tag:blogger.com,1999:blog-55155764903230196802024-02-06T20:23:13.842-08:00Science universe: Physics articlesLearn about science,health and physics.Physics laws and theories explained- with illustrated images and worked examples.Your guide to be a scientist!.Ahmed Farahathttp://www.blogger.com/profile/18408185056533495895noreply@blogger.comBlogger1002125tag:blogger.com,1999:blog-5515576490323019680.post-47225910054597539602015-03-06T09:24:00.001-08:002015-03-06T09:24:23.671-08:00Transmission LinesIntroduction A transmission line is a system of conductors connecting one point to another and along which electromagnetic energy can be sent. Thus telephone lines and power distribution lines are typical examples of transmission lines; in electronics, however, the term usually implies a line used for the transmission of radio-frequency (r.f.) energy such as that from a radio transmitter to the Ahmed Farahathttp://www.blogger.com/profile/18408185056533495895noreply@blogger.comtag:blogger.com,1999:blog-5515576490323019680.post-63880655042148172812015-03-06T08:18:00.001-08:002015-03-06T08:18:12.942-08:00AttenuatorsIntroduction An attenuator is a device for introducing a specified loss between a signal source and a matched load without upsetting the impedance relationship necessary for matching. The loss introduced is constant irrespective of frequency; since reactive elements (L or C) vary with frequency, it follows that ideal attenuators are networks containing pure resistances. A fixed attenuator Ahmed Farahathttp://www.blogger.com/profile/18408185056533495895noreply@blogger.comtag:blogger.com,1999:blog-5515576490323019680.post-96675072771566282015-03-06T08:16:00.001-08:002015-03-06T08:16:34.118-08:00ModulationIntroduction to Modulation The transmission of information such as speech, music and data over long distances requires the use of a carrier channel. It is common practise to ‘carry’ different communications, called signals, at different frequencies to stop one signal from interfering with another. A signal can be shifted bodily from its original band to another, this being achieved by ‘Ahmed Farahathttp://www.blogger.com/profile/18408185056533495895noreply@blogger.comtag:blogger.com,1999:blog-5515576490323019680.post-19342441178767153622015-03-06T08:15:00.001-08:002015-03-06T08:15:40.328-08:00Filter NetworksIntroduction A filter is a network designed to pass signals having frequencies within certain bands (called passbands) with little attenuation, but greatly attenuates signals within other bands (called attenuation bands or stopbands). A filter is frequency sensitive and is thus composed of reactive elements. Since certain frequencies are to be passed with minimal loss, ideally the inductors andAhmed Farahathttp://www.blogger.com/profile/18408185056533495895noreply@blogger.comtag:blogger.com,1999:blog-5515576490323019680.post-10230671170306706982015-03-06T07:40:00.001-08:002015-03-06T07:40:41.726-08:00Field TheoryIntroduction Electric fields, magnetic fields and conduction fields (i.e. a region in which an electric current flows) are analogous, i.e. they all exhibit similar characteristics. Thus they may all be analysed by similar processes. In the following the electric field is analysed. Figure 79.1 shows two parallel plates A and B. Let the potential on plate A be CV volts and that on plate B be ðV Ahmed Farahathttp://www.blogger.com/profile/18408185056533495895noreply@blogger.comtag:blogger.com,1999:blog-5515576490323019680.post-52648575760899026702015-03-06T07:34:00.001-08:002015-03-06T07:34:07.021-08:00Dielectrics and Dielectric LossElectric Fields, Capacitance and Permittivity Any region in which an electric charge experiences a force is called an electrostatic field. Electric fields, Coulombs law, capacitance and permittivity are discussed in chapter 44 — refer back to page 233. Summarising the main formulae: Polarisation When a dielectric is placed between charged plates, the capacitance of the system increases. The Ahmed Farahathttp://www.blogger.com/profile/18408185056533495895noreply@blogger.comtag:blogger.com,1999:blog-5515576490323019680.post-26826671051280763882015-03-06T07:03:00.001-08:002015-03-06T07:03:19.155-08:00A Numerical Method of Harmonic AnalysisIntroduction Many practical waveforms can be represented by simple mathematical expressions, and, by using Fourier series, the magnitude of their harmonic components determined. For waveforms not in this category, analysis may be achieved by numerical methods. Harmonic analysis is the process of resolving a periodic, non-sinusoidal quantity into a series of sinusoidal components of ascending Ahmed Farahathttp://www.blogger.com/profile/18408185056533495895noreply@blogger.comtag:blogger.com,1999:blog-5515576490323019680.post-24286052357116787382015-03-06T06:55:00.001-08:002015-03-06T06:55:51.609-08:00Maximum Power Transfer Theorems and Impedance MatchingMaximum Power Transfer Theorems A network that contains linear impedances and one or more voltage or current sources can be reduced to a The´venin equivalent circuit as shown in chapter 73. When a load is connected to the terminals of this equivalent circuit, power is transferred from the source to the load. A Thevenin equivalent circuit is shown in Figure 75.1 with source internal impedance, zAhmed Farahathttp://www.blogger.com/profile/18408185056533495895noreply@blogger.comtag:blogger.com,1999:blog-5515576490323019680.post-66812596461833840942015-03-06T06:53:00.001-08:002015-03-06T06:53:34.695-08:00Delta-star and Star-delta TransformationsIntroduction By using Kirchhoff’s laws, mesh-current analysis, nodal analysis or the super- position theorem, currents and voltages in many network can be determined as shown in chapters 70 to 72. Thevenin’s and Norton’s theorems, introduced in chapter 73, provide an alternative method of solving networks and often with considerably reduced numerical calculations. Also, these latter theorems areAhmed Farahathttp://www.blogger.com/profile/18408185056533495895noreply@blogger.comtag:blogger.com,1999:blog-5515576490323019680.post-88105290127244642312015-03-06T06:52:00.001-08:002015-03-06T06:52:46.382-08:00The´venin’s and Norton’s TheoremsIntroduction Many of the networks analysed in Chapters 70 to 72 using Kirchhoff’s laws, mesh-current and nodal analysis and the superposition theorem can be analysed more quickly and easily by using The´venin’s or Norton’s theorems. Each of these theorems involves replacing what may be a complicated network of sources and linear impedances with a simple equivalent circuit. A set procedure may beAhmed Farahathttp://www.blogger.com/profile/18408185056533495895noreply@blogger.comtag:blogger.com,1999:blog-5515576490323019680.post-17224727659126068882015-03-06T06:17:00.001-08:002015-03-06T06:17:31.904-08:00The Superposition Theorem.Introduction The superposition theorem states: ‘In any network made up of linear impedances and containing more than one source of e.m.f. the resultant current flowing in any branch is the phasor sum of the currents that would flow in that branch if each source were considered separately, all other sources being replaced at that time by their respective internal impedances’ Using the Ahmed Farahathttp://www.blogger.com/profile/18408185056533495895noreply@blogger.comtag:blogger.com,1999:blog-5515576490323019680.post-56867286503592747942015-03-06T05:42:00.001-08:002015-03-06T05:42:27.808-08:00Mesh-current and Nodal AnalysisMesh-current Analysis Mesh-current analysis is merely an extension of the use of Kirchhoff’s laws, explained in chapter 70. Figure 71.1 shows a network whose circulating cur- rents I1, I2 and I3 have been assigned to closed loops in the circuit rather than to branches. Currents I1 , I2 and I3 are called mesh-currents or loop-currents. In mesh-current analysis the loop-currents are all arranged Ahmed Farahathttp://www.blogger.com/profile/18408185056533495895noreply@blogger.comtag:blogger.com,1999:blog-5515576490323019680.post-55902283001680561212015-03-06T05:41:00.001-08:002015-03-06T05:41:06.045-08:00Introduction to Network AnalysisIntroduction Voltage sources in series-parallel networks cause currents to flow in each branch of the circuit and corresponding volt-drops occur across the circuit components. A.c. circuit (or network) analysis involves the determination of the currents in the branches and/or the voltages across components. The laws which determine the currents and voltage drops in a.c. networks are: (a), Ahmed Farahathttp://www.blogger.com/profile/18408185056533495895noreply@blogger.comtag:blogger.com,1999:blog-5515576490323019680.post-19178012747152277802015-03-05T09:01:00.001-08:002015-03-05T09:01:32.485-08:00D.c. TransientsIntroduction When a d.c. voltage is applied to a capacitor C and resistor R connected in series, there is a short period of time immediately after the voltage is connected, during which the current flowing in the circuit and voltages across C and R are changing. Similarly, when a d.c. voltage is connected to a circuit having inductance L connected in series with resistance R, there is a short Ahmed Farahathttp://www.blogger.com/profile/18408185056533495895noreply@blogger.comtag:blogger.com,1999:blog-5515576490323019680.post-79043630578801249482015-03-05T08:12:00.001-08:002015-03-05T08:12:12.814-08:00Single-phase Parallel a.c. CircuitsIntroduction In parallel circuits, such as those shown in Figures 56.1 and 56.2, the voltage is common to each branch of the network and is thus taken as the reference phasor when drawing phasor diagrams. For any parallel a.c. circuit: These formulae are the same as for series a.c. circuits as used in chapter 55. R-L Parallel a.c. Circuit In the two branch parallel circuit containing Ahmed Farahathttp://www.blogger.com/profile/18408185056533495895noreply@blogger.comtag:blogger.com,1999:blog-5515576490323019680.post-49076888622679490422015-03-05T08:00:00.001-08:002015-03-05T08:00:48.660-08:00Single-phase Series a.c. CircuitsPurely Resistive a.c. Circuit In a purely resistive a.c. circuit, the current IR and applied voltage VR are in phase. See Figure 55.1. Purely Inductive a.c. Circuit In a purely inductive a.c. circuit, the current IL lags the applied voltage VL by See Figure 55.2. In a purely inductive circuit the opposition to the flow of alternating current is called the inductive reactance, XL For Ahmed Farahathttp://www.blogger.com/profile/18408185056533495895noreply@blogger.comtag:blogger.com,1999:blog-5515576490323019680.post-48951991687930185212015-03-05T07:56:00.001-08:002015-03-05T07:56:07.811-08:00D.c. Circuit TheoryIntroduction The laws that determine the currents and voltage drops in d.c. networks are: (a) Ohm’s law (see chapter 40), (b) the laws for resistors in series and in parallel (see chapter 43), and (c) Kirchhoff’s laws. In addition, there are a number of circuit theorems that have been developed for solving problems in electrical networks. These include: (i) the superposition theorem (ii) The´Ahmed Farahathttp://www.blogger.com/profile/18408185056533495895noreply@blogger.comtag:blogger.com,1999:blog-5515576490323019680.post-9170280187946389192015-03-05T07:15:00.001-08:002015-03-05T07:15:31.231-08:00Alternating Voltages and CurrentsIntroduction Electricity is produced by generators at power stations and then distributed by a vast network of transmission lines (called the National Grid system) to industry and for domestic use. It is easier and cheaper to generate alternating current (a.c.) than direct current (d.c.) and a.c. is more conveniently distributed than d.c. since its voltage can be readily altered using Ahmed Farahathttp://www.blogger.com/profile/18408185056533495895noreply@blogger.comtag:blogger.com,1999:blog-5515576490323019680.post-86878471260835529152015-03-05T06:42:00.001-08:002015-03-05T06:42:28.900-08:00TransistorsThe Bipolar Junction Transistor The bipolar junction transistor consists of three regions of semiconductor material. One type is called a p-n-p transistor, in which two regions of p-type material sandwich a very thin layer of n-type material. A second type is called an n-p-n transistor, in which two regions of n-type material sandwich a very thin layer of p-type material. Both of these types of Ahmed Farahathttp://www.blogger.com/profile/18408185056533495895noreply@blogger.comtag:blogger.com,1999:blog-5515576490323019680.post-19080776503383107682015-03-05T05:29:00.001-08:002015-03-05T05:29:14.550-08:00Electrical Measuring Instruments and MeasurementsIntroduction Tests and measurements are important in designing, evaluating, maintaining and servicing electrical circuits and equipment. In order to detect electrical quantities such as current, voltage, resistance or power, it is necessary to transform an electrical quantity or condition into a visible indication. This is done with the aid of instruments (or meters) that indicate the magnitude Ahmed Farahathttp://www.blogger.com/profile/18408185056533495895noreply@blogger.comtag:blogger.com,1999:blog-5515576490323019680.post-26895976590176876952015-03-05T05:22:00.001-08:002015-03-05T05:22:55.058-08:00Semiconductor DiodesTypes of Materials Materials may be classified as conductors, semiconductors or insulators. The classification depends on the value of resistivity of the material. Good conductors are usually metals and have resistivities in the order of 10Ð7 to 10Ð8 Qm. Semiconductors have resistivities in the order of 10Ð3 to 3 ð 103 Qm. The resistivities of insulators are in the order of 104 to 1014 Qm. Some Ahmed Farahathttp://www.blogger.com/profile/18408185056533495895noreply@blogger.comtag:blogger.com,1999:blog-5515576490323019680.post-86302217379014785652015-03-04T09:22:00.001-08:002015-03-04T09:22:29.784-08:00Magnetically Coupled CircuitsIntroduction When the interaction between two loops of a circuit takes place through a magnetic field instead of through common elements, the loops are said to be inductively or magnetically coupled. The windings of a transformer, for example, are magnetically coupled (see Chapter 60). Mutual Inductance Mutual inductance is said to exist between two circuits when a changing current in one Ahmed Farahathttp://www.blogger.com/profile/18408185056533495895noreply@blogger.comtag:blogger.com,1999:blog-5515576490323019680.post-32031437166185115882015-03-04T08:56:00.001-08:002015-03-04T08:56:53.739-08:00Electromagnetic Induction and InductanceIntroduction to Electromagnetic Induction When a conductor is moved across a magnetic field so as to cut through the lines of force (or flux), an electromotive force (e.m.f.) is produced in the conductor. If the conductor forms part of a closed circuit then the e.m.f. produced causes an electric current to flow round the circuit. Hence, an e.m.f. (and thus current) is ‘induced’ in the conductor Ahmed Farahathttp://www.blogger.com/profile/18408185056533495895noreply@blogger.comtag:blogger.com,1999:blog-5515576490323019680.post-66408719003031817452015-03-04T08:50:00.001-08:002015-03-04T08:50:29.419-08:00ElectromagnetismMagnetic Field due to an Electric Current Magnetic fields can be set up not only by permanent magnets, as shown in chapter 45, but also by electric currents. Let a piece of wire be arranged to pass vertically through a horizontal sheet of cardboard on which is placed some iron filings, as shown in Figure 47.1(a). If a current is now passed through the wire, then the iron filings will form a Ahmed Farahathttp://www.blogger.com/profile/18408185056533495895noreply@blogger.comtag:blogger.com,1999:blog-5515576490323019680.post-2443518112228880372015-03-04T08:34:00.003-08:002015-03-04T08:34:21.990-08:00Magnetic MaterialsMagnetic Properties of Materials The full theory of magnetism is one of the most complex of subjects. How- ever the phenomenon may be satisfactorily explained by the use of a simple model. Bohr and Rutherford, who discovered atomic structure, suggested that electrons move around the nucleus confined to a plane, like planets around the sun. An even better model is to consider each electron as Ahmed Farahathttp://www.blogger.com/profile/18408185056533495895noreply@blogger.com