Introduction to electronics:Part 1

Electronics:

Electronics is defined as the branch of science and technology which relates to the conduction of electricity through vacuum by electrons alone or through gases by electrons and ions.Basically electronics refers to the study of flow and control of electrons in vacuum,gases,semi conductors,conductors and the devices which uses these electrons.

To understand the concept of electronics,we need to study the basic structure of atom and its related concepts. 

                                                                                                                                                                     Atom is defined as the smallest particle of matter that possesses the  chemical properties of chemical element. Initially, the concept atom has evolved from the research on electricity.Electricity is a form of energy resulting from the existence of electric charge.Charge can be either positive or negative.The movement of these charged particles gives electric current. so by doing research on electricity, has motivated scientists to find out the concept of atom.

First Michael Faraday has the studied the passage of electricity through the liquid solutions . Then with this phenomena,the study of electrical nature of matter was made by J.J.Thomson .

J.J Thomson model:

He has focused his attention on the passage of electricity through gases.The studies made by Thomson on the electrical discharge through gases at low temperature established that atom composed of charged particles - electrons and protons.

He described his atomic model of an atom as a sphere of  completely positively charged matter in which electrons were embedded.His model could not explain all the properties of optical spectra of hydrogen and other elements.

Rutherford model:

In the year 1911, Rutherford has found that an atom consists of  a positively charged nucleus around which,the negative charged particles called electrons are moving by surrounding this central positive core. 

As a specific illustration of this model,let us consider a hydrogen atom.This simple hydrogen atom consists of a positively charged nucleus (a proton) and an electron.The charge of the proton is positive and is equal to the magnitude of electron.Therefore,an atom as a whole is electrically neutral.

Practically,as the proton carries the entire mass of the atom,it will substantially remain immobile,where as the electron will move about it in a closed orbit. Therefore,there exist a force of attraction between the proton and electron.This force of attraction between the proton and the electron,follows Coulomb's law.From the classical mechanics,the resultant closed path will be a circle or an ellipse under the action of such a force.Assume that the orbit of electron in the planetary model of an atom is a circle and the nucleus is supposed to be fixed in space.

Let us  calculate the radius of an electron in an orbit in terms of total energy(W) of the electron:

The force of attraction between the electron and nucleus is given by,
  
where,
the electronic charge e is in coulomb's
the seperation r between the two particles is in meters
force is in newtons
  is the permitivity of free space.

By Newton's second law of motion,this force of motion,this force must be set equal to the product of electronic mass 'm' in kilograms,electonic mass 'm' in kilograms and the acceleration towards the nucleus.Then
  ---------------> (1)

Further more,the potential energy of the electron at a distance 'r' from the nucleus is and the kinetic energy is given by . Then according to the law of conservation of energy,
-------------> eqn (2) where W is the energy in Joules.

Now, we can write eqn (1) as,

------------------------------> eqn(3)


substitute eqn (3) in eqn (2),we get


------------------------> eqn (4)
which gives the desired relationship between the radius and the energy of electron.

Eqn (4) shows that the total energy of electron is always negative.The negative sign arises because the potential energy has been chosen to zero when r is infinite.This expression also shows that the energy of electron becomes smaller as it approaches closer to the nucleus.

Rutherford model couldn't explain two points which are contradicting to what is normally happening in an atom:

1. If the accelerated electron loses energy by radiation,then the total energy of the electron continually decreases and it must spiral down into the nucleus.Then hydrogen atom cannot be stable,but most of the atoms are stable.

2.according to the classical electromagnetic theory,an accelerating electron must radiate energy at frequency equal to the mechanical frequency of orbiting electron and hence proportional to the angular velocity f the orbiting electron and hence proportional to the angular velocity of the electron. Therefore, as the electron spirals towards the nucleus,the angular velocity tends to infinity and hence frequency will also tend to infinity.This will result in a continuous spectrum with all possible wavelengths but many other atoms like hydrogen emit line spectra of fixed wavelength only.

Bohr's model:
Bohr model could successfully explain many of the atomic phenomena  which Rutherford model couldn't explain.Bohr took Rutherford model of an atom and tried to overcome the defects of the model. Bohr proposed the laws of classical mechanics and electromagnetic broke down within the atom.

The basic postulates of Bohr's theory are a combination of the ideas of classical theory and Planck's quantum theory of radiation.Bohr's postulates are as follows:

• Postulate 1:Not all energies as given by classical mechanics are possible,but the atom can possess only certain energies.While in states corresponding to discrete energies,the electron does not emit radiation,and the electron is said to be in a stationary or in a non-radiating state.
• Postulate 2:In a tranistion from one stationary state corresponding to a higher energy state to a lower energy state,with an associated energy W,radiation will be emitted.The frequency of this radiant energy will be given by,
• Postulate 3:Any stationary or non-radiating state is determined by the condition that is angular momentum of the electron in this state is quantized and must be integral mutiple of    .Thus 

Now, on combining these two with eqn (1) and (4) in rutherford model,we get,
 
   
For nth orbit,let the radius be    ,then,substitute the above equation in (1),







 -----> This equation represents the radius of electron in n th orbit.

To calculate the energy of the nth orbit,substitute the value of  in the equation(4) :





 ---------> This equation represents the total energy of an electron in nth orbit.

Atomic energy levels:

Though it is theoretically possible to calculate to calculate various energy states of the atom of simple elements,these levels must be determined indirectly for spectroscopic and other data for more complicated atoms.the experimentally determines energy level diagram for hydrogen is as shown below.,

fig., atomic energy levels of hydrogen atom

The lowest energy level (n=1) corresponds to normal unexcited state of hydrogen.This state is also called as ground state.In energy level diagram the lower energies(more negative) at the bottom while higher energies(less negative) are at the top.so it is easier to remove an electron from higher orbit than lower one.  By such a consideration,various electrons jumps between allowed orbits will be vertical arrows between the different  energy levels.The energy of radiated photon is greater when the length of the arrow is greater.The grater is the length of the arrow,higher is the frequency of emitted radiation. It is obvious from the figure that the various energy levels are separated  by forbidden energy gaps where the electrons cannot exist.

The photon nature of light:
In the transition process,the electron in an atom must lose an amount of energy equal to the difference of energy between the two states that is successively occupied.This energy appears in the form of radiation.According to Bohr's postulates,this energy is emitted in the form of photon of light.The term photon denotes the amount of radiant energy equal to the constant "h" times of the frequency.This quantized nature of an electromagnetic wave was first introduced by Planck in 1901,in order to theoretically verify black body radiation formula which is obtained experimentally.

The concept of photon radiation may be difficult to comprehend at first.Classically,it was believed that the atoms were systems that emitted radiation continuously in all directions.However it is not true from further concepts as the emission of light by an atom being a discontinuous process,so the atom is radiated only when the transition takes place from one energy level to another energy level.In this transition,it emits a definite amount of energy from one particular frequency,namely,one photon hf of light.





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