Note for Basic Electronics - BE By Aishwarya Lenka
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Basic Electronics Tutorials 1.0 ELECTRICAL THEORY 1.1 THE STRUCTURE OF ATOMS Atoms are not solid but composed of three fundamental particles: electrons, protons, and neutrons arranged in various combinations. The electron is defined as being negatively ( - ) charged and revolves around the nucleus of the atom in various concentric paths called orbits. The proton is defined as being positively ( + ) charged. Neutrons are defined as being uncharged or neutral. Protons and neutrons are tightly bound together within the atoms nucleus and are not free to orbit. In an atom the number of negative electrons and positive protons are the same making it electrically neutral. The number of protons present within the atoms nucleus specifies its atomic number. The corresponding numbers of electrons are arranged in different elliptical orbits, called shells, around the nucleus. Electrons in different orbits can rotate around the nucleus in all directions, thus producing a three-dimensional atom. The electrons in the nearest orbit having a great force of attraction while the electrons in the farthest orbit having the least force of attraction. Electrons in the farthest orbit which are loosely held to the nucleus are called valence electrons and therefore rotate around the valence shell. Copper for instance, has one valence electron. An Atoms Structure Valence shell ( one electron) atoms nucleus elliptical orbits (shell) + fixed electrons valence electron The loosely held electrons in the outer shell often break free due to an input of energy such as heat allowing them to move randomly around through the space in between the various orbits of the other atoms. Such loose electrons are called "free electrons". An atom that loses an electron in this way is left positively charged since it now has an excess of protons. If more electrons attach themselves to the valence shell, then there are more electrons than protons and the atom becomes negatively charged. 1.2 THE UNIT OF CHARGE The unit of charge is the Coulomb, C. The symbol of electric charge is Q. The -19 charge of one electron is given as: 1.6x10 , so one Coulomb of charge is -19 18 equal to 1/ 1.6x10 or 6.25x10 electrons. Thus: The more free electrons the valence shell has the better the conduction of current by the atom Q = 1 Coulomb = charge of 6.25 x 1018 Electrons EElectrelectrons ©2013 Basic Electronics Tutorials | www.electronics-tutorials.ws Page 3
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Basic Electronics Tutorials 1.3 ELECTRIC CURRENT Electric current is the flow of electric charge in the form of free electrons. Current is measured by the number of free electrons passing a particular point within a circuit per second. Therefore the flow of charge per unit second 18 defines the amount of electric current. When the charge moves at the rate of 6.25 x 10 electrons flowing past a given point per second, the value of the current is one ampere. This is the same as saying one coulomb of charge per second. The SI (International System of Units) unit of current is the ampere with letter symbol A. A constant current has symbol I, while a time-varying current has a symbol i for intensity. Mathematically we can define the relationship between charge (Q) and electric current (I) as: Where: I (amperes) = Q (coulombs) t (seconds) I = Average Current flowing Q = Total Charge passing a fixed point t = Time taken to pass point Electric current has a direction associated with it. Conventional current flow is in the direction of positive charge movement from positive to negative. Electron flow is in the opposite direction from negative to positive. The arrow in a circuit specifies the direction of positive current flow. In solid metals only negatively charged free electrons move to produce a current flow, the positive protons can not move. But in a liquid or a gas, both the positive protons and negative electrons move to produce a current flow. An electric current of one ampere flows in a circuit when a charge of one coulomb passes a given point in one second Since electric circuits consist almost entirely of solid metal conductors such as copper wire, only negatively charged electrons produce a current flow. Current is also a measure of how intense or concentrated the electron flow is. 1.4 POTENTIAL DIFFERENCE When two positive charges or two negative charges are brought near to each other they repel while a positive and negative charge are attracted to each other. Then a charged particle has the ability to do work. The ability of a charged particle to do work is called an electric potential. Thus two dissimilar charges have a difference of potential and the unit of potential difference (pd) is called the volt. The volt unit of potential difference, named after Alessandro Volta (1745–1827), involves work which is the measure of the amount of work required to move an electric charge, which in turn involves force and distance. The SI unit of work is the joule with unit symbol J, the SI unit of force is the newton with unit symbol N, and the SI unit for distance is the meter with unit symbol m. W (joules) = N (newtons) x m (meters) Energy is the capacity to do work. Potential energy, which is the energy a body has because of its physical position. ©2013 Basic Electronics Tutorials | www.electronics-tutorials.ws Page 4
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Basic Electronics Tutorials 1.5 AC WAVEFORMS Alternating current, (AC) is current that changes direction (usually many times per second) cyclically, passing first in one direction, then in the other through a circuit. Such alternating currents are produced by generators and other such voltage sources whose polarities alternate between a positive direction and a negative direction rather than being fixed in a constant direction as with DC sources. By convention, alternating currents are called AC currents and alternating voltages are called AC voltages. The most common AC source is the commercial AC power system that supplies energy to your home. The variation of an AC voltage or an AC current over time is called a waveform. Since these waveforms vary with time, AC supplies are designated by lowercase letters v(t) for voltage, and i(t) for current instead of uppercase letters V and I for DC values. Note that the subscript (t) represents time. There are many different types and shapes of waveforms but the most fundamental is the sine wave (also called sinusoid). The sine wave or sinusoidal AC waveform is the voltage and current waveform shape at the wall socket outlets in your home. Sinusoidal waveforms are produced by rotating a coil of wire at a constant angular velocity within a fixed magnetic field The waveform starts at zero, increases to a positive maximum (called the peak), and then decreases to zero, changes polarity, increases to a negative maximum, then returns again back to zero. One complete variation between the same points on the waveform is referred to as a cycle. Since the waveform repeats itself at regular intervals over time, it is called a periodic waveform. A(max) = 1 +1 A(rms) = A(max) x 0.707 A(avg) = A(max) x 0.636 Amplitude +Amax 2 (rads/ s) o 135 90o 180o 2 360o negative -Amax Sinusoidal Waveform A ( RMS) A ( AVG ) q -1 ( Hz ) T A(t) = Am sin (t) positive 0 1 Apk - pk A max 2 0 . 7 0 7 1Vm a x 2 A max Form Fact or 0 . 6 3 7 Vm a x 1.11 2 2 Periodic Time (T) C rest Fact or 2 1.414 The calculation of Average, R.M.S, Form Factor and Crest Factor can be use with any type of periodic waveform including Triangular, Square, Sawtoothed or any other irregular or complex voltage or current waveform shape. For a pure sinusoidal waveform the effective or R.M.S. value will always be equal to 1/√2 x Amax which is equal to 0.707 x Amax and this relationship also holds true for RMS values of current. The RMS value for a sinusoidal waveform is always greater than its Average value. ©2013 Basic Electronics Tutorials | www.electronics-tutorials.ws Page 6
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Basic Electronics Tutorials In electronics, potential difference is commonly referred to as voltage, with the symbol V. Sometimes the symbol U or E for emf (electromotive force) is used, but the standard symbol V represents any potential difference. This applies either to the voltage generated by a source such as a battery or solar cell, or to the voltage dropped across a passive component such as a resistor. The voltage difference (also called potential difference) between two points is the work in joules required to move one coulomb of charge from one point to the other. The SI unit of voltage is the volt and is given as: Where: V (volts) = W (joules) Q (coulombs) V = is the Voltage in Volts W = is the Work done Q = Total Charge passing a fixed point A constant voltage is called a DC voltage and a voltage that varies sinusoidally with time is called an AC voltage. A voltage source such as a battery or solar cell provides a constant DC voltage, for example 12 VDC. A voltage source such as an alternator or generator provides an alternating AC voltage, so for example 240 VAC. If point a is positive with respect to point b, moving a positive charge around a closed circuit from a to b (or a negative charge from b to a) requires work. The difference between the two points is the voltage polarity. This voltage polarity is indicated by a positive sign ( + ) at point a and a negative sign ( - ) at point b. a I = +2A + V = +6V VR R - The potential difference between two points is one volt if one joule of work is done displacing one coulomb of charge In the circuit the arrow indicates the direction of current flow. If the arrow points in the same direction of the positive charge carriers (conventional current flow) the numeric vale of the current receives a positive sign, +2A. If current flow is opposite (electron flow), the numerical value receives a minus sign, -2A. The sign indicates the direction of current flow with the arrow ( + ) or in reverse ( - ). Then Conventional Current Flow gives the flow of electrical current from positive to negative and Electron Current Flow around a circuit from the negative terminal to the positive. The item which provides a path for the electrons to flow is called a conductor. b a I = -2A - V = -6V VR R The polarity of the voltage is also indicated by the direction of an arrow. If the arrow points from more positive to more negative potential, the numerical value of the voltage has a positive sign, +6V. If it points from a more negative to a more positive potential, then the numerical value receives a minus sign, -6V. + The battery symbol is often used to denote a DC voltage source, but it may not always be a battery. Usually the positive ( + ) and negative ( - ) signs are not b shown because, by convention, the long end line denotes the positive terminal (the Anode) and the short end line the negative terminal (the Cathode). Thus, it is not necessary to put + and - signs on the diagram. For more information visit our website at: www.electronics-tutorials.ws ©2013 Basic Electronics Tutorials | www.electronics-tutorials.ws Page 5
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