Read an Excerpt
Electronics Simplified
Previously published as Electronics Made Simple
By Ian Sinclair
Newnes
Copyright © 2011 Ian Sinclair
All right reserved. ISBN: 978-0-08-097111-7
Chapter One
Electricity, Waves, and Pulses
Fundamental Electricity
Definition
Electrical engineering is the study of generation of electrical power from other forms of power (mainly heat, but now including mechanical systems such as wave and wind), its transmission from one place to another, and its use, both industrial and domestic. Electronics is the branch of electrical engineering concerned with the control of individual particles called electrons whose movements we call electric current.
Electrical engineering is concerned with making use of electricity as a way of transmitting and using power. There are natural sources of electrical power, such as lightning and some living creatures, such as the electric eel, but the main concern of electrical engineering is directed to man-made systems that generate, distribute, and use electricity. Our main uses of electricity depend on converting electrical power to other forms, mainly into mechanical effort, heat and light. These other forms are also our main sources of electrical power.
The first electrical effects to be discovered were those of static electricity, observed when amber was rubbed with silk. The ancient Greeks first discovered these effects, and because amber is called electron in Greek, we have coined the words such as electricity and electronics that are so familiar today. Considerably later, in the eighteenth century, experimenters discovered other effects that also seemed to be electrical, but these were entirely different. Typically, these effects use a chemical action to generate what we now call electric current in a closed path that we call a circuit. As an aid to imagining what was happening, electric current can be compared to the flow of water in pipes.
Definition
Electric current is the flow of electricity through a metal such as a cable. Electric voltage is the electrical form of pressure that forces the current to flow.
Think for a moment about a water circuit, such as is used in central heating. The path for the water is closed (Figure 1.1) and the water is moved by using a pump. No water is lost from the circuit and none is added. Turning off a tap at the tank (breaking the circuit) would make the water level rise, because of the pressure of the pump, in the vertical (overflow) piece of pipe. The pump maintains the pressure that makes the water move, and we can use this movement to transmit power because the flowing water can turn a turbine wheel at some other part of the circuit. Hydraulic machines depend on this idea of a liquid in a circuit, though the modern development of hydraulics came later than our use of electricity.
It is not surprising, then, that early experimenters thought that electricity was some kind of invisible liquid. Static electricity effects were explained as being caused by the pressure of this liquid, and current electricity by the flow of the liquid. Electrical engineering is mainly concerned with flow, but the later science of electronics has been based as much on static electricity as on current flow, because we now know much more about what is flowing and why it can flow so easily in metal wires. The fundamental quantity of electricity is electrical charge, and that is what moves in a circuit.
Summary Electrical effects were once thought to be caused by an invisible liquid that could flow through metals and accumulate on non-metallic materials. Electrical engineering is concerned with the effects of electrical flow, and most of our early applications of electricity have used the effects of flowing electricity.
Definition
All of the effects we call electrical are due to electric charge. Electrostatic (static electricity) effects are caused by charge at rest, and electric current effects (including magnetism) are caused by charge that is moving.
That definition does not tell you much unless you know something about electric charge. No-one knows precisely what charge is (though we are slowly getting there), but we do know a lot about what charge does, and what we know about what charge does is knowledge that has been accumulated since the time of the ancient Greeks. We can summarize what charge does (the properties of charge):
• When you rub two non-metallic objects together they will both usually become charged.
• These charges are of two opposite types, one called positive, the other called negative.
• Two charges of the same type (two positives or two negatives) repel each other; two opposite charges (one positive, one negative) attract each other.
• The natural state of any substance is not to have any detectable charge, because it contains equal quantities of positive and negative charges.
What we know about the way that charge behaves has led to finding out more about what it is, and we know now that charge is one of the most important effects in the Universe. Like gravity, charge is a way of distorting space, so that it appears to cause force effects at a distance from the cause of the charge. What we call charge is the effect of splitting atoms, separating small particles called electrons from the rest of each atom. Each electron is negatively charged, and the amount of charge is the same for each electron. The other main part of an atom, the nucleus, carries exactly as much positive charge as the electrons around it carry negative charge (Figure 1.2), if we picture the atom as looking like the sun and its planets. For example, if there are six electrons then the nucleus must carry six units of positive charge, exactly balancing the total negative charge on the electrons.
Note Modern physics has long abandoned pictures of atoms as sun-and-planet systems, but this type of picture of the unimaginable is good enough for all purposes concerned with electrical engineering, and for most concerned with electronics.
When an electron has become separated from the atom that it belongs to, the attraction between the electron and its atom is, for such tiny particles, enormous, and all the effects that we lump together as electricity, ranging from lightning to batteries, are caused by these force effects of charge. The forces between charges that are at rest are responsible for the effects that used to be called static electricity (or electrostatics), and these effects are important because they are used in several types of electronics devices.
Note The forces are so enormous that we can usually separate only one electron from a nucleus, and we can separate all of the electrons only at enormous temperatures, such as we find within the sun or in an exploding hydrogen bomb (which is what the sun actually is).
Another option for an electron that has become separated from an atom is to find another atom that has lost its electron (and is therefore positively charged). The movement of electrons from one atom to another causes a large number of measurable effects such as electric current, magnetism, and chemical actions like electroplating. Of these, the most important for electronics purposes are electric current, and one of its effects, magnetism. Materials that allow electrons to move through them are called conductors; materials that do not allow electrons to move easily through them are called insulators.
Note In some types of crystals, compressing the crystal will separate charges, generating a high voltage. These crystals are termed piezoelectric, and a typical application is as an ignition device for a gas fire or cooker. The effect is also reversible, so that a piezoelectric crystal can be used to convert an electrical pulse into a mechanical compression or expansion of the crystal. This effect is used in ultrasonic cleaners.
The movement of electrons that we call electric current takes place in a circuit, a closed path for electrons that has been created using conducting material. All circuits for current are closed circuits, meaning that electrons will move from a generator through the circuit and back to the generator again. This is essential because unless electrons moved in a closed path like this many atoms would be left without electrons, and that condition could not exist for long because of the large forces that draw the electrons back to the atoms.
Definition
Electric current is the amount of charge that passes per second any point in a circuit. Electric voltage is the amount of work that each charge can do when it moves.
These are formal definitions. We cannot easily count the number of electrons that carry charge along a wire, and we cannot easily measure how much work is done when a charge moves. We can, however, measure these quantities by making use of the effects that they cause. Current along a wire, for example, will cause a force on a magnet, and we can measure that force. The voltage caused by some separated charges can be measured by the amount of current that will flow when the charges are allowed to move. The unit of current is called an ampere or amp, and the unit of voltage is a volt. These terms come from the names of the pioneers Ampere and Volta, and the abbreviations are A and V, respectively.
Note We can create less formal definitions for ourselves. Voltage is like a propelling force for current, and current itself can be thought of as like the current of a river. If we continue with this idea, voltage corresponds to the height of the spring where the river starts.
All substances contain electrons, which we can think of as being the outer layer of each atom. Some materials are made out of atoms that hold their electrons tightly, and when electrons are moved out of place it is not easy for them to return to their positions. In addition, other electrons cannot move from their own atoms to take up empty places on other atoms. We call these materials insulators, and they are used to prevent electric current from flowing. In addition, insulators can be charged and will remain charged for some time.
A good example is the party balloon which is charged by rubbing it against a woolen sweater and which will cling to the wall or the ceiling until its charge is neutralized. Surprisingly high voltages can be generated in this way on insulators, typically several kilovolts (kV), where kilo means one-thousand. For example, 5 kV means five-thousand volts. A very small current can discharge such materials, and we use the units microamp (µA), meaning a millionth of an amp, nanoamp (nA), meaning a thousandth of a millionth of an amp, and picoamp (pA), meaning a millionth of a millionth of an amp. There is an even smaller unit, the femtoamp (fA), one-thousandth of a picoamp.
Note As a comparison, the electrical supply to a house in the UK is at 240 V ± 10%, 50 Hz and currents of 1 A to 13 A are used in domestic equipment (though electric cookers can use up to 30 A). In the USA, the minimum supply voltage is 110V to 115V, and the maximum is 120V to 125V, at 60 Hz, with higher currents (requiring thicker wiring).
Now let's look at electricity with lower voltages and higher currents. This is the form of electricity that we are most familiar with and which we use daily.
(Continues...)
Excerpted from Electronics Simplified by Ian Sinclair Copyright © 2011 by Ian Sinclair . Excerpted by permission of Newnes. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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