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24.010 Electricity is a fundamental constituent of the atoms that make up all matter

Doug Lowe and Dickon Ross
Electronics All-In-One for Dummies, 2014
Pages 12, 23-33, 262, electricity is a fundamental constituent of the atoms that make up all matter

Page 12

Electric charge

Electric charge refers to a fundamental property of matter still being discussed by the cleverest of physicists. Although a huge simplification, in essence two of the tiny particles that make up atoms – protons and electrons – are the bearers of electric charge. Two types of charge exist: positive and negative. Protons have positive charge and electrons have negative charge.

Electric charge is one of the basic forces of nature that hold the universe together. Positive and negative charges are irresistibly attracted to each other. Thus, the attraction of negatively-charged electrons to positively-charged protons holds atoms together.

If an atom has the same number of protons as it has electrons, the positive charge of the protons balances out the negative charge of the electrons, and the atom itself has no overall charge.

If an atom loses one of its electrons, however, the atom has an extra proton, which gives the atom a net positive charge. When an atom has a net positive charge, it ‘goes looking’ for an electron to restore its balanced charge.

Similarly, if an atom somehow picks up an extra electron, the atom has a net negative charge. When this happens, the atom ‘goes looking’ for a way to get rid of the extra electron to restore balance.

Atoms don’t really ‘look’ for anything. They don’t have eyes and they don’t have minds that are troubled when they’re short an electron or have a few too many. But the natural attraction of negative to positive charges causes atoms that are short an electron to be attracted to atoms that are long an electron. When they find each other, something almost magical happens. The atom with the extra electron gives its electron to the atom that’s missing an electron. Thus, the charge represented by the electron moves from one atom to another, which brings us to the second important concept: current.

Electric current:

Electric current refers to the flow of the electric charge by electrons as they move between the atoms. The concept is very familiar: when you turn on a light switch, electric current flows from the switch through the wire to the light and the room is instantly illuminated.

Pages 23-33

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.

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 chumped 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, 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.

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.

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 negative polarity and 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.

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)

Strong-arming protons

You may be wondering how the nucleus of an atom can stay together if it consist 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. The strong force holds protons (and neutrons) together in spite of the proton’s natural tendency to avoid each other.

The strong force doesn’t affect electrons, and 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.

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.

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 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).

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 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 book). 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.

Amping things up

The strength on 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 6.24 trillion electrons per second in the continental European numbering system (6.24 quintillion electrons per second in the American system). Either way, it’s a lot.

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 to 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 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 voltages 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.

Are you positive about that?

For the first 150 years or so of serious research into the nature of electricity, scientists had the concept of electric current backwards: 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.

Not until around 1900 did scientists begin to unravel the structure of atoms. They figured out that electrons have a negative charge and that current is the flow of these negatively-charged electrons. In other words, current flows in the opposite direction to what they’d previously thought.

Old ideas die hard, however, and to this day most people think of electric current as flowing from positive to negative (sometimes called conventional current). Modern electronic circuits are almost always described in terms of conventional current, encouraging the assumption that current flows from positive to negative, even though the reality is that electrons in the circuit are 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 (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.)

Page 262

Examining elements and atoms

The electrons in an atom are organised in layers, kind of like the layers of an onion. These layers are called shells. The outermost shell is called the valence shell. The electrons in this shell are the ones that form bonds with neighbouring atoms. Such bonds are called covalent bonds because they share valance electrons, which are also the electrons that sometimes go wandering off in search of other atoms.

Most conductors, including copper and silver, have just one electron in the valence shell. Atoms with just one valence electron have a hard time keeping that electron, which is what makes copper and silver such good conductors. When valence electrons travel, they create moving electric fields that push other electrons out of their way. That’s what causes current to flow.

Semiconductors, on the other hand, typically have four electrons in their valence shell. The best known elements with four valence electrons rarely lose one of them. However, they do like to share them with neighbouring atoms.

If all the neighbouring atoms are of the same type, all the valence electrons are able to bind with valence electrons from other atoms. When that happens, the atoms arrange themselves into neat orderly structures called crystals. Semiconductors are made out of such crystals.

The most plentiful element with four valence electrons is carbon. However, carbon crystals are rarely used as semiconductors because they have other uses, such as in diamond rings. So instead, semiconductors are usually made from crystals of silicon and occasionally germanium.

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