TRANSISTOR

Transistor:
Transistor is an electronic device that can be used as an amplifier or as a switch.Transistor is the next version to diode.Diode can simply pass through current in uni direction and can act as switch,but transistor can also amplify.They can control a large current flowing through 2 regions of semiconductor like si to a small current applied to a middle region.There are many kinds of transistors typical is bipolar transistor. Transistor has 3 layers. Transistor is designed by appending another region on diode(PN-diode) either left or right,so the transistor configurations are PNP,NPN . The layers are Emitter,Collector and Base.In transistor current flow from Emitter to Collector and that flow is controlled by current applied to middle terminal called Base.

Demonstration:  
When current is applied to Emitter electrons cross over to P-type base filling the electrons holes ,when holes are filled resulting negative charges so repell further electrons to come from Emitter and transistor doesnt conduct.But a small amount of current is applied to base that will produce additional holes ,this will further invite much electrons from Emitter to pass through base to collector because Emitter to Collector flow is several 100 times more than Emitter to Base thus making transistor a power amplifier.
As base current is controlling the amplification process transistor is a current controlled voltage source.
Transistor is also called Bipolar junction transistor.since both holes and electrons are operating this is a bipolar device and since junction is present between PN and NP.This is referred as bipolar junction transistor.

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Machine - Motor

As already we discussed about the generator now we see the action of motor.Actually motor is a electronic device which converts electrical energy into mechanical energy.

HOW DOES IT ACTUALLY WORK?

When ever a current carrying conductor is wounded around a cock and is placed between the magnets,electric field(due to flow of electrons in the conductor) and magnetic field(due to the magnets) interact with each other resulting in the motion of the cock.The direction can be known by Fleming left hand rule and can be changed by changing the direction of current(flow of electrons) . This is how the electrical energy is converted into mechanical energy.

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Machine-Generator

Generator is a great invention.Normally machine can be a machine or a motor. In this section we will learn about a generator. Actually a generator is a electronic device which is used to convert mechanical energy into  electrical energy.

How does it actually do?

The answer to the question is when a conductor is placed between a strong magnetic fields and when it is rotating (i.e when applied mechanical force ),the conductor cuts the flux lines(magnetic lines of force between two poles) an EMF(electro motive force or simply voltage)will be induced into the conductor. With this the electrons travel causing an electric current.In this way mechanical energy is converted into electrical energy 

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RECTIFIER

In the last article we studied about a P N junction diode,how it works ,whats it nature in forward and reverse bias condition and one application that is as a switch. Today we learn about an another application that is a diode can act as a "RECTIFIER".

         In this section we observe how a diode act as a rectifier.
RECTIFIER  :   It  is a device which is used to convert AC(continuous variable with time) into a DC(constant with time).

NEED TO CONVERT AC TO DC:

1. We cannot store an AC signal as it is continuously varying with time.But we can store a DC signal like in batteries etc.
2. Our all electronic goods require DC because a continuous change in signal will affect the device.
3. Our renewable energy sources like hydro electric ,wind turbine generate AC so there is a need to convert to DC.

There are 3 types of rectifiers using diode.

• half wave rectifier
• full wave rectifier
• bridge rectifier

1. Half wave rectifier:

In this half wave rectifier only half of the input AC signal into DC signal.

OPERATION:

                      Here when the input AC is in positive state, diode becomes ON and allows for rectification but for negative input of AC diode will be in OFF state so open circuit(no output will be resulted). 

Efficiency is around 40.6%.

Transformer is to allow only AC and reject DC.

output resistance is used to measure the output DC signal.

2. Full wave rectifier:

In the full wave rectifier both positive and negative cycles of input AC is converted in DC.

OPERATION:

                      Here for the positive input cycle, diode D1 becomes ON so the output DC is resulted and for the negative input cycle diode D2 becomes ON so same process continues and DC signal is resulted.

*Hence in the full wave rectifier, input AC signal is completely converted into DC signal.

*Efficiency is around 81.2%(double of half wave rectifier).

3. Bridge rectifier:

                        similar to the full wave rectifier bridge rectifier also converts AC into DC,it is one of the design process using 4 diodes.

OPERATION:

If we name the diodes as D1,D2,D3,D4 respectively in clockwise direction from right top then for positive AC signal D1 & D3 will be in ON condition remaining will be in OFF state where as for negative AC signal D2 & D4 will be ON and remaining will be in OFF state.

*This is how AC signal is completely converted into DC signal.

*Efficiency is also the same.

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Introduction to PN junction diode

As we know metals are classified into 3 types they are : conductors,insulators and semiconductors. conductors allow the flow of electrons, where as the insulators doesn't allow the flow.
Reason is that in conductors the distance between conduction band and valence band is small where as in insulators it is high .when we look at semiconductors  which  allow partially the flow of electrons. These semiconductors are necessary in these days as they play an important role in modern day electronic goods.
Examples of semiconductors are silicon and germanium which have four valence electrons in their outermost shell.But we mostly use silicon because it has low leakage current(around nA) due to high forbidden gap between conduction band and valence band,and silicon has high temperature withstand capacity(around 1425c) compared to germanium .
construction:
Take a silicon piece and to the left most dope with p-type impurities and to the right, dope with n-type impurities.
Doping means adding of impurities(other metals) to the present one.
Ex:silicon with boran,aluminium or with phosphorus .By proper doping we can design the diode.
working:

when doping is done, a small junction is formed between the P and N regions before biasing .Reason is P-type has majority holes and minority electrons where as the N-type has majority electrons and minority holes due to force of attraction and repulsion small layer is formed and is called as "depletion layer".

=>when the potential at anode is greater than cathode then we can say diode is in "forward bias" and act like as closed switch with threshold voltage of 0.6V

Reason is that when positive potential(greater) is applied at anode due to force of repulsion between positive charges and holes(positive charges) the depletion layer contracts and allow the electrons to recombine with holes.

=>If we apply lower potential at anode compared to cathode then the diode acts as a "Reverse bias" and is  in open state that is it offers infinite resistance.
  Reason is that lesser voltage applied at anode will attract the holes(positive charges) towards negative charge and in turn increases the depletion layer so conduction stops.
=>Applications of a diode is it can act as a open or closed switch.

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wave propagation

Normally in the conductor free electrons are present due to which we have electric fields and if these are moving we also have the magnetic fields around the conductor. These electric and magnetic fields are responsible for the wave to propagate.normally we observe it in the case of antenna propagation,when antenna is fed by the current source due to charge and as this charge is flowing we have electric and magnetic fields which help in wave propagation.we have many types of antenna's in the past like yagi-uda antenna which has a dipole for which the current is fed,in this antenna another two parasitic elements are present called directors and reflectors. reflector is used to reflect back the back side radiation(opposite to the wave propagation) directors are used to place the radiation in the desired direction.we also have reflector antennas,horn antenna,microstrip antenna(mostly used in mobile phones),helical antenna and many more designing up.But the principle remains same.
The antenna height is determined by the wave frequency.if the wave is of smaller frequency it requires very long antenna(since wavelength is inversly proportional to frequency) and if the wave has higher frequency we can easily transmit,but due to tech development we developed some modulation techniques to transmit the wave having lower frequency.
Suppose assume that wave is propagating along x-axis,then we have the electric and magnetic fields in perpendicular direction to x-axis that is it may be along y or z-axes.In the direction of propagation we dont have any types of fields.further if electric field is along is y-axis,it will be independent of 'y' but is a function of 'x'.similarly if magnetic field is along z-axis,it will be independent of 'z' but will be function of 'x'.
 Based on the maxwell's equations we can decide the nature of wave in the charge ,free and conducting region.The nature of the EM waves vary from medium to medium and also upon the angle of incidence whether it is oblique or normal incidence.

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End of world

Now a days we may hearing the word about how the world is going to be destroyed. There are so many ways but i think these are only few that we should fear.
1.water :our planet earth consists of 75 percent of water and 25 percent of land . So we should think the major one is going to affect the minor in which we are leading the life. Among this most of the impure water is present in oceans and seas and the minimum quantity of pure water is present as ice caps. The land near to these would think that they are in danger because the water is melting due to this the land will get destroy. So that every individual should take part in controlling the global warming.
2.Earthquakes:now a days we are watching in our tv's that so many people are dying due to earthquakes . These days it is common the earthquake is occured at this place and at another place . We have device to estimate the amount of earthquake occured but we dont have the machines to discover it first. Animals are able to discover but man could not. So now there is a need to discover the machine and to minimise the destroy.
3.valcones:Due to this the region around is being damaged. Normally the region is forest . If forest gets destroy the equilibrium will be damaged so that at last man suffers. So we should careful in controlling this.
4.brimstone and fire from heaven:The fire coming from the heaven is the major possibility the our living planet will get destroy. This brimstone normally contains 98 percent of sulphur . The amount of damage we can see in the holy bible which is happened in the city called sodom. So the man should learn from this.
>conclusion:If we want to survive in the we should be in control of this four ways and we should search a way in protecting the beautiful planet earth.

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