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.

Van De Graaff Generator

Van De Graaff Generator 

In this electrostatic accelerator, electric charge is carried continuously to an insulated hollow spherical conductor (S) and delivered to it.

The potential of the hollow sphere can thus be gradually raised.

When the potential has attained a sufficiently high value, it can be used to accelerate positive ions in the accelerator tube.

A well-insulated endless belt B is rapidly rotated between two pulleys P₁  and P₂ by means of a motor [Fig 1].

Fig 1

The upper pulley P₂ is placed inside a hollow spherical conductor S.

Near the lower pulley P₂, there is a metallic comb C connected to a source of steady voltage of 50 kV.

Positive charge is sprayed over the moving belt by the metallic comb C.

The belt moving upward carries this charge into the hollow sphere.

The positive charge, on entering the sphere S. induces an equivalent negative charge on its inside and a corresponding positive charge on its outside.

The negative charge is sprayed by the comb C on the the belt, thus neutralizing the positive charge on it.

The net result is that the positive charges are in effect transferred from the moving belt to the outside of the sphere. The belt, returning to the bottom, is charged again and delivers up the charge, when inside the sphere.

Charges are thus accumulated and distributed on the surface of the sphere. As the charges on the sphere accumulates, its potential rises to a very high value.

The charged particles which are to be accelerated are produced by a gas discharge ion source I placed inside the hollow sphere S over the acceleration tube G.

The accelerating tube is evacuated and contains a number of insulated metal cylinders (C) arranged with short gaps between them [Fig 2].

 

Fig 2

 

The H.T. is applied to the topmost cylinder near the ion source and the bottom-most cylinder is earthed.

 A potential gradient exists from the top to the bottom. The ions are accelerated at the gaps between the cylinders, where intense electric field exists.

The highly accelerated ions are deviated by a suitable and adjustable magnetic field M and made to strike the target (T) at the bottom of the tube.

By reversing the potential of the spray voltage, the Van De Graaff generator can also be used to accelerate electrons.

Atom bomb

Atom Bomb

The  principle of fission is made use of in the construction of the atom bomb. As atom bomb consists essentially of two pieces of ₉₂U²³⁵ (or ₉₂Pu²³⁹) each smaller than the critical size and a source of neutrons. The subcritical masses of U²³⁵ in the form of hemispheres are kept apart by using a separator aperture (Fig 1).

Fig 1 

 

When the bomb has to be exploded. A third well fitting cylinder of  U²³⁵ ( whose mass is also less then critical mass) is propelled so that it will fit in or fuse together with the other two pieces. Now the total quantity of U²³⁵ is greater than the critical mass. Hence an uncontrolled chain reaction takes place resulting in a terrific explosion.

The explosion of an atom bomb releases tremendously large quantity of energy in the form of heat, light and radiation. A temperature of millions of degrees and a pressure of millions of atmospheres are produced. Such explosions produce shock wave, They are every dangerous because the waves spread radioactivity in air and cause loss of life. 

The release of dangerously radio active γ-rays, neutrons and radioactive materials presents a health hazard over the surroundings for a long time. 

The radioactive fragments and isotopes formed out of explosion adhere to dust particles thrown into space and fall back to earth causing a radiation “fall-out”, even at very distant places.

 

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.

Quantum mechanics 

Quantum mechanics can be thought of roughly as the study of physics on very small length scales, although there are also certain macroscopic systems it directly applies to. The descriptor “quantum” arises because, in contrast with classical mechanics, certain quantities take on only discrete values. However, some quantities still take on continuous values, In quantum mechanics, particles have wavelike properties, and a particular wave equation, the Schrodinger equation, governs how these waves behave.

The Schrodinger equation is different in a few ways from the other wave equations we’ve seen in this book. But these differences won’t keep us from applying all of our usual strategies for solving a wave equation and dealing with the resulting solutions.

In some respect, quantum mechanics is just another example of a system governed by a wave equation. However, although it is fairly straightforward to deal with the actual waves, there are many things about quantum mechanics that are a combination of subtle, perplexing, and bizarre. To name a few: the measurement problem, hidden variables along with Bell’s theorem, and wave-particle duality. 

Even though there are many things that are highly confusing about quantum mechanics, the nice thing is that it’s relatively easy to apply quantum mechanics to a physical system to figure out how it behaves.

 Classical mechanics

Classical mechanics deals with the question of how an object moves when it is subjected to various forces, and also with the question of what forces act on an object which is not moving. The word classical indicates that we are not discussing situations in which an object moves with a velocity which is an appreciable fraction of the velocity of light or phenomena on the atomic scale.

The description of atomic phenomena requires quantum mechanics, and the description of phenomena at very high velocities requires Einstein’s Theory of Relativity. Both quantum mechanics and relativity were invented in the twentieth century; the laws of classical mechanics were stated by Sir Isaac Newton in 1687. 

The laws of classical mechanics enable us to calculate the trajectories of bullets and baseballs, planets and space vehicles. Using these laws we can predict the position-versus-time relation for a cylinder rolling down an inclined boat or for an oscillating pendulum and can calculate the tension in the wire when a picture is hanging on a wall.

The practical importance of the subject hardly requires demonstration in a world which contains buildings, automobiles, aero planes, ballistic missiles, and bridges, for a person who does not have any professional reason there is a compelling intellectual reason to study classical mechanics: this is the example par excellence of a theory which explains an incredible multitude of phenomena on the basis of a minimal number of simple principles.

 

Important Questions for Class 12 Physics

1. Why do the electrostatic field lines not form closed loops?

Answer:

Electric field lines do not form closed loops because the direction of an electric field is from positive to negative charge. So one can regard a line of force starting from a positive charge and ending on a negative charge. This indicates that electric field . lines do not form closed loops.

2. Why do the electric field lines never cross each other?

Answer:

The electric lines of force give the direction of the electric field. In case, two lines of force intersect, there will be two directions of the electric field at the point of intersection, which is not possible.

3. What is the electric flux through a cube of side 1 cm which encloses an electric dipole? 

Answer:

Zero because the net charge of an electric dipole (+ q and – q) is zero.

4. Why are electric field lines perpendicular at a point on an equipotential surface of a conductor?

Answer:

If the electric field lines were not normal to the equipotential surface, it would have a non-zero component along the surface. To move a unit test charge against the direction of the component of the field, work would have to be done which means this surface cannot be equipotential surface.
Hence, electric field lines are perpendicular at a point on an equipotential surface of a conductor.

5. How does the electric flux due to a point charge enclosed by a spherical Gaussian surface get affected when its radius is increased? 

Answer:

The electric flux due to a point charge enclosed by a spherical gaussian surface remains ‘unaffected’ when its radius is increased.

6 .Does the charge given to a metallic sphere depend on whether it is hollow or solid? Give reason for your answer. 

Answer:

No, it does not, because the charge resides only on the surface of the conductor.