Basic Electronics Mv Rao Pdf Free Download
Avis Whitelow
The goal of this chapter is to provide some basic information about electroniccircuits. We make the assumption that you have no prior knowledge of electronics,electricity, or circuits, and start from the basics. This is an unconventional approach,so it may be interesting, or at least amusing, even if you do have some experience. So,the first question is ``What is an electronic circuit?'' A circuit is a structure thatdirects and controls electric currents, presumably to perform some useful function. Thevery name "circuit" implies that the structure is closed, something like a loop.That is all very well, but this answer immediately raises a new question: "What is anelectric current?" Again, the name "current" indicatesthat it refers to some type of flow, and in this case we mean a flow of electric charge,which is usually just called charge because electric charge is really the only kind thereis. Finally we come to the basic question:
No one knows what charge really is anymore than anyone knows what gravity is.Both are models, constructions, fabrications if you like, to describe and representsomething that can be measured in the real world, specifically a force. Gravity is thename for a force between masses that we can feel and measure. Early workers observed thatbodies in "certain electrical condition" also exerted forces on one another thatthey could measure, and they invented charge to explain their observations. Amazingly,only three simple postulates or assumptions, plus some experimental observations, arenecessary to explain all electrical phenomena. Everything: currents, electronics, radiowaves, and light. Not many things are so simple, so it is worth stating the threepostulates clearly.
We just invent the name to represent the source of the physical force that can beobserved. The assumption is that the more charge something has, the more force will beexerted. Charge is measured in units of Coulombs, abbreviated C. The unit was named tohonor Charles Augustin Coulomb (1736-1806) the French aristocrat and engineer who firstmeasured the force between charged objects using a sensitive torsion balance he invented.Coulomb lived in a time of political unrest and new ideas, the age of Voltaire andRousseau. Fortunately, Coulomb completed most of his work before the revolution andprudently left Paris with the storming of the Bastille.
We call the two styles positive charge, + , and (you guessed it) negative charge, - .Charge also comes in lumps of 1.6 10-19C , which is about twoten-million-trillionths of a Coulomb. The discrete nature of charge is not importantfor this discussion, but it does serve to indicate that a Coulomb is a LOT of charge.
You cannot create it and you cannot annihilate it. You can, however, neutralize it.Early workers observed experimentally that if they took equal amounts of positive andnegative charge and combined them on some object, then that object neither exerted norresponded to electrical forces; effectively it had zero net charge. This experimentsuggests that it might be possible to take uncharged, or neutral, material and to separatesomehow the latent positive and negative charges. If you have ever rubbed a balloon onwool to make it stick to the wall, you have separated charges using mechanical action.
First we return to the basic assumption that forces are the result of charges.Specifically, bodies with opposite charges attract, they exert a force on eachother pulling them together. The magnitude of the force is proportional to the product ofthe charge on each mass. This is just like gravity, where we use the term "mass"to represent the quality of bodies that results in the attractive force that pulls themtogether (see Fig. 4.1).
Electrical force, like gravity, also depends inversely on the distance squared between thetwo bodies; short separation means big forces. Thus it takes an opposing force to keep twocharges of opposite sign apart, just like it takes force to keep an apple from falling toearth. It also takes work and the expenditure of energy to pull positive andnegative charges apart, just like it takes work to raise a big mass against gravity, or tostretch a spring. This stored or potential energy can be recovered and put to work to dosome useful task. A falling mass can raise a bucket of water; a retracting spring can pulla door shut or run a clock. It requires some imagination to devise ways one might hook onto charges of opposite sign to get some useful work done, but it should be possible.
The potential that separated opposite charges have for doing work if they are releasedto fly together is called voltage, measured in units of volts (V). (Sadly, the unit voltis not named for Voltaire, but rather for Volta, an Italian scientist.) The greater theamount of charge and the greater the physical separation, the greater the voltage orstored energy. The greater the voltage, the greater the force that is driving the chargestogether. Voltage is always measured between two points, in this case, the positive andnegative charges. If you want to compare the voltage of several charged bodies, therelative force driving the various charges, it makes sense to keep one point constant forthe measurements. Traditionally, that common point is called "ground."
them apart, and an opposing force is necessary to hold them together, like holding acompressed spring. Work can potentially be done by letting the charges fly apart, justlike releasing the spring. Our analogy with gravity must end here: no one has observednegative mass, negative gravity, or uncharged bodies flying apart unaided. Too bad, itwould be a great way to launch a space probe. The voltage between two separated likecharges is negative; they have already done their work by running apart, and itwill take external energy and work to force them back together.
So how do you tell if a particular bunch of charge is positive or negative? You can'tin isolation. Even with two charges, you can only tell if they are the same (they repel)or opposite (they attract). The names are relative; someone has to define which one is"positive." Similarly, the voltage between two points A and B , VAB, is relative. If VAB is positive you know the two points are oppositelycharged, but you cannot tell if point A has positive charge and point B negative, orvisa versa. However, if you make a second measurement between A and another point C ,you can at least tell if B and C have the same charge by the relative sign of the twovoltages, VAB and VAC to your common point A . You can evendetermine the voltage between B and C without measuring it: VBC = VAC- VAB . This is the advantage of defining a common point, like A , as groundand making all voltage measurements with respect to it. If one further defines the chargeat point A to be negative charge, then a positive VAB means point B ispositively charged, by definition. The names and the signs are all relative, and sometimesconfusing if one forgets what the reference or ground point is.
Charge is mobile and can flow freely in certain materials, called conductors. Metalsand a few other elements and compounds are conductors. Materials that charge cannot flowthrough are called insulators. Air, glass, most plastics, and rubber are insulators, forexample. And then there are some materials called semiconductors, that, historically,seemed to be good conductors sometimes but much less so other times. Silicon and germaniumare two such materials. Today, we know that the difference in electrical behavior ofdifferent samples of these materials is due to extremely small amounts of impurities ofdifferent kinds, which could not be measured earlier. This recognition, and the ability toprecisely control the "impurities" has led to the massive semiconductorelectronics industry and the near-magical devices it produces, including those on yourRoboBoard. We will discuss semiconductor devices later; now let us return to conductorsand charges.
There is a force between them, the potential for work, and thus a voltage. Now we connecta conductor between them, a metal wire. On the positively charged sphere, positive chargesrush along the wire to the other sphere, repelled by the nearby similar charges andattracted to the distant opposite charges. The same thing occurs on the other sphere andnegative charge flows out on the wire. Positive and negative charges combine to neutralizeeach other, and the flow continues until there are no charge differences between anypoints of the entire connected system. There may be a net residual charge if the amountsof original positive and negative charge were not equal, but that charge will bedistributed evenly so all the forces are balanced. If they were not, more charge wouldflow. The charge flow is driven by voltage or potential differences. After things havequieted down, there is no voltage difference between any two points of the system and nopotential for work. All the work has been done by the moving charges heating up the wire.
The flow of charge is called electrical current. Current is measured in amperes (a),amps for short (named after another French scientist who worked mostly with magneticeffects). An ampere is defined as a flow of one Coulomb of charge in one second past somepoint. While a Coulomb is a lot of charge to have in one place, an ampere is a commonamount of current; about one ampere flows through a 100 watt incandescent light bulb, anda stove burner or a large motor would require ten or more amperes. On the other hand lowpower digital circuits use only a fraction of an ampere, and so we often use units of 1/1000 of an ampere, a milliamp, abbreviated as ma, and even 1/1000 of a milliamp, or amicroamp, a . The currents on the RoboBoard are generally in the milliamp range, exceptfor the motors, which can require a full ampere under heavy load. Current has a direction,and we define a positive current from point A to B as the flow of positive charges inthe same direction. Negative charges can flow as well, in fact, most current is actuallythe result of negative charges moving. Negative charges flowing from A to B would be anegative current, but, and here is the tricky part, negative charges flowing from B to A would represent a positive current from A to B . The net effect is thesame: positive charges flowing to neutralize negative charge or negative charges flowingto neutralize positive charge; in both cases the voltage is reduced and by the sameamount.
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