To the beginner

Before getting into the technical stuff, we might wish to ask ourselves what electricity is. My first recollection of thinking about it (though I did not know that I was) is of being a child and dismantling a prized radio to find out that no one was inside; just a mind-boggling collection of small coloured objects which evidently were not sweets, though some looked like them. My first direct experience of electricity was more sudden; an electric shock from a bar fire while trying to melt some plastic on it. I knew it was hot, but hadn’t expected that! Later I came across the manual for a record player and amplifier in the house, which was old-fashioned enough to come with a circuit diagram. I was fascinated by it. It meant something to someone, but these hieroglyphics were like no language that I could figure out.

Later I took a technician’s course, and a confusing set of concepts was presented to me as explanation for this unseen and magical force. Electricity, I had now realised, is used in a huge range of ways; recording sound, reproducing pictures, lighting our darkness and a lot else besides. All this was, they told me, due to unseen little balls which whiz around in some materials, creating equally unseen lines which can cause little balls in other places to whiz around as well. And this the mental territory of staid, rational looking people who would probably claim that they don’t believe in magic.

I have to confess that, some years later, I have still not seen the little balls or the lines which they fling about, and I’m not very sure that I really understand them. But I have managed a reasonable career as a technician and an engineer, and I believe that I would have a fair chance of fixing my cassette machine, TV or house wiring should they misfunction. On the whole, I have not thought about the antics of little balls very much (though I suppose that it helps to know that they are there). Electricity looks, in my mind, more like water, wires like pipes, resistors like very thin pipes, capacitors like pairs of half-filled balloons which squash against each other, and inductors like – well something else. (I never said that the analogy was complete or accurate.)

I say all this as, I hope, a source of comfort to the new student of the subject who may be suffering the same pangs of insecurity about atoms, electrons and so on that I went through. Unless you feel strongly to do otherwise, let them live their own strange lives. You can be a very competent and effective user of the principles of electricity and electronics without needing to know much about them (though the same might not be said of completing your college course). Leave it to the physicists. What helps a great deal is having a good and clear grasp of the laws of electricity, and being able, as much as is possible, to prove them to yourself.

What is electricity?

We are never concerned with the behaviour of an individual atom in electronics; but with their movement in vast numbers, as they fling themselves, lemminglike, around the wires of your circuit. And yet we hardly think of them at all. We think a great deal about ‘voltage’, ‘current’ and about the golden rule of electronics, Ohm’s Law. You will meet these soon.

Actually it is not atoms that move, but mostly bits of them, even tinier particles called electrons which separate themselves somehow from the larger atom which is their usual home. These electrons are now called ‘free electrons’. Though unimaginably small, they are pretty powerful. They carry something which is called a ‘negative charge’ which is another way of saying that if, having been separated from the parent atom, they should ever encounter another with a vacancy then they will be in there like a shot and pretty difficult to move. And the feeling is mutual; once it has lost an electron the atom feels the loss and will try to bring close to it any waifs that it senses nearby. Being much bigger, it moves much less easily, or not at all. All of which physicists understate quite magnificently when they describe atoms which are an electron short as being ‘holes’. Where a material has very few free electrons but many holes it is said to be ‘positively charged’. The movement of charge is called ‘electric current’.

If we can’t see electricity, how do we measure it?

The answer is that we don’t, directly. We measure voltage and current, we read the values written on the sides of components, and we work the rest out from there. But that’s jumping the gun. Who built the meters that we use for measurement? What did they use?

Electricity is unseen. It is only through its ability to affect things physically that we know anything about it, be that people a hundred and more years ago, watching thunderstorms and making the legs of dead frogs jump, or us with our loudspeakers and computer screens. There are two ways of converting electrical energy to mechanical energy that we are concerned with.

MMF and current

MMF is most familiar to us as magnetism. The physical force that you feel between magnets is called MMF, or ‘magnetomotive force’. A less familiar manifestation occurs when two wires with electric current passing through them are placed near to each other. A physical force exists between them. (It’s usually too small to feel but the effect is exploited in electric motors.) An MMF is a result of a magnetic field, and the study of magnetic fields is called electromagnetics, which we won’t get involved in here.

If we were to make the wires a given length and separate them by a given distance we could increase the current until we had some given measurable force. Then we would have a unit value of electrical current, the ampere or amp. The amp is abbreviated to A. The symbol which we use for electrical current in equations is I.

This in turn gives rise to a unit of measurement for electric charge, the coulomb. The coulomb is defined as the amount of charge passing a point in a wire that carries one amp of current in one second. This gives us our first equation:

image (1.1)

where I is current in amps, Q is charge in coulombs and t is time in seconds. (The symbol Δ means ‘change in’, so the term image means ‘change in charge divided by change in time’–or the amount of charge which passed the point divided by the time period that charge was measured for.)

This tells us that a charge of one coulomb passing a point in one second equates to a current of one amp; two coulombs in a second, or half a coulomb in one second would be two amps; and so on. A coulomb is a lot of charge. Typically we deal in charges of thousandths of coulomb per second (mC/s), or mA. (The prefix ‘m’ denotes a thousandth. We will use these prefixes increasingly as we progress. They are described fully in Appendix 1.)

Electron current’ and ‘conventional current’ There is something of a mix-up which has taken place over history with regard to the direction of current flow, and no one has ever bothered to sort it out. You would think from what we’ve said above that, as the electrons do most or all of the running around in circuits, the current flow would be from the negative end of a battery to the positive end. It is, but we have a convention of always arrowing diagrams with the direction of current flow reversed. When people were discovering the laws of electricity they had some idea about things moving in wires but no way of seeing in which direction, so they had to guess. They guessed wrongly. Later this was discovered, but by this time people had been labelling currents on circuit diagrams for ages, quite happily and without any problem. The numbers all work out the same; its like deciding to rewrite your bank statement with the signs reversed. A minus sign means you are in credit or have deposited cash, and a plus that you took money out or are in debt. So long as you know what the symbols mean there is no problem. So it was with current flow, and they left it at that. It was too much hassle to change the labels on everything. If you hear talk about ‘electron current’ (which goes from negative to positive) and ‘conventional current’ (positive to negative) this is what it means. Most people, and this book, use conventional current.

EMF and voltage

Electric charges of the same polarity repel and of opposite polarity attract. The attraction is the second physical force, called the ‘electromotive force’ or EME This is how electrons and holes know about each others’ presence and are caused to move towards each other. EMF’s are caused by electric fields. The study of electric fields is electrostatics and, like electromagnetics, we won’t get into it here.

EMF is often called ‘potential difference’ or ‘voltage’. It is measured in volts, abbreviated to V. When we need a symbol for voltage in equations we will also use V.

Voltage is a comparison between two places that are charged. We say that one place on a circuit is ‘10 V positive with respect to’ another or that a potential difference of 10 Vexists. The important thing to remember is that the number is always a difference between two places, or it means nothing. Generally we take one point, the metal chassis of equipment, mains earth or perhaps a piece of wire connected to the power supply, and we call it ‘0 V’ or ‘earth’. This becomes our reference for other measurements, unless we state otherwise.

The volt is defined by its ability to make charge do work (by pushing it more or less quickly through something and generating another form of energy, like heat or light). The amount of potential required to make one coulomb of charge do one joule of work is one volt:

image (1.2)

where V is voltage in volts, J is work in joules and Q is charge in coulombs. Ajoule is a measurement of energy or of work done which is derived from physical quantities. We will not actually use this but we will develop it into something more useful in Chapter 2. For now it is enough to know that a volt is a definite measurable quantity.

How fast does electricity move? When a current flows through a wire, the same electron doesn’t go in one end and come out of the other; one bounces a little way along, until it is able to find a home, which it often does by dislodging another and sending it on its way down the wire. (In doing so it generates heat.) The average speed of electron movement in a wire is extremely low, fractions of millimetres per second, even for currents which are, by our standards, large.

However, what we generally mean by this question is something very different; when we apply a voltage to one end of a wire, how long is it before the voltage is felt at the other end of the wire? The answer is that the impulse travels at the speed of light – instantly for all our purposes.

An analogy is a string of marbles in a narrow tube. If you shove the end marble very hard, the impulse (the voltage) travels along the tube like a wave and reaches the other end quickly, say one metre in a second. In that time, any one marble may have only moved 1 cm. So the average marble speed (the current) is 1 cm/s while the impulse speed is 1 m/s, 100 times greater. The average marble speed changes at the other end of the tube just as quickly – they start falling out of the tube at a rate of 1 cm/s, a second later. With electrons the effect is the same, but unimaginably more exaggerated.

If that seems a bit confusing, please don’t worry – it is to most people. The important thing is that changes in voltage or current travel instantly but electrons have different physical speeds. (The second fact is only relevant if you want to understand how power is dissipated in components. Even if you don’t, you can get by quite well just knowing that it is. We look at this in Chapter 3.)

..................Content has been hidden....................

You can't read the all page of ebook, please click here login for view all page.