferredto a doorknob. This is, in effect, the current (flow) moving between a highpotential (you) to a lower potential (the doorknob), much like water flowsdown a waterfall or a rock falls down the side of a hill. The otherwiseuninteresting static charge suddenly becomes very interesting (or at leastit should get your attention). When a charge is not in motion, it is calledthe potential, and yes, we can make an analogy between electrical potentialand mechanical potential energy, as you’ll see shortly.

Current flow arises when the atoms that make up the conductors and componentsof electrical circuits transfer electrons from one to another. Electrons movetoward things that are positive, so if you have a small light bulb attachedto a battery with some wires (sometimes also known as a flashlight), theelectrons move out of the negative terminal of the battery, through the lightbulb, and return back into the positive terminal. Along the way, they causethe filament in the lamp to get white-hot and glow.

Figure 1-2, a simplified diagram of some copper atoms ina wire, shows one way to visualize the current flow. When an electron isintroduced into one end of the wire, it causes the first atom to becomenegatively charged. It now has too many electrons. Assuming acontinuous source of electrons, the new electron cannot exit the way it camein, so it moves to the next available neutral atom. This atom is now negativeand has a surplus electron. In order to become neutral again (the preferredstate of an atom), it then passes an extra electron to the next (neutral)atom, and so on, until an electron appears at the other end of the wire. Solong as there is a source of electrons under pressure connected to the wireand a return path for the electrons back to the source, current will flow. Thepressure is called voltage, which “Current Flow in a Basic Circuit” will discuss in more detail.


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Figure 1-2. Electrons moving in a wire

Figure 1-3 shows another way to think about current.In this case, we have a tube (a conductor) filled end to end with marbles(electrons).


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Figure 1-3. Modeling electrons with marbles in a tube

When we push a marble into one end of the tube in Figure 1-3,a marble falls out the opposite end. The net number of marbles in the tuberemains the same. Note that the electrons put into one end of a conductor arenot necessarily the ones that come out the other end, as you can see from Figures 1-2 and 1-3. In fact, if the conductor is long enough, the electrons introduced at one end might not be the ones that appear at the other end, but electrons would appear, and you would still be able to measure electron movement in the conductor.


Current Flow in a Basic Circuit

Electricity flows when a closed circuit allows for the electrons to move froma high potential to a lower potential in a closed loop. Stated another way, current flow requires a source of electrons with a force to move them, as well as a return point for the electrons.

Electric current flow (a physical phenomenon) is characterized by fourfundamental quantities: voltage, current, resistance, and power. We’ll usethe simple circuit shown in Figure 1-4 as our baseline for thefollowing discussion. Notice that the circuit is shown both in picture andschematic form. For more about schematic symbols, refer to Appendix B.


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Figure 1-4. A simple DC circuit

A few words about the term current are in order here. The word has more thanone meaning in electronics, which can be confusing at first. In one sense,current refers to the flow of electrons through a conductor of some kind. Itis a reference to the movement of charge carried by the electrons. In the othersense, current refers to the number of electrons moving through the conductor.In this sense, it specifies the volume of electrons moving past some pointin the circuit at some point in time. In other words, the measurement of currentis the determination of the quantity of electrons in motion.

One way to think about current is to remember that it cannot be measured without movement, so when you see or hear the word current, it is usuallyreferring to movement. To make the distinction clear, the term current flow isoften used to mean movement of electrical charges. Static charges, even if justat the terminals of a common battery, have no current flow and hence no measurablecurrent.

Current that flows in only one direction, as in Figure 1-4, is calleddirect current (DC). A common battery produces DC, as does the DC power supplyin a typical computer system. Current that changes direction repeatedly is calledalternating current (AC). AC is what comes out of a household wall socket (inthe US, for example). It is also the type of current that drives the loudspeakersin a stereo system. The rate at which the current changes direction is called thefrequency and is measured in cycles per second in units of Hertz (abbreviated Hz).So, a 60 Hz signal is made up of a current flow changing direction 60 times persecond. When AC is used to drive a loudspeaker, a signal with afrequency of 440 Hz will be A above middle C to our ears.

By convention, DC is described as flowing from positive to ground(negative), whereas in reality, electrons flow from the negative terminal to thepositive terminal of the power source. In Figure 1-4, the arrows showthe electron flow. Basically, the discrepancy stems from an erroneous assumption made byBenjamin Franklin, who thought that electrons had a positive charge and flowedfrom positive to negative terminals. He guessed wrong, but we ended up with aconvention that was already well ingrained by the time physicists figured outwhat was really going on. Hence we have conventional current flow and electroncurrent flow. Although you should be aware of this discrepancy, from this pointonward, we’ll use conventional current flow, since that is what most of theelectronics industry uses.

A volt (V) is the unit of measurement used for electric potential difference, electricpotential, and electromotive force. When the term voltage is used, it usually refersto the electric potential difference between two points. In other words, we say that astatic charge has a value of some number of volts (potential), but there is acertain amount of voltage between two points in a circuit (potential difference).

Voltage can be visualized as a type of pressure, or driving force (although it is notactually a force in a mechanical sense). This is the electromotive force (emf) producedby a battery or a generator of some type, and the emf can drive a current through a circuit.And even though it may not look like a generator, a power supply (like the one that plugsinto the wall socket to charge a cell phone) is really nothing more than a converter forthe output of a generator at a power plant somewhere.

Another way to think of voltage is as the electric potential difference between two pointsin an electric field. It is similar to the difference in the potential energy of a cannonball at the top of a ladder as opposed to one at the top of a tall tower. Both cannonballsexist in the earth’s gravitational field, they both have potential energy, and it took somework to get them both into position. When they are released, the cannonball on the top of thetower will have more energy when it hits the ground than the cannonball dropped from thetop of the ladder, because it had a larger potential energy due to its position.

These two descriptions of voltage are really just opposite sides of the same coin. Inorder to create a potential difference between two points, work must be done. When that energyis lost or used, there is a potential drop. When the cannonball hits the ground, all of theenergy put into getting it into position against the pull of gravity is used to make anice dent in the ground.

The main point here to remember is that a high voltage has more available electrical energy(pressure) than a low voltage. This is why you don’t get much more than a barely visiblespark when you short out a common 9-volt battery with a piece of wire, but lightning, ataround 10,000,000 volts (or more!), is able to arc all the way between a cloud and theground in a brilliant flash. The lightning has more voltage and hence a larger potentialdifference, so it is able to overcome the insulating effects of the intervening air.

Whereas voltage can be viewed as electrical pressure, current is the measure of the quantity,or volume, of electrons moving through a circuit at some given point. Remember that the term current can have two different meanings: electron movement (flow) andthe volume of the electron flow. In electronics, the word current usually meansthe quantity of electrons flowing through a conductor at a specific point at a single instantin time. In this case, it refers to a physical quantity and is measured in units ofamperes (abbreviated as A).

Now that we’ve looked at voltage and current, we can examine some of the thingsthat happen while charge is in motion (current flow) at some particular voltage.No matter how good a conventional conductor happens to be, it will never passelectrons without some resistance to the current flow (superconductors getaround this, but we’re not going to deal with that topic here). Resistance isthe measure of how much the current flow is impeded in a circuit, and it ismeasured in units of ohms, named after German physicist Georg Simon Ohm. “Resistance”has more details about the physical properties of resistance, but for now, let’s consider howresistance interacts with current flow.

You might think of resistance as an analog of mechanical friction (but the analogyisn’t perfect). When current flows through a resistance, some of the voltagepotential difference is converted to heat, and there will be a voltage drop acrossthe resistor. How much heat is generated is a function of how much current is flowingthrough the resistance and the amount of the voltage drop. We’ll look at this more closelyin “Power”.

You can also think of resistance as the degree of “stickiness” that an atom’s valenceshell electrons will exhibit. Atoms that can give up or accept electrons easily will havelow resistance, whereas those that want to hold onto their electrons will exhibit higherresistance (and, of course, those that don’t readily give up electrons under normalconditions are good insulators).

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Carbon, for example, will conduct electricity, but not as easily as copper. Carbonis a popular material for fabricating the components called resistors used inelectronic circuits. Chapter 8 covers passive components, such as resistors.