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Old Thursday, October 27, 2005
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--15.CERAMICS:

Ceramics include a vast variety of inorganic ,non mettalic materials which require high temperature heating for preparation.The most famous forms of ceramic are pottery,bricks,tiles and sanitary ware.Infact the ceramics have a wide variety of usage in most sectors especailly in the industrial sector.

RAW MATERIAL:

The raw material available for ceramic preparation is usually the clay which is is found benetah the top soil .This material is formed through the break down process of rocks affected from the chemical procees and the weather conditions.This raw material is very cheap and can be found in huge quatity.
This raw material when mixed with water can be shaped and moulded in any form according to the requirements or usage.It becomes hard at heating thus making it more durable and strong.

KINDS:

Three kinds of pottery are in use at the present times.
1.EARTHENWARE
2.STONEWARE
3PORCELAIN

EARTHENWARE:One of the cheapest and common style of pottery is earthenware.It is easy to prepare and is heated at low temperature.However it is not durable and is dull,porous and absorbent.

STONEWARE:This kind of pottery is ectremely hard and strong as is emplyed in the making of heavy dishes or jugs ,sanitary wares etc.Howver it can hold liquids and is semi vertious and glass like unlike earthenware.

PORCELAIN:This is the finest and the most refined form of pottery.It is translucent and strong light can shine through it.Porcealin is also called chinaware as the original technique originated from China.


--17.SEMI CONDUCTORS:

Semi-conductors are materials with an electrical conductivity that increases with increasing temperature, a trend that is opposite to that of metals. Semi-conductors characteristically have a band gap between the valence and conduction bands that is smaller than that found in the insulators. The reason the conductivity increases is because as the temperature increases more electrons become thermally excited and are able to jump the band gap between the valence and conduction band. An example of this is silicon.

n-Type Conductivity


When a foreign atom with an excess of electrons is added to a pure semi-conductor, the result is a n-type semi-conductor, so named because the charge carriers are negative. This increases the conductivity because a donor band, which is filled with electrons, is introduced near to the conduction band in the band gap. This greatly decreases the band gap which the electrons must jump. Therefore, more electrons are able to get to the conduction band and hence a greater conductivity is the result. An example of an n-type semi-conductor is germanium doped with phosphorous.

p-Type Conductivity

When foreign atoms with less than 2N electrons are added, the result is a p-type semi-conductor, so called because the charge carrier is a positive hole. The foreign atoms create an acceptor band very close to the valence band that is empty. The result is that the band gap is deceased between a full and empty band. Electrons are then able to easily jumpfromthe valence band into the acceptor bands where they are trapped creating positive holes in the valence band. These positive create a means for the electrons to move within the valence band, thus increasing the conductivity.

--18.RADIO:

Radio is based on the principle that electrical signals have the capacity of travelling without the wire.Radio signals are carried by electromagnetic waves which travel through space at a speed of light.

The sound waves enter through a microphone in which a coil ,a metal ribbon vibrates to change spund in an electric current.This signal of sound combines with a carrier signal which is at higher radio frequncy.The carrier is modulated by audio frequency signal.This modulated carrier signal get transfers to transmitting aerial where radio waves are emitted in all directions.

The received waves are fed into a radio frequency amplifier to stengthen,They pass to a detector which seperates the audio frequency signal from the carrier wave.The currents that are obtained ae identical to those that left the microphone at the broadcasting station.They are amplified and fed to the loudspeaker.The loudspeaker acts like a microphone,similar to the process that produces sounds like the original sound.
--
19.TELEVISION


The tv works through the electromagnetic signals that are transmitted,received and converted back to original patterns.The sound transmission is very much similar to radio.In picture transmission the fundamental component is the camera which is responsible for changing the image into electrical impulses..The cathode ray tube at teh other end converts the apttern of electrical impulses into visible images.

Inside the TV camera an illuminated plate emits electrons.The electrons travel to a plate which is called a target plate.The electrical pattern that produces afterwards is transmitted to to the transmitter where the synchronizing pulses are added.Before the final output is fed to the transmitting aerials the sound signal is added.

For transmission VHF and UHF frequencies are used .The receiver is based on the superheterodyne principle,the sound,the vision are received at the seperate intermediate frequency amplifiers,detectors and output stages.the electron beam is made to scan the screen of teh cathode,ray tube and in step with the beam in the tv camers.A picture is then received on the screen.
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