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هاي
ترجمة النصوص من الانكليزي الى اللغة العربية بدقة من اجل الامتحان
ومشكورين جدا
1-
the cathode ray tube
the cathode ray tube ( crt ) is used in oscilloscopes , radar receivers and television sets . the type described here is that used in oscilloscopes . by means of a crt , an oscilloscope not only shows the size of signal . but also how the signal varies with time . in other words it shows the waveform of the signal .
the crt operates as follows . first electrons are emitted from a heated cathode . then these electrons are accelerated to give them velocity . next they are formed into a beam which can be deflected vertically and horizontally . finally they are made to strike a screen coated on its inner surface with a phosphor .
the crt comprises an electron gun and a deflection system enclosed in a glass tube with a phosphor coated screen . the electron gun form the electrons into a beam . it contains a cathode which is heated to produce a stream of electrons . on the some axis as the cathode is a cylinder known as the grid . by varying the negative potential on the grid . the intensity of the beam can be varied .
a system of three anodes follows . these accelerate the beam and also operate as a lens to focus the beam on the screen as a small dot . varying the potential on the central anode . a2 . allows the focus to be adjusted .
on leaving the electron gun , the beam passes through two sets of plates which are at right angles to each other . the first set of plates are the Y plates as these are nearer the anodes , they have a greater effect on the beam . therefore the signal is applied to this set . they control the vertical deflection of the beam . the second set are the X plates . on an oscilloscope the output from a timebase oscillator is applied across these plates as a means of moving the beam horizontally at regular intervals . hence the horizontal axis of an oscilloscope is the time axis . by means of the deflection system , then , the beam strikes the screen , the phosphor coated screen . when the electron beam strikes the screen , the phosphor coating fluoresces . various colours of light are produced on the phosphor used .
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circuit elements
current moves from a point of high potential energy to one of low potential . it can only do so if there is a path for in to follow . this path is called an electric circuit . all circuit contain four elements : a source , a load , a transmission system and a control .


the source provides the electromotive force . this establishes the difference in potential which makes current flow possible . the source can be any device which supplies electrical energy . for example , it may be a generator or a battery the load converts the electrical energy from the source into some other form of energy . for instance , a lamp change electrical energy into light and heat . the load can be any electrical device .
the transmission system conducts the current round the circuit . any conductor can be part of a transmission system . most systems consist of wires it is often possible , however , for the metal frame of a unit to be one section of its transmission system . for example , the metal chassis of many electrical devices are used to conduct current . similarly the body of a car is part of its electrical transmission system .
the control regulates the current flow in the circuit . it may control the current by limiting it , as does a rheostat , or by interrupting it , as does switch .
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Conductors , insulators and semiconductors
If we connect a battery across a body, there is a movement of free electrons towards the positive end . this movement of electrons is an electric current . all materials can be classified into three groups according to how readily they permit an electric current to flow . these are: conductors and semiconductor .


In the first category are substances which provide an easy path for an electric current . all metals are conductors , however some metals do not conduct well . manganin , for example, is a poor conductor . copper is a good conductor , therefore it is widely used for cables . a non-metal which conductors well is carbon .salt water is an example of a liquid conductor .
A material which does not easily release electrons is called an insulator rubber , nylon , porcelain and air are insulators . there are no perfect insulators . all insulators will allow some flow of electrons , however this can usually be ignored because the flow they permit is so small.
Semiconductor are midway between conductor and insulators . under certain conditions they allow a current to flow easily but under others they behave as insulators . germanium and silicon are semiconductors . mixtures of certain metallic oxides also act as semiconductors . there are known as thermistors . the resistance of thermistors falls rapidly as their temperature rises . they are therefore used in temperature-sensing devices .


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The DC motor



An electric motor is a machine for converting electrical energy into mechanical energy . motors can be designed to run on direct (dc) or alternating current (ac) . the motor shown in figure 1 is a dc motor . its most important parts are the rotor , the stator and the brushgear .



The rotor is the moving part . it contains an armature , which is a set of wire loops wound on a steel core . when current is fed to the armature , these windings produce a magnetic field . the armature and core are mounted on a shaft which runs on bearings . it provides a means of transmitting power from the motor .



The rotor also contains a commutator . this consists of a number of copper segments .insulated from one another .



The armature windings are connected to these segments . carbon brushes are held in contact with the commutator by springs . these brushes allow current to pass to the armature windings . as the rotor turns , the commutator acts as a switch making the current in the armature alternate .



The stator does not move . it consists of magnetic and electrical conductors . the magnetic circuit is made up of the frame and poles . wound round the poles are the field coils . these form the stator is electrical circuit . when current is fed to them , a magnetic field is set up is the stator .



The motor operates on the principle that when a current-carrying conductor is placed in a magnetic field , a force is produced on the conductor . the interaction of the forces produced by the magnetic field of the rotor and the stator makes the rotor spin .



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