Basic Electronics – Computer Science Notes – For W.B.C.S. Examination.

বেসিক ইলেক্ট্রনিক্স – কম্পিউটার সায়েন্স নোট – WBCS পরীক্ষা।

Basic Electronics   

The goal of this chapter is to provide some basic information about electronic circuits. 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 that directs and controls electric currents, presumably to perform some useful function. The very 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 an electric current?” Again, the name “current” indicates that 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 there is. Finally we come to the basic question.Continue Reading Basic Electronics – Computer Science Notes – For W.B.C.S. Examination.

What is Charge?

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 represent something that can be measured in the real world, specifically a force. Gravity is the name for a force between masses that we can feel and measure. Early workers observed that bodies in “certain electrical condition” also exerted forces on one another that they could measure, and they invented charge to explain their observations. Amazingly, only three simple postulates or assumptions, plus some experimental observations, are necessary to explain all electrical phenomena. Everything: currents, electronics, radio waves, and light. Not many things are so simple, so it is worth stating the three postulates clearly.

Charge exists.   

We just invent the name to represent the source of the physical force that can be observed. The assumption is that the more charge something has, the more force will be exerted. Charge is measured in units of Coulombs, abbreviated C. The unit was named to honor Charles Augustin Coulomb (1736-1806) the French aristocrat and engineer who first measured 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 and Rousseau. Fortunately, Coulomb completed most of his work before the revolution and prudently left Paris with the storming of the Bastille.

Charge comes in two styles.

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 two ten-million-trillionths of a Coulomb. The  discrete nature of charge is not important for this discussion, but it does serve to indicate that a Coulomb is a LOT of charge.

Charge is conserved.

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 and negative charge and combined them on some object, then that object neither exerted nor responded to electrical forces; effectively it had zero net charge. This experiment suggests that it might be possible to take uncharged, or neutral, material and to separate somehow the latent positive and negative charges. If you have ever rubbed a balloon on wool to make it stick to the wall, you have separated charges using mechanical action.

Those are the three postulates. Now we will present some of the experimental findings that both led to them and amplify their significance.

Voltage  

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 each other pulling them together. The magnitude of the force is proportional to the product of the 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 them together.

Electrical force, like gravity, also depends inversely on the distance squared between the two bodies; short separation means big forces. Thus it takes an opposing force to keep two charges of opposite sign apart, just like it takes force to keep an apple from falling to earth. It also takes work and the expenditure of energy to pull positive and negative charges apart, just like it takes work to raise a big mass against gravity, or to stretch a spring. This stored or potential energy can be recovered and put to work to do some useful task. A falling mass can raise a bucket of water; a retracting spring can pull a door shut or run a clock. It requires some imagination to devise ways one might hook on to 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 released to fly together is called voltage, measured in units of volts (V). (Sadly, the unit volt is not named for Voltaire, but rather for Volta, an Italian scientist.) The greater the amount of charge and the greater the physical separation, the greater the voltage or stored energy. The greater the voltage, the greater the force that is driving the charges together. Voltage is always measured between two points, in this case, the positive and negative charges. If you want to compare the voltage of several charged bodies, the relative force driving the various charges, it makes sense to keep one point constant for the measurements. Traditionally, that common point is called “ground.”

Early workers, like Coulomb, also observed that two bodies with charges of the same type, either both positive or both negative, repelled each other . They experience a force pushing

them apart, and an opposing force is necessary to hold them together, like holding a compressed spring. Work can potentially be done by letting the charges fly apart, just like releasing the spring. Our analogy with gravity must end here: no one has observed negative mass, negative gravity, or uncharged bodies flying apart unaided. Too bad, it would be a great way to launch a space probe. The voltage between two separated like charges is negative; they have already done their work by running apart, and it will 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’t in 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 , V_AB , is relative. If V_AB is positive you know the two points are oppositely charged, but you cannot tell if point A has positive charge and point B negative, or visa 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 two voltages, V_AB and V_AC to your common point A . You can even determine the voltage between B and C without measuring it: V_BC = V_AC – V_AB . This is the advantage of defining a common point, like A , as ground and making all voltage measurements with respect to it. If one further defines the charge at point A to be negative charge, then a positive V_AB means point B is positively charged, by definition. The names and the signs are all relative, and sometimes confusing if one forgets what the reference or ground point is.

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