Chapter 2

Understanding Electricity

In This Chapter

Unravelling the mystery of matter

Discovering current, voltage and power

Before you can do anything interesting with electronics, you need to have a basic understanding of what electricity is and how it works. Unfortunately, understanding electricity is a tall order, and so frankly the title of this chapter is a bit ambitious. Don’t let this discourage or dissuade you, though: the smartest physicists in the world are still making discoveries about electricity.

In this chapter, you take a look at the very nature of electricity: what it is and what causes it. This section may remind you of a school science lesson, as you delve into atoms, protons, neutrons and electrons.

We also introduce you to three things you have to know about electricity if you want to design and build your own circuits: current, voltage and power – the Huey, Dewey and Louie of electricity, or the Groucho, Harpo and Chico, or the – well, you get the idea.

Wondering about the Nature of Electricity

The exact nature of electricity is one of the mysteries of the universe. Although even experts don’t know everything about electricity, they do know a lot about what it does and how it behaves.

Strange as it sounds, your understanding of electricity improves right away if you avoid using the term ‘electricity’ to describe it, because this word isn’t sufficiently precise. People use this word to refer to any of several different but related things, each of which has a more accurate name, such as electric charge and electric current (check out the later sections ‘Charging ahead’ and ‘Keeping current’), electric energy, electric field and so on. All these things are commonly called electricity.

But electricity is less a specific thing and more a phenomenon with many different faces. So to avoid confusion, wherever possible we use more precise terms such as ‘charge’ or ‘current’ in this book.

We prefer to think of electricity more as a wonder than a phenomenon, because the latter sounds so scientific. Electricity really does qualify as one of the great wonders of the universe. To the so-called Seven Wonders of the Ancient World we’d like to add a list called ‘The Seven Wonders of the Universe’, including matter, gravity, time, light, life, pizza and electricity.

Looking for electricity

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

Consider a common misconception about electric current. Many people think that wires carry electricity from place to place. When you plug in a vacuum cleaner and turn on the switch, you may think 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, grime and dog hair. But that’s not the case.

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The truth is that the electricity is already in the wire. It’s always in the wire, even when the vacuum cleaner is turned off or the power cord not 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 that the rubber keeps from touching the wires.

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

Peering inside atoms

All matter is made up of unbelievably tiny bits called atoms. They’re so tiny that the full stop at the end of this sentence contains several trillion of them.

Getting your head around numbers as large as trillions is tricky. For the sake of comparison, suppose that you were able to enlarge that full stop until it was about five times the size of England. Then, each atom would be about the size of – you guessed it – the full stop at the end of this sentence.

The word ‘atom’ comes from an Ancient Greek fellow named Democritus. Contrary to what you may expect, the word doesn’t mean ‘really small’: 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, imagine that 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 keep dividing your piece of copper into ever smaller halves. But eventually, you 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 aren’t copper. Instead, you have a collection of the basic particles that make up atoms: the three such particles are neutrons, protons and electrons.

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

Although even today children are still taught that electrons orbit around the nucleus much like planets orbit around the sun in the solar system (building models like the one in Figure 2-1), in fact that’s 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 figuring out exactly where in its cloud an electron is at any given moment is next to impossible.

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

Examining the elements

Here’s the deal with elements: 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 and atoms with three protons are called lithium.

The number of protons in the nucleus of an atom is the atomic number. Thus, the atomic number of hydrogen is 1, the atomic number of helium is 2 and lithium is 3. 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 play that big a role in the way electric current works, and so we can safely ignore them in this chapter (phew!). Suffice it to say that in addition to protons, the nucleus of each atom (except hydrogen) contains neutrons – and most atoms have a few more neutrons than protons.

The third particle that makes up atoms is the electron. Electrons are the most interesting particle for this book, because they’re 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 full stop at the end of this sentence, a proton would be almost the size of a football pitch.

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’s ‘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 we explain in the next section.

Charging ahead

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 and protons have a positive polarity.

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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.

Charge is a property of one of the fundamental forces of nature known as electromagnetism. The other three forces are gravity, the strong force (check out the nearby sidebar ‘Strong-arming protons’ for a little more) and the weak force. As we say in the preceding section, an atom normally has the same number of electrons as protons, 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. In this case, it’s said to be neutral.

An atom can, however, pick up an extra electron. When it does, the atom has a net negative charge because of that extra electron. An atom can also lose an electron, which causes the atom to have a net positive charge, because it has more protons than electrons.

Conducting and Insulating Elements: Current, Voltage and Power

Some elements (which we introduce earlier in ‘Examining the elements’) don’t hold on to some of their electrons as tightly as other elements. These elements (called conductors) frequently lose electrons or pick up extra electrons, and so they often get bumped off neutral and become negatively or positively charged (check out the preceding section for more on charges). The metals silver, copper and aluminium are the best conductors.

In contrast, other elements hold on to their electrons more tightly. In these elements (called insulators), prying loose an electron or forcing another electron in is harder. These elements almost always stay neutral.

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 we call Atom B. This movement creates a net positive charge in Atom A and a net negative charge in Atom B. But almost immediately, an electron jumps out of another nearby atom – 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 may 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 the 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 organised, the result is electric current (the subject of the next section).

Keeping current

Electric current is what happens when the random exchange of electrons that occurs constantly in a conductor becomes organised 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 – around a few millimetres per 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 bulb.

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 almost immediately moves. The other balls move a little, but not much (assuming you line them up straight and strike the cue ball straight).

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

This effect 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!).

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

Most electric incandescent light bulbs have about a quarter of an amp of current flowing through them when they’re turned on. A hair dryer uses about 5 A.

Current in electronic circuits is usually much smaller than current in electrical devices such as light bulbs and hair dryers (if you’re unclear of the difference between electronic and electrical devices, flip to Chapter 1 of this minibook). The current in an electronic circuit is often measured in thousandths of amps (milliamps, abbreviated mA).

Current is often represented by the letter I (for intensity) in electrical equations.

The fact that moving water is also called current is no coincidence. Many 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.

Pushing electrons around: 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). You know it more commonly as voltage.

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A voltage is nothing more than a difference in charge between two places. For example, suppose that 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 the two clumps. If you connect the two clumps with a conductor, such as a copper wire, you create a circuit through which electric current can 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.

Although current stops flowing when the two sides of the circuit have been neutralised, 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. With no voltage to push them along, they move about randomly.

Whenever a difference in charge exists between two locations, a current may flow between the two locations if they’re connected by a conductor. Because of this possibility, the term potential is often used to describe voltage. Without voltage, you can’t have current. Thus, voltage creates the potential for a current to flow.

If we compare current to the flow of water through a hose, you can then see voltage as the water pressure at the tap. Water pressure causes the water to flow in the hose.

Here are some facts and figures about voltage:

Voltage is measured using a unit called, naturally, the volt (usually abbreviated V). The voltage available in a standard electrical outlet in the UK is about 230 V. The voltage available in a battery is about 1.5 V and a car battery provides about 12 V.

To find out how much voltage exists between two points, you use a voltmeter, a device with two wire test leads that you touch to different points in a circuit to measure the voltage between those points. Figure 2-3 shows a typical voltmeter. (In fact, this meter is a multimeter, which simply means that it can measure things other than voltage as well. In the figure, the multimeter is functioning as a voltmeter. 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 battery the voltage at the positive terminal is +1.5 V relative to the negative terminal. The voltage at the negative terminal is –1.5 V relative to the positive terminal.

Figure 2-3: Measuring voltage with the voltmeter function of a multimeter.

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Comparing direct and alternating current

An electric current that flows continuously in a single direction is called a direct current (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 DC circuit needs to be constant, or at least relatively constant, to keep the current flowing in a single direction. Thus, the voltage provided by a torch 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.

The other common type of current is alternating current (AC). In an AC circuit, voltage periodically reverses itself. When the voltage reverses, so does the direction of the current flow. In the most common form of AC, used in most power-distribution systems throughout the world, the voltage reverses itself 50 or 60 times per second, depending on the country. In the UK, the voltage is reversed 50 times per second.

Alternating current is used in nearly all the world’s power-distribution systems for the simple reason that AC 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 ¹⁄₅₀ of a second, and then turn around and go the other direction for ¹⁄₅₀ of a second. The net effect is that they don’t really go anywhere.

A popular toy called Newton’s Cradle can help you understand how AC 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.

Figure 2-4: This Newton’s Cradle works like alternating current.

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.

To see a Newton’s Cradle in action, go to YouTube and search for ‘Newton’s Cradle’.

The reversal of voltage in a typical AC circuit isn’t instantaneous. Instead, the voltage swings smoothly from one polarity to the other. Thus, the voltage in an AC circuit is always changing. It starts out at zero, increases in the positive direction for a bit until it reaches its maximum positive voltage and then 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. (Flick to Book I Chapter 9 to see what this voltage swing looks like on a graph.)

The fact that the amount of voltage in an AC circuit is always changing is incredibly useful. (To discover how, flip to Book IV, Chapter 1, where we take a deeper look at AC.)

Working out with power

The third of the three key concepts about electricity, in addition to current and voltage (see the earlier sections ‘Keeping current’ and ‘Pushing electrons around: Voltage’ respectively), is power (abbreviated P in equations).

Simply put, power is the work done by an electric circuit. Electric current becomes useful when the energy it carries 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. When this happens, the circuit is said to dissipate power.

Calculating the power dissipated by a circuit is often a very important part of circuit design, because electrical components such as resistors, transistors, capacitors and integrated circuits have maximum power ratings. For example, the most common type of resistor can dissipate at most ¹⁄₄ watt. If you use a ¹⁄₄-watt resistor in a circuit that dissipates more than ¹⁄₄ watt of power, you run the risk of burning up the resistor.

As you can see, power is measured in units called watts (W). The definition of 1 W is simply the amount of work done by a circuit in which 1 A of current is driven by 1 V.

This relationship lends itself to a simple equation. Although we use as few equations in this book as possible, we have to include some basic ones. Fortunately, this one is pretty simple:

P = V × I

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

This formula is sometimes called Joule’s Law, after the person who discovered it.

Confusingly, current is represented by the letter I (for intensity), not the letter C. But at least power is represented by P. Table 2-1 may prove helpful in keeping these abbreviations sorted out.

Table 2-1 The Three Central Concepts of Electricity

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

To use the equation correctly, make sure that you measure power, voltage and current using their standard units: watts, volts and amperes. For example, suppose that you have a light bulb connected to a 10-V 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 P = V × I formula as follows:

P = 10 V × 0.1 A = 1 W

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

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

For example, if you want to determine how much current flows through a lamp with a 60-W light bulb when it’s plugged into a 230-V electrical outlet, use the formula like this:

Thus, the current through the circuit is 0.26 A.

In the earlier section ‘Pushing electrons around: Voltage’, we say that you need to know what power is in order to define 1 V. Now you can see that 1 V is the amount of electromotive force necessary to do 1 W of work at 1 A of current.