## Thursday, January 26, 2012

### Slideshow on Radiation detectors given to GET 2012 batch.

GET 2012 batch can download the pdf file and can make any queries on subject in the comments box below.

## Saturday, January 21, 2012

### How does a Light Emitting diode (LED) work?

Light Emitting Diode (LED) is always operated in forward bias mode.

Direct band semiconductors are used for LEDs.

Before applying voltage

After applying voltage

At sufficient voltage, electrons gain sufficient voltage so that they combine with holes to give light which has color that depends on energy gap of chosen semiconductor.

The wavelength of emitted light depends on energy gap and is expressed as 1.24 / Eg; where Eg is energy gap.

## Friday, January 20, 2012

### Equivalent circuit of a capacitor

The equivalent circuit of an actual capacitor shown below indicates that there are other characteristics besides capacitance to be considered.

i) Inductance (L) of the leads and plates
ii) Resistance (R1) of dielectric (leakage resistance)
iii) Resistance (R2) of leads and plates (effective series resistance)
iv) C is measured capacitance

## Sunday, January 15, 2012

### Is there energy associated with intrinsic inertia, the rest mass?

We could have the object at rest to annihilate with an "anti-object", producing radiation possessing energy.
So, yes, there is energy associated with the rest mass.

while creating an atom, various particles like electrons, protons, neutrons are to be brought in together. Every bit of energy that goes into creating the object would be accompanied by an increase in inertia.

According to relation E = ΜC^2

The following figure sketches this symbolically. Starting from zero for both the object's energy and its inertia and adding up increments, we emerge with E=MC^2.

The attribute energy is always accompanied by attribute inertia.

In 1905, Einstein summarized his theoretical discovery with the sentence,

" The mass [i.e inertia] of a body is a measure of its energy content."

Energy , in all forms has property of inertia, the reluctance to undergo a change in velocity. The more energy that went into forming a body, the more inertia the body has.

## Friday, January 13, 2012

### Definition of Ampere

It is the constant current which, if maintained in two straight parallel conductors of infinite length and of negligible cross-section and placed one meter apart in vacuum, would produce between these conductors a force equal to 2 x 10 ^ -7 N/m of length.

## Monday, January 2, 2012

### How is it proved that alpha rays are nuclei of Helium atoms?

The identification is made by Rutherford and Royds in 1909. By spectroscopic method, they found traces of Helium in a pure sample of Radon gas, which is an alpha emitter. When an electric discharge was passed through a tube containing pure radon gas, initially only characteristic Radon lines appeared.

After a day, Rutherford found that Radon lines became somewhat weaker and new lines started appearing. These new lines were identified with Helium spectrum. As the days passed, the Radon spectrum began weakening while Helium lines grow brighter.

Thus, for first time, people saw decay of an element (Radon) and birth of a new element, Helium nuclei. Such a transformation in which a parent element gives rise to a new element called daughter product by emitting radioactive rays is called radioactive transformation.

### What are nano materials?

These are materials whose dimensions are less than 100nm. They act as one dimensional systems or zero dimensional.For comparison, a human hair is about 50,000nm in diameter, while a smoke particle is about 1000nm in diameter.

The term nano materials is less specific since it refers to coatings that are less than 100nm.

The nano particles consist only thousands of atoms. They are called quantum dots.

Methods of Synthesis of Nano Structured Materials:

Synthesis and assembly strategies can be accommodated from Liquid, Solid or Gas Phase

i) By employing chemical or physical deposition approaches.

ii) Relying on either chemical reactivity or physical compaction to integrate nano structure building blocks to form final nano material structure.

Variety of techniques are
i) Assembling from nano building blocks
ii) Powder/Aerosol compaction
iii) Chemical Synthesis
iv) Mechanical attrition
v) Lithography/Etching

These above techniques can be classified into two approaches:

i) Bottom up Approach: Here first building blocks are formed and then assembled to form final material.

ex:- Aerosol /Powder technique in which powder components are compacted into final material.

This approach has been widely used in formation of structural composite materials.

ii) Top-Down Approach: Here first a suitable nano material is taken and then sculpted according to functionality of material. This approach is very similar to techniques employed semiconductor industries in forming devices from electric substrate by etching a pattern from it.Lithography/etching, Ball milling fall in this category.

Classification of Nano Crystalline Materials:

i) Nano crystalline materials are single phase or multi phase poly crystals, the crystal size of which is of order of few (1-100nm) is at least one dimension.

ii) They can be equiaxed and can be formed nano strucuture Crystallites -3D nano structures

iii) They can consist of lamellar structure and will be termed as layered nano structure-1D

iv) They can be filamentary in nature - 2D

v) They can be clusters or cluster assembly - 0D

### Why statistical mechanics was introduced?

When we consider bodies at macroscopic level they consist of uncountable atoms or molecules i.e about 10^23 atoms/gm mole. In such cases we cannot predict the result of interactions between atoms with the help of ordinary classical laws of motion. In order to solve such a situation, a new branch of science called statistical mechanics is introduced.

Statistical mechanics is the branch of science which establishes the interpretation of macroscopic behavior of system in terms of its microscopic properties. It doesn't deal with motion of each particle but it takes into account the average or most probable properties of system with out going into interior details of characteristics of its constituents.

The larger the number of particles in the physical system considered, the more nearly correct are statistical predictions.

Before the advent of quantum theory, Maxwell, Boltzmann, Gibbs etc applied statistical methods making the use of classical physics. These statistical methods are known as "classical statistics" or "maxwell boltzmann statistics".

These statistics explained successfully many observed physical phenomenon like temperature, pressure, energy etc; but couldn't explain adequately several other experimentally observed phenomenon like black body radiation, specific heat at low temperature etc.

In order to explain such phenomenon "quantum statistics" was introduced and developed by Fermi, Dirac, Bose, Einstein with new approach by using new quantum idea of discrete exchange of energy between system.