Ohms law Ohm’s law states that the current through a conductor between two points is directly proportional to the potential difference across the two points. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equation that describes this relationship: Where I is the current through the conductor in units of amperes, V is the potential difference measured across the conductor in units of volts, and R is the resistance of the conductor in units of ohms. More specifically, Ohms law states that the R in this relation is constant, independent of the current. The law was named after the German physicist Georg Ohm, who, in a treatise published in 1827, described measurements of applied voltage and current through simple electrical circuits containing various lengths of wire. Its relation with respect to Current I, V and R For a moment, think about what resistance looks like from the electrons’ point of view. Imagine you are an electron being pushed from atom to atom from the pressure of all your fellow electrons pushing on you. Think about your electron self, moving through the mix of glue binder to carbon particles, where you could either collide with an atom and be given off as heat in the form of atomic vibrations, or at other times, you are allowed to pass through the resistor to continue your path through the circuit. It would stand to reason that if you had a certain pressure of electron friends pushing on you, lets say 9 volts (V or E) worth, and a certain amperage (I) working its way through the resistor of a given resistance, that your movement as an electron would be based on the relationship of all your pushy electron friends. Georg Ohm (1789-- 1854) defined an important relationship between voltage, current, and resistance, now called Ohm’s law which states: a potential difference of one volt will force a current of one ampere through a resistance of one ohm. Mathematically, Ohms law can be written as: I = E/R, where I is current, E is applied voltage, and R is the resistance in ohms. It can also be written as E = I ´ R or R = E/I. The diagram below allows you to easily memorize this important law. By putting your finger on the value you are trying to solve, you will see the solution. So by putting your finger on the E you solve for I and R or E = I x R or solving for I would be I = E divided by R. Ohms Law To understand Ohm’s law and the relationship between voltage, resistance, and current flow, the water analogy is one way to think about this relationship. (Directly below). Visualization of electron flow with resistor as faucet analogy. More resistance means less electron flow. The liquid flow decreases when the faucet is closed. This is analogous to high value resistance constricting the width of the electron pipe, so fewer electrons pass in a given period. Therefore, high resistance decreases the flow of electrons, but the water analogy also allows identification of the idea of electron pressure or electron quantity as it relates to the possible flow. With Ohm’s law, you can now calculate what size resistor you will need to protect a sensitive electronic part that is not able to withstand large quantities of electrons in a given period of time. Remember how we defined a fuse as a thin wire that would burn up and melt if too many electrons are being pushed through in a given period of time? Well, resistors can prevent sensitive parts from burning up by absorbing and giving off as heat some of that electron energy. With Ohms law if you are given any two values in a circuit, you can always solve for the third value using simple algebra. Often when you purchase parts the data sheet on the back of the package will say that the electronic device will work with a certain voltage like 5-volts DC. The package will also say that the electronic part only can withstand a certain amount of amperage or mill amperage. With Ohm’s law, you can now calculate that a resistor of a particular size will allow a 5-volt source to deliver a certain amount of amperage in one second of time. Fortunately, Ohm’s law works for AC, DC, and radio frequencies (RF). When working with electronics, it is important that we have a language to communicate electronic designs. The excitement of being able to read the maps will become apparent as you learn electronics since maps of possible circuits are freely available. A schematic is a map that tells us how to build circuits. They are constructed of symbols, which refer to different electronic parts. When I first started studying electronics it was intimidating until I realized that artists and engineers are visual thinkers and can see things three dimensionally however engineers have also constructed a marvelous symbolic language to describe circuits and their connections, and this language also makes a lot of sense to artists. For example, (image below) shows the symbol for the resistor and a 3D model of a resistor to the right. The crooked line communicates that as the electrons enter the resistor they are reduced in quantity or slowed down, as in a crooked road that may slow your car down. You can also think of the resistor symbols, crooked line, as a wave of heat which the resistor is getting hot with all the quantum vibrations of electrons striking the nuclei of the carbon atoms within the resistor. 470-ohm resistor: part on right and schematic drawing on left. Following this paragraph is an example of schematic that has a resistor, a lamp and a battery in a complete circuit. We will use this circuit schematic and some changing values to do some Ohm’s law calculations. LED (light emitting diode) A light-emitting diode (LED) is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for general lighting. Appearing as practical electronic components in 1962, early LEDs emitted low-intensity red light, but modern versions are available across the visible, ultraviolet, and infrared wavelengths, with very high brightness. When a light-emitting diode is switched on, electrons are able to recombine withholds within the device, releasing energy in the form of photons. This effect is called electroluminescence, and the color of the light (corresponding to the energy of the photon) is determined by the energy band gap of the semiconductor. An LED is often small in area (less than 1 mm2), and integrated optical components may be used to shape its radiation pattern. LEDs have many advantages over incandescent light sources including lower energy consumption, longer lifetime, improved physical robustness, smaller size, and faster switching. However, LEDs powerful enough for room lighting are relatively expensive, and require more precise current and heat management than compact fluorescent lamp sources of comparable output. Light-emitting diodes are used in applications as diverse as lighting, automotive, advertising, general lighting, and traffic signals. LEDs have allowed new text, video displays, and sensors to be developed, while their high switching rates are also useful in advanced communications technology. Infrared LEDs are also used in the remote control units of many commercial products including televisions, DVD players and other domestic appliances. LEDs are also used in seven-segment display. Construction and Working Principle of LED: Light Emitted Diode: LED stands for Light Emitted Diode. It is a P-N junction diode which emits the radiations of light when it is forward. Symbol of LED (Light Emitted Diode). Working Principle of LED (Light Emitted Diode): Working of the Light Emitted Diode is based on the Principle that when diode is forward biased the electron and hole recombination takes place in the depletion region of P-N junction diode and when the electron hole recombination takes place the radiations of light are emitted in the form of photons of light by the recombination of the electrons and holes. So, Light Emitted Diode is a semiconductor source of light. This effect is called as electroluminescent and the color off the light which is emitted by the Light Emitted Diode will depend on the Energy band gap between the semiconductors. Construction of LED (Light Emitted Diode): Light Emitted Diode consists of the heavily doped P-N junction diode and this diode is forward biased. During the forward biased the large number of the majority carriers moves across the junction where number of electron and holes will combine with each others. Where the electrons and holes combination takes place if the diode is made of the Silicon (Si) or Germanium (Ge) then the Infrared Radiations are emitted out by the diode and if the diode is made of the Gallium Arsenite (GaAs) OR Gallium Indium Phosphate (GaInP) then the visible radiations are emitted out from the diode. The Vacuum Tube A vacuum tube is just that: a glass tube surrounding a vacuum (an area from which all gases has been removed). What makes it interesting is that when electrical contacts are put on the ends, you can get a current to flow though those vacuums. Thomas Edison noticed this first in 1883. While fiddling with light bulbs he saw that he could get current to jump from the hot filament to a metal plate at the bottom. What Edison discovered (and it was promptly dubbed the Edison effect) was that electrical current doesnt need a wire to move through. It can travel right through a gas or even a vacuum. The Edison effect, incidentally, is the only piece of scientific work Edison ever did. He was not a scientist but an inventor, a tinkered. This kind of thinking would be as important as science for the invention of the transistor. Edisons discovery that current can travel through a vacuum didnt turn out to be very useful information until 1904. Thats when a British scientist named John A. Fleming made a vacuum tube known today as a diode. Then the diode was known as a valve, because it forced current in the tube to travel exclusively in one direction. Getting that single directional flow was critical for radio sets which needed to turn alternating current into direct current. The vacuum tube didnt reach its full maturity until Lee De Forest came along a decade later. De Forest invented something he called the audion. Not only did it force current to move in a single direction, but it could be used to increase the current along the way. De Forest put a metal grid in the middle of the vacuum tube. By using a small input current to change the voltage on the grid, De Forest could control the flow of a second, more powerful current, through the tube. The strength of two currents was not necessarily related -- a weak current might be applied to the tubes grid, but a much stronger current could come out the main electrodes of the tube. Turning weak currents into strong currents was crucial for a number of new technologies at that time. Bell Labs made use of it for its coast to coast phone system and vacuum tubes soon found their way into everything from hearing aids to radios to televisions Sources. Construction of Vacuum Tube Diode: electrons from the hot cathode flow towards the positive anode, but not vice versa Triode: voltage applied to the grid controls plate (anode) current. A vacuum tube consists of two or more electrodes in a vacuum inside an airtight enclosure. Most tubes have glass envelopes, though ceramic and metal envelopes (atop insulating bases) have been used. The electrodes are attached to leads which pass through the envelope via an airtight seal. On most tubes, the leads, in the form of pins, plug into a tube socket for easy replacement of the tube (tubes were by far the most common cause of failure in electronic equipment, and consumers were expected to be able to replace tubes themselves). Some tubes had an electrode terminating at a top cap which reduced inter electrode capacitance to improve high-frequency performance, kept a possibly very high plate voltage away from lower voltages, and could accommodate one more electrode than allowed by the base. The earliest vacuum tubes evolved from incandescent light bulbs, containing a filament sealed in an evacuated glass envelope. When hot, the filament releases electrons into the vacuum, a process called thermion emission. A second electrode, the anode or plate, will attract those electrons if it is at a more positive voltage. The result is a net flow of electrons from the filament to plate. However, electrons cannot flow in the reverse direction because the plate is not heated and does not emit electrons. The filament (cathode) has a dual function: it emits electrons when heated; and, together with the plate, it creates an electric field due to the potential difference between them. Such a tube with only two electrodes is termed a diode, and is used for rectification. Since current can only pass in one direction, such a diode (or rectifier) will convert alternating current (AC) to pulsating DC. This can therefore be used in a DC power supply, and is also used as a demodulator of amplitude modulated (AM) radio signals and similar functions. Work of Vacuum Tube A vacuum tube works to control and convert electric current. A vacuum tube is usually shaped like a cylinder and has a small light bulb inside that burns off electrons and converts them from alternating current to direct current. Old televisions and transistor radios used the vacuum tubes. When one went bad, it had to be replaced in order to maintain the flow pattern of current and the ability to produce electronic amplification. Most electronic applications that were build on the use of vacuum tubes have been replaces with transistors and semiconductor devices for more efficiency but are still used for high powered radio transmitters. Reference: All materials have been taken from internet through wedipedia .
Posted on: Fri, 22 Nov 2013 08:34:12 +0000
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