Chapter 2

Understanding Electricity

IN THIS CHAPTER

Unraveling the deep mysteries of the matter and energy (well, only a little bit)

Learning about three important aspects of electric circuits: current, voltage, and power

Looking at the difference between direct and alternating current

Learning your first electrical equation (don’t worry; it’s simple)

Frankly, the title of this chapter is a bit ambitious. Before you can do much that’s very interesting with electronics, you need to have a basic understanding of what electricity is and how it works, but unfortunately, understanding electricity is a tall order. Don’t let this discourage or dissuade you: Even the smartest physicists in the world don’t really understand it.

At the start of this chapter, you examine the very nature of electricity: what it is and what causes it. This first part of the chapter will remind you of a seventh- or eighth-grade science class, as you delve into the insides of atoms and learn about protons, neutrons, and electrons.

The second part of this chapter introduces you to three things you have to know about electricity if you want to design and build circuits: current, voltage, and power — the Manny, Moe, and Jack of electricity. Or if you prefer, the Huey, Dewey, and Louie, or the Groucho, Chico, and Harpo, or the — you get the idea.

Pondering the Wonder of Electricity

The exact nature of electricity is one of the core mysteries of the universe. Although we don’t really know exactly what electricity is, we do know a lot about what it does and how it behaves.

Strange as it may sound, your understanding of electricity will improve right away if you avoid using the term electricity to describe it. That’s because the term electricity isn’t very precise. We use the word electricity to refer to any of several different but related things. Each has a more precise name, such as electric charge, electric current, electric energy, electric field, and so on. All these things are commonly called electricity.

Electricity isn’t so much a specific thing, but a phenomenon that has many different faces. So to avoid confusion, I try to avoid the word electricity in the rest of this book. Instead, I use more precise terms such as charge or current.

I really hate to use the word phenomenon here because it sounds so scientific. I feel like I should wear a bow tie whenever I say the word phenomenon. I consulted my thesaurus to see if there was a simpler word I could use instead. None of the suggestions really seemed to fit, but the one that came closest was wonder .

Wonder isn’t a bad substitute. When it comes down to it, the phenomenon we call electricity is pretty amazing. It really does qualify as one of the great wonders of the universe.

Remember the so-called “Seven Wonders of the Ancient World,” which included the Great Pyramid of Giza, the Hanging Gardens of Babylon, the Temple of Artemis at Ephesus, the Statue of Zeus at Olympia, the Mausoleum of Halicarnassus, the Colossus of Rhodes, and the Lighthouse of Alexandria? If we were to make a list called “The Seven Wonders of the Universe,” I suppose it would have to include Matter, Gravity, Time, Light, Life, Pizza, and Electricity.

WHERE DID THE WORD ELECTRICITY COME FROM?

Do you remember the movie Jurassic Park , where scientists discovered the DNA of dinosaurs locked inside bits of amber? Amber is fossilized tree resin, and it played a key role in the history of our knowledge about electricity.

Since the days of the ancient Greeks, people have known if you rubbed sticks of amber with fur, the amber could then be used to raise the hair on your head and that lightweight objects like feathers would stick to it. They had no idea why this happened, but they knew that it did happen.

The Greek word for amber is elektron . The Latin version of the word was electricus.

At the beginning of the seventeenth century, an English scientist named William Gilbert began to study electricity. He used these ancient words to describe the phenomena he was investigating, including the Latin term electricus . The influence of Gilbert’s book, which was written in Latin, led to the word electricity in the English language.

Looking for Electricity

One of the most amazing things about electricity is that it is, literally, everywhere. By that I don’t mean that electricity is commonplace or plentiful, or even that the universe has an abundant supply of electricity. Instead, what I mean is that electricity is a fundamental part of everything.

To get an idea of what I mean, consider a common misconception about electric current. Most of us think that wires carry electricity from place to place. When we plug in a vacuum cleaner and turn on the switch, we believe that electricity enters the vacuum cleaner’s power cord at the electrical outlet, travels through the wire to the vacuum cleaner, and then turns the motor to make the vacuum cleaner suck up dirt and grime and dog hair. But that’s not the case. The truth is that the electricity was already in the wire. The electricity is always in the wire, even when the vacuum cleaner is turned off or the power cord isn’t plugged in. That’s because electricity is a fundamental part of the copper atoms that make up the wire inside the power cord. Electricity is also a fundamental part of the atoms that make up the rubber insulation that protects you from being electrocuted when you touch the power cord. And it’s a fundamental part of the atoms that make up the tips of your finger which the rubber keeps from touching the wires.

In short, electricity is a fundamental part of the atoms that make up all matter. So, to understand what electricity is, we must first look at atoms.

Peering Inside Atoms

As you probably learned in grade school, all matter is made up of unbelievably tiny bits that are called atoms . They’re so tiny that the period at the end of this sentence contains several trillion of them.

It’s hard for us to comprehend numbers as large as trillions. For the sake of comparison, suppose you could enlarge the period at the end of this sentence until it was about the size of Texas. Then, each atom would be about the size of — you guessed it — the period at the end of this sentence.

The word atom comes from an ancient Greek fellow named Democritus. Contrary to what you might expect, the word atom doesn’t mean “really small.” Rather, it means “undividable.” Atoms are the smallest part of matter that can’t be divided without changing it to a different kind of matter. In other words, if you divide an atom of a particular element, the resulting pieces are no longer the same thing.

For example, suppose you have a handful of some basic element such as copper and you cut it in half. You now have two pieces of copper. Toss one of them aside, and cut the other one in half. Again, you have two pieces of copper. You can keep doing this, dividing your piece of copper into ever smaller halves. But eventually, you’ll get to the point where your piece of copper consists of just a single copper atom.

If you try to cut that single atom of copper in half, the resulting pieces will not be copper. Instead, you’ll have a collection of the basic particles that make up atoms. There are three such particles, called neutrons, protons, and electrons.

The neutrons and protons in each atom are clumped together in the middle of the atom, in what is called the nucleus . The electrons spin around the outside of the atom.

When I first learned about atoms as a kid, I was taught that the electrons orbit around the nucleus much like planets orbit around the sun in our solar system. Even today, kids are taught this. School children are still being taught to create models of atoms using Styrofoam balls and wires, like the one shown in Figure 2-1 .

FIGURE 2-1: A common model of an atom.

That turns out to be a really bad analogy. Instead, the electrons whiz around the nucleus in a cloud that’s called, appropriately enough, the electron cloud. Electron clouds have weird shapes and properties, and strangely enough, it’s next to impossible to figure out exactly where in its cloud an electron actually is at any given moment.

Examining the Elements

Several times in this chapter, I use the term element without explaining it. So here’s the deal: An element is a specific type of atom, defined by the number of protons in its nucleus. For example, hydrogen atoms have just one proton in the nucleus, an atom with two protons in the nucleus is helium, atoms with three protons are called lithium, and so on.

The number of protons in the nucleus of an atom is called the atomic number . Thus, the atomic number of hydrogen is 1, the atomic number of helium is 2, lithium is 3, and so on. Copper — an element that plays an important role in electronics — is atomic number 29. Thus, it has 29 protons in its nucleus.

What about neutrons, the other particle found in the nucleus of an atom? Neutrons are extremely important to chemists and physicists. But they don’t really play that big of a role in the way electric current works, so we can safely ignore them in this chapter. Suffice it to say that in addition to protons, the nucleus of each atom (except hydrogen) contains neutrons. In most cases, there are a few more neutrons than protons.

The third particle that makes up atoms is the electron. Electrons are what we’re most interested in when we work with electricity because they are the source of electric current. They’re unbelievably small; a single electron is about 200,000 times smaller than a proton. To gain some perspective on that, if a single electron were the size of the period at the end of this sentence, a proton would be about the size of a football field.

Atoms usually have the same number of electrons as protons, and thus an atom of the element copper has 29 protons in a nucleus that is orbited by 29 electrons. When an atom picks up an extra electron or finds itself short of an electron, things get interesting because of a special property of protons and electrons called charge, which I explain in the next section.

Minding Your Charges

Two of the three particles that make up atoms — electrons and protons — have a very interesting characteristic called electric charge . Charge can be one of two polarities : negative or positive. Electrons have a negative polarity, while protons have a positive polarity.

The most important thing to know about charge is that opposite charges attract and similar charges repel. Negative attracts positive and positive attracts negative, but negative repels negative and positive repels positive.

As a result, electrons and protons are attracted to each other, but electrons repel other electrons and protons repel other protons.

The attraction between protons and electrons is what holds the electrons and the protons of an atom together. This attraction causes the electrons to stay in their orbits around the protons in the nucleus.

Here are a few more enlightening details about charge:

• Charge is a property of one of the fundamental forces of nature known as electromagnetism . The other three forces are gravity , the strong force , and the weak force.

• As I say in the previous section, an atom normally has the same number of electrons as protons. This is because the electromagnetic force causes each proton to attract exactly one electron. When the number of protons and electrons is equal, the atom itself has no net charge. It is then said to be neutral.

  However, it’s possible for an atom to pick up an extra electron. When it does, the atom has a net negative charge because of the extra electron. It’s also possible for an atom to lose an electron, which causes the atom to have a net positive charge because it has more protons than electrons.

• If you’ve been paying attention, you may have wondered how it can be that the nucleus of an atom can stay together if it consists of two or more protons that have positive charges. After all, don’t like charges repel? Yes they do, but the electrical repellent force is overcome by a much more powerful force called, for lack of a better term, the strong force . Thus, the strong force holds protons (and neutrons) together in spite of the protons’ natural tendency to avoid each other.

• The strong force doesn’t affect electrons, so you never see electrons clumped together the way protons do in the nucleus of an atom. The electrons in an atom stay well away from each other.
• If one were so inclined, one might liken the strong force to the patriotic force that binds the citizens of a nation together in spite of their differences. It’s this force that keeps a country together in spite of the fact that its political parties seem to hate each other. Let’s hope the strong force remains strong.

Conductors and Insulators

Some elements don’t hold on to their outermost electrons very tightly. These elements frequently lose electrons or pick up extra electrons, and so they frequently get bumped off of neutral and become either negatively or positively charged. Such elements are called conductors. The best conductors are the metals silver, copper, and aluminum.

Other elements hold on to their electrons tightly. In these elements, it’s hard to pry loose an electron or force another electron in. These elements almost always stay neutral. They’re called insulators.

In a conductor, electrons are constantly skipping around between nearby atoms. An electron jumps out of one atom — call it Atom A — into a nearby atom, which I’ll call Atom B. This creates a net positive charge in Atom A and a net negative charge in Atom B. But almost immediately, an electron will jump out of another nearby atom – call it Atom C — into Atom A. Thus, Atom A again becomes neutral, and now Atom C is negative.

This skipping around of electrons in a conductor happens constantly. Atoms are in perpetual turmoil, giving and receiving electrons and constantly cycling their net charges from positive to neutral to negative and back to positive.

Ordinarily, this movement of electrons is completely random. One electron might jump left, but another one jumps right. One goes up, another goes down. One goes east, the other goes west. The net effect is that although all the electrons are moving, collectively they aren’t going anywhere. They’re like Keystone Kops, running around aimlessly in every direction, bumping into each other, falling down, picking themselves back up, and then running around some more. When this randomness stops and the Keystone Kops get organized, the result is electric current, as explained in the next section.

Understanding Current

Electric current is what happens when the random exchange of electrons that occurs constantly in a conductor becomes organized and begins to move in the same direction.

When current flows through a conductor such as a copper wire, all those electrons that were previously moving about randomly get together and start moving in the same direction. A very interesting effect then happens: The electrons transfer their electromagnetic force through the wire almost instantaneously. The electrons themselves all move relatively slowly — on the order of a few millimeters a second. But as each electron leaves an atom and joins another atom, that second atom immediately loses an electron to a third atom, which immediately loses an electron to the fourth atom, and so on trillions upon trillions of times.

The result is that even though the individual electrons move slowly, the current itself moves at nearly the speed of light. Thus, when you flip a light switch, the light turns on immediately, no matter how much distance separates the light switch from the light.

Here are a few additional points that may help you understand the nature of current:

• One way to illustrate this principle is to line up 15 balls on a pool table in a perfectly straight line, as shown in Figure 2-2 . If you hit the cue ball on one end of the line, the ball on the opposite end of the line will almost immediately move. The other balls will move a little, but not much (assuming you line them up straight and strike the cue ball straight).

  This is similar to what happens with electric current. Although each electron moves slowly, the ripple effect as each atom loses and gains an electron is lightning fast (literally!).

• It’s no coincidence that moving water is also called current. Many of the early scientists who explored the nature of electricity believed that electricity was a type of fluid, and that it flowed in wires in much the same way that water flows in a river.

• The strength of an electric current is measured with a unit called the ampere , sometimes used in the short form amp or abbreviated A . The ampere is nothing more than a measurement of how many charge carriers (in most cases, electrons) flow past a certain point in one second. One ampere is equal to 6,240,000,000,000,000,000 electrons per second. That’s 6,240 quadrillion electrons per second. (That’s a huge number, but remember that electrons are incredibly small. To give it some perspective, though, imagine that each electron weighed the same as an average grain of sand. If that were true, one amp of current would be equivalent to the movement of nearly 350 tons of sand per second.)

• Most electric incandescent light bulbs have about one amp of current flowing through them when they are turned on. A hair dryer uses about 12 amps.
• Current in electronic circuits is usually much smaller than current in electrical devices like light bulbs and hair dryers. The current in an electronic circuit is often measured in thousandths of amps, or milliamps (abbreviated mA ).
• Current is often represented by the letter I in electrical equations. The I stands for intensity.

FIGURE 2-2: Electrons transfer current through a wire much like a row of pool balls transfers motion.

Understanding Voltage

In its natural state, the electrons in a conductor such as copper freely move from atom to atom, but in a completely random way. To get them to move together in one direction, all you have to do is give them a push. The technical term for this push is electromotive force, abbreviated EMF, or sometimes simply E. But you know it more commonly as voltage.

A voltage is nothing more than a difference in charge between two places. For example, suppose you have a small clump of metal whose atoms have an abundance of negatively charged atoms and another clump of metal whose atoms have an abundance of positively charged atoms. In other words, the first clump has too many electrons and the second clump has too few. A voltage exists between those two clumps. If you connect those two clumps with a conductor such as a copper wire, you create what is called a circuit through which electric current will flow.

This current continues to flow until all the extra negative charges on the negative side of the circuit have moved to the positive side. When that has happened, both sides of the circuit become electrically neutral and the current stops flowing.

Here are some additional points to ponder concerning voltage:

• Whenever there’s a difference in charge between two locations, there’s a possibility that a current will flow between the two locations if those locations are connected by a conductor. Because of this possibility, the term potential is often used to describe voltage. Without voltage, there can be no current. Thus, voltage creates the potential for a current to flow.
• If current can be compared to the flow of water through a hose, voltage can be compared to water pressure at the faucet. It’s water pressure that causes the water to flow in the hose.
• Voltage is measured using a unit called, naturally, the volt , usually abbreviated V. The voltage that’s available in a standard electrical outlet in the United States is about 117 V. The voltage available in a flashlight battery is about 1.5 V. A car battery provides about 12 V.
• You can find out how much voltage exists between two points by using a device known as a voltmeter , which has two wire test leads that you can touch to different points in a circuit to measure the voltage between those points. Figure 2-3 shows a typical voltmeter. (Actually, the meter shown in the figure is technically called a multimeter because it can measure things other than voltage. For more information about using a voltmeter, refer to Chapter 8 of this minibook.)
• Voltages can be considered positive or negative, but only when compared with some reference point. For example, in a flashlight battery, the voltage at the positive terminal is relative to the negative terminal. The voltage at the negative terminal is relative to the positive terminal.

• I’d like to tell you the exact definition of a volt, but I can’t — at least not yet. The definition of a volt won’t make any sense until you learn about the concept of power , which is described later in this chapter in the section titled “Understanding Power .”

• Although current stops flowing when the two sides of the circuit have been neutralized, the electrons in the circuit don’t stop moving. Instead, they simply revert to their natural random movement. Electrons are always moving in a conductor. When they get a push from a voltage, they move in the same direction. When there’s no voltage to push them along, they move about randomly.

• In electrical equations, voltage is usually represented by the letter E , which stands for electromotive force .

FIGURE 2-3: You can use a multimeter like this to measure voltage.

ARE YOU POSITIVE ABOUT THAT?

For the first 150 years or so of serious research into the nature of electricity, scientists had electric current backward: They thought that electric current was the flow of positive charges and that electric current flowed from the positive side of a circuit to the negative side.

It wasn’t until around 1900 that scientists began to unravel the structure of atoms. They soon figured out that electrons have a negative charge, and current is actually the flow of these negatively charged electrons. In other words, they discovered that current flows in the opposite direction from what they had long thought.

Old ideas die hard, and to this day most people think of electric current as flowing from positive to negative. This concept of current flow is sometimes called conventional current . Modern electronic circuits are almost always described in terms of conventional current, so the assumption is that current flows from positive to negative, even though the reality is that the electrons in the circuit are actually flowing in the opposite direction.

Comparing Direct and Alternating Current

An electric current that flows continuously in a single direction is called a direct current, or DC. The electrons in a wire carrying direct current move slowly, but eventually they travel from one end of the wire to the other because they keep plodding along in the same direction.

The voltage in a direct-current circuit must be constant, or at least relatively constant, to keep the current flowing in a single direction. Thus, the voltage provided by a flashlight battery remains steady at about 1.5 V. The positive end of the battery is always positive relative to the negative end, and the negative end of the battery is always negative relative to the positive end. This constancy is what pushes the electrons in a single direction.

Another common type of current is called alternating current, abbreviated AC. In an alternating-current circuit, voltage periodically reverses itself. When the voltage reverses, so does the direction of the current flow. In the most common form of alternating current, used in most power distribution systems throughout the world, the voltage reverses itself either 50 or 60 times per second, depending on the country. In the United States, the voltage is reversed 60 times per second.

Alternating current is used in nearly all the world’s power distribution systems, for the simple reason that AC current is much more efficient when it’s transmitted through wires over long distances. All electric currents lose power when they flow for long distances, but AC circuits lose much less power than DC circuits.

The electrons in an AC circuit don’t really move along with the current flow. Instead, they sort of sit and wiggle back and forth. They move one direction for 1/60th of a second, and then turn around and go the other direction for 1/60th of a second. The net effect is that they don’t really go anywhere.

For your further enlightenment, here are some additional interesting and useful facts concerning alternating current:

• A popular toy called Newton’s Cradle might help you understand how alternating current works. The toy consists of a series of metal balls hung by string from a frame, such that the balls are just touching each other in a straight line, as shown in Figure 2-4 . If you pull the ball on one end of the line away from the other balls and then release it, that ball swings back to the line of balls, hits the one on the end, and instantly propels the ball on the other end of the line away from the group. This ball swings up for a bit, and then turns around and swings back down to strike the group from the other end, which then pushes the first ball away from the group. This alternating motion, back and forth, continues for an amazingly long time if the toy is carefully constructed.

  Alternating current works in much the same way. The electrons initially move in one direction, but then reverse themselves and move in the other direction. The back and forth movement of the electrons in the circuit continues as long as the voltage continues to reverse itself.

• If you want to see a Newton’s Cradle in action, go to YouTube and search for Newton’s Cradle .

• The reversal of voltage in a typical alternating current circuit isn’t instantaneous. Instead, the voltage swings smoothly from one polarity to the other. Thus, the voltage in an AC circuit is constantly changing. It starts out at zero, then increases in the positive direction for a bit until it reaches its maximum positive voltage, and then it decreases until it gets back to zero. At that point, it increases in the negative direction until it reaches its maximum negative voltage, at which time it decreases again until it gets back to zero. Then the whole cycle repeats itself.
• The fact that the amount of voltage in an AC circuit is always changing turns out to be incredibly useful. You learn why in Book 4 , Chapter I when I give you a deeper look at alternating current.

FIGURE 2-4: Newton’s Cradle works much like alternating current.

Understanding Power

At the start of this chapter, I mention the three key concepts you need to know about electricity before you can start to work with your own circuits. The first two — current and voltage — are described earlier in this chapter. To recap, current is the organized flow of electric charges through a conductor, and voltage is the driving force that pushes electric charges to create current.

The third piece of the puzzle is called power (abbreviated P in equations). Simply put, power is the work done by an electric circuit. Electric current, in and of itself, isn’t all that useful. It becomes useful only when the energy carried by an electric current is converted into some other form of energy, such as heat, light, sound, or radio waves. For example, in an incandescent light bulb, voltage pushes current through a filament, which converts the energy carried by the current into heat and light.

Power is measured in units called watts (abbreviated W ). The definition of one watt is simple: One watt is the amount of work done by a circuit in which one ampere of current is driven by one volt.

This relationship lends itself to a simple equation. I promised myself when I started this book that I would use as few equations as possible, but I knew I’d have to include at least some of the basic equations. Fortunately, this one is pretty simple:

In other words, power (P) equals voltage (E) times current (I).

To use the equation correctly, you must make sure that you measure power, voltage, and current using their standard units: watts, volts, and amperes. For example, suppose you have a light bulb connected to a 10-volt power supply, and one-tenth of an ampere is flowing through the light bulb. To calculate the wattage of the light bulb, you use the formula like this:

Thus, the light bulb is doing 1 watt of work.

Often, you know the voltage and the wattage of the circuit and you want to use those values to determine the amount of current flowing through the circuit. You can do that by turning the equation around, like this:

For example, if you want to determine how much current flows through a lamp with a 100-watt light bulb when it’s plugged into a 117-volt electrical outlet, use the formula like this:

Thus, the current through the circuit is 0.855 amperes.

Here are some final thoughts concerning the concept of power:

• The term dissipate is often used in association with power. As the energy carried by an electric current is converted into another form such as heat or light, the circuit is said to dissipate power.

• Did you notice that current and voltage are represented by the letters I and E, not the letters C or V as you might expect, but power is represented by the letter P? Sometimes you wonder if the people who make the rules are just trying to confuse everyone.

  Maybe the following table will help you keep things sorted out:

  Concept	Abbreviation	Unit
  Current	I	amp (A)
  Voltage	E or EMF	volt (V)
  Power	P	watt (W)

• Earlier in this chapter, in the section “Understanding Voltage ,” I say that I can’t define “one volt” until you know what power is. Now that you know, you can see that the definition of a volt is simple: One volt is the amount of electromotive force (EMF) necessary to do one watt of work at one ampere of current.

• The formula is sometimes called Joule’s Law , named after the person who discovered it. This little factoid will not be on the test.

• Calculating the power dissipated by a circuit is often a very important part of circuit design. That’s because electrical components such as resistors, transistors, capacitors, and integrated circuits all have maximum power ratings. For example, the most common type of resistor can dissipate at most watt. If you use a -watt resistor in a circuit that dissipates more than watt of power, you run the risk of burning up the resistor.