THERMOELECTRIC GENERATOR

Introduction: 

A working of Thermoelectric Generator - (TEG) is based on the direct interconversion of heat and electricity, The Seebeck Effect produces an electric current when dissimilar metals are exposed to a variance in temperature. 

Principle:

when two materials are exposed to different temperatures there is a resultant voltage between the two. Thermoelectric generators take heat input at one end of the material and then induce a voltage due to the difference in temperatures between them. 

There have been different materials that are being used in the thermoelectric generators and a majority of them are now using semiconductors. Thermoelectric power generators have the same basic configuration, as shown in the figure.

WORKING MODULE:

 A circuit containing thermoelectric materials which generate electricity from heat directly. A thermoelectric module consists of two dissimilar thermoelectric materials joined at their ends: an n-type (with negative charge carriers), and a p-type (with positive charge carriers) semiconductor. Direct electric current will flow in the circuit when there is a temperature difference between the ends of the materials.

Spark Chamber

       A Spark chamber consists of a set of conducting plates alternately connected to a source of high DC voltage - Fig 1

Fig - 1

      The chamber is filled with an inert gas. Sudden application of very high voltages to alternate plates. While the other are left at ground potential. results in very high electrical fields across the gaps. Electrical breakdown then occurs along the trails of ions. So the trajectory of a given particle through the system is marked by a series of sparks. The spark trails are photographed stereoscopically. If the chamber is located in a magnetic field, the charge and momentum of the particle can be determined from the curvature of the track. Vidicons are frequently used in place of photography.

      The spark chamber has an important advantage over the bubble chamber in that it can be rendered sensitive for only a very short time. Suppose, for example, that in an intense beam from a high-energy particle accelerator a rather rare particle is produced. Scintillation or Cerenkov counters can signal the production of this kind of particle. The pulse voltage is then applied to the spark chamber. The track of this particular particle can thus be determined. There is little likelihood that one of the much more numerous background particle will cause another spark track in the short time that the chamber is sensitive. Thus rare events and processes can be studied.

       The Scintillation Counters 

     One of the earliest methods of radiation detection was the spinthariscope (Fig-1). 

Fig - 1

      It consists of a small wire, the tip of which is dipped in Radium bromide(R) or any other radioactive salt. It is placed in front of a zinc sulphide screen  S and viewed through a microscope. When and α or β-particle falls on the zinc sulphide  screen. they produce light flashes which can by seen by a microscope (M) in a dark room. The visible luminescence excited in zinc sulphide by α-particles was used by Rutherford for counting the particles. The process of counting these scintillations through a low power  microscope is a tedious one and the limitations of observation with the eye restrict the counting rate to about 100 per minute. This process, whereby the energy of the particle is converted to light, is the basis of scintillation counter.

 

Fig - 2

       The main parts of a scintillation counter are shown in Fig. 2, the atoms of the phosphor are excited or ionised by the energy loss of an impinging  α,β or γ ray. When the  atoms return to their ground states, photons are emitted, in the blue and ultraviolet regions of the optical spectrum. The phosphors optically coupled to the envelope of a photomultiplier tube. The photons strike the photocathode, causing the ejection of photo-electrons Fig - 3. As these photo-electrons leave the photocathode, they are directed by a focusing electrode to the first multiplier electrode or dynode. The electrode has the property of emitting three, four or five electrons for every single electron which strikes its surface.

Fig - 3

    There may be from 10 to 14 such multiplier stages in a given tube. Hence, from the emission of one single electron from the cathode, a burst of one million electrons may impinge on the final stage in the tube ( the anode.) The output pulse from the photomultiplier is fed to a pulse amplifier followed by a scaler circuit.

 

Bubble Chamber

Bubble Chamber

Principle :

      We know that normally the liquid boils with the evolution of bubbles of vapour at the boiling point. If the liquid is heated under a high pressure to a temperature well above its normal boiling point. a sudden release of pressure will leave the liquid in a superheated state. If an ionizing particle passes though the liquid within a few milliseconds after the pressure is released. the ions left in the track of a particle act as condensation centres for the formation of vapour bubbles. The vapour bubbles grow at a rapid  rate and attain a visible size in a time of the order of 10 to 100μs. Thus in a bubble chamber, a vapour bubble forms in a superheated liquid. Whereas in a cloud chamber, a liquid drop forms in a supersaturated vapour. Thus an ionizing particle passing through the superheated liquid leaves in its wake a trail of bubbles which can be photographed.  

    A schematic diagram of a liquid hydrogen bubble chamber, operating at temperature of 27 K is shown in Fig 1.       

Fig 1.

   A box  of thick glass walls is filled with liquid hydrogen and connected to the expansion pressure system. To maintain the chamber at constant temperature, it is surrounded by liquid nitrogen and liquid  hydrogen shields. High energy particles are allowed to enter the chamber from the side window W.A sudden release of pressure from  the expansion valve is followed by light flash and camera takes the stereoscopic view of the chamber.

   The incoming beam triggers the chamber. The charge of the tracks can be identified by the direction of their curvature in the magnetic field applied over the bubble chamber. From the curvature and length of the track. the momentum and energy of the particle can be found. The bubble chamber is used to study particle interaction and to detect very high energy particles. 

Advantages :

 1. The density of a liquid is very large when compared to that of a gas of even high pressure. Hence the chances of collision of a high energy particle with a molecule of the liquid are very much greater.  Consequently there is a greater chance of their track being recorded. So the chances of recording events like cosmic ray phenomena are improved when compared with cloud chambers.

    

 2. The bubbles grow rapidly and as a result the tracks  are not likely to get distorted due to convection currents in the liquid.

   

 3. The bubble chamber is sensitive even to particles of low ionizing power.

Diffusion Cloud Chamber

Diffusion Cloud Chamber.....

The disadvantage of the cloud chamber lies in the fact that it needs a definite time to recover after an expansion. Hence it is not possible to have a continuous. Hence it is not possible to have a continuous record of events taking place in the chamber. This difficulty was removed by the introduction of the diffusion cloud chamber.

The outline of the apparatus is shown in diagram...

diagram    

If consists of a chamber containing a heavy get which is kept warm at the top and cold  at the bottom. Thermal gradient is maintained between the bottom and top of the chamber by external heating or cooling. The liquid (methyl alcohol) vaporises in the warm region. Where the vapour pressure is high.

The vapour  diffuses downwards continuously where the vapour pressure is low and condensation takes place. In available ions. The chamber remains continuously sensitive to lionizing particles until the supply of volatile liquid is exhausted. The system is illuminated by a strong source of light and the track of the particle is photographed by camera.

Nuclear weapons

Nuclear weapons

A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or from a combination of fission and fusion reactions

Nuclear weapons

Fission weapons. All existing nuclear weapons derive some of their explosive energy from nuclear fission reactions. Weapons whose explosive output is exclusively from fission reactions are commonly referred to as atomic bombs or atom bombs (abbreviated as A-bombs).



How does a nuclear bomb explode?

"This is known as a chain reaction and is what causes an atomic explosion. When a uranium-235 atom absorbs a neutron and fissions into two new atoms, it releases three new neutrons and some binding energy. Two neutrons do not continue the reaction because they are lost or absorbed by a uranium-238 atom."


How did the atomic bomb work?

"Nuclear fission produces the atomic bomb, a weapon of mass destruction that uses power released by the splitting of atomic nuclei. When a single free neutron strikes the nucleus of an atom of radioactive material like uranium or plutonium, it knocks two or three more neutrons free."


Why was the A bomb dropped?

"The reasons Hiroshima was chosen as the target for the A-bombing are assumed to be the following. The size and the shape of the city was suited to the destructive power of the A-bombs. Because Hiroshima had not been bombed, ascertaining the effects of the A-bomb would be relatively easy."


What is a nuclear weapon?


A nuclear weapon is an explosive device that derives its destructive force from nuclearreactions, either fission (fission bomb) or a combination of fission and fusion (thermonuclear weapon