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Modern Electron Theory

     Modern research has ebtablished that all matter whether solid, liquid, or gaseous, consists of minute particles called molecules which are them selves made up of still minute particles known as atoms. Those substances whose molecules consist of similar atoms  are known as elements (as shown in figure 1)
Figure 1 ( http://gb.scientificgems.wordpress.com/)
and those whose molecules consist of dissimilar atoms are called compounds (as shown in figure 2). 
Figure 2 (https://prodiipa.wordpress.com/)
An atom is taken to consist of the following: 
1). It has a hard central core known as nucleus. It contains two types of particlesor; one is known as proton and carries positive charge, the other is neutron (discovered by Chadwick in 1932), which is electrically neutral i.e. it carries no charge though it is as haevy as proton. The protons and neutrons are very closely held together with tremendous forces. 
2). Revolving round the relatively massive nucleus, in more or less elepptical orbits (or shells) are infinitesimally small particles known as electrons. These electrons carry the smallest negative charge and have negligible mass.  The mass of an electron is approximately 1/1890 that of a proton.
 Figure 3
     Such a view of an atom, known as Bohr-Rutherford model, is shown figure 4. It has been found that an atom is like a miniature solar system,  a heavy positively charged nucleus taking the place of the sun at the center with orbital electrons acting the planets. The planetary electrons revolve at distance which are much greater than the size of nucleus or electron themselves, hence most of the space occupied by an atom is empty, In fact, a solid may be thought of as a sponge-like sturucture in which nucleus and electron occupy but little of space taken up by solid. 

  Figure 4
    It has also been found that the effective diameters of the atoms are of the order of 10­­-10 metre and the diameter of their nuclie of the order 10-15 metre. Hence the diameter of an atom is roughly 100,000 times greater than the diameter of the nucleus. Some roughly idea of the vast emptiness existing with in an atom can be got by imagining that model of hydrogen atom may consist of cricket ball with a small soap bubble revolving round it at a distance of 3 km or so! 



    The particles discussed above form the fundamental bricks of which all matter is made. Atoms of all substances consist of identical protons, neutrons, electrons, and mesons etc., the only difference being in their number and relative configuration. 
     It has been found that the positive charge on a proton is numerically equal to the negative charge of an electron. Normally an atom is electrically neutral, becouse it consists of as many protons as electrons. The number of proton in the nucleus of an atom gives the atomic number of the substance whose atom it is. The total weigh of nucleus (i.e. protons plus neutrons) is called the atomic weight. If the number of protons in a nucleus is changed, than transmutation of one element into another can be achieved.
      The simpliest atom is that of hydrogen. It consist of one positive proton and one orbital electron as shown in figure 5. The haviest atom is that of an element which is called Kurchatovium by Russians and Rutherfordium (104 360) by americans. It has 104 electrons, 104 protons and 156 neutrons as shown in figure 3. 
Figure 5
The following importan points about atomic structure should be understood clearly: 
(1) the mass of a proton is1,66 x 10-37kg and that of an electron 9,02 x 10-31kg. Though the charge carried by an electron is natural unit of electricity, yet it is so extremely small that to adopt it as a unit of electricity would be like adopting the grain as unit for measuring sand. The unit of charge or quantity of electricity is one coloumb which is equal to the charge of 6,242 x 1018 electrons. Hence the charge of a single electron is 1/6,242x1018 i.e. 1,602 x 10-19 coulumb. 
(2) The orbit are more or less elliptical in shape and lie in all planes and not in one, though for convinience. 
(3) The maximum number of the electrons possibel in one extranuclear orbit or shell is fixed. Counting these orbits from nucleus outward, the firs orbit can have a maximum number of 2 electrons; second orbit 8 electrons; third orbit 18 electrons; fourth orbit 32 electrons and so on. For example copper atom has 29 electrons which will be distributed as follows.
1st orbit : 2 electrons         2nd orbit : 8 electrons 
3nd orbit : 16 electrons      4nd orbit : 1 electron

"Read also: Nature of Electric"

   The centripetal force necessery to keep electrons rotating in electrical orbit round the nucleus is supplied by the force of attraction between their charges as given by coloumb's laws. it is obvious that nearer the the electron is to the neuleus, greater is the force with which it is bound to it. the electron in the outermost or bit experience a very weak force of attraction for two reason: 
(a) force varies inversely as the square of the distance between two charges. 
(b) the presence of large nuber of electrons inthe intermediate or bits act as a partial screen berween        the nucleus and the outermost electrons. this screening or shield action result in reduced attraction between the two. in fact, they can be hardly said to be attached to one parent atom. they freely move from one atom to another and behave beru much like the molecules of a gas in a draught-free room. they wander about with random motion between atoms, continously colliding with one another but not movinf in any particular directions. these free-moving and unattached electrons form what is known as electron gas. the condition of these electrons in a copper wire is shown drammatically in figure 6 below. 
Figure 6
the atoms are arranged in a particular pattern called crystal lattic. the blu sign indicate that they are charged positevely (becouse of having lost come electrons) i.e. they are now ions. although these ions can oscillate about mean position, yet for the present, they are shown stationary. The electrons shown as black dots with arrows, wander about in all sorts of manner and directions. When this wire is joined across the terminals of a battery, the electrons experience an attractive force due to anode and a repulsive force due to cathode, with the result that they start drifting from cathode to anode as showing in figure 7
Figure 7
when some external force (potential different) is applied to these atoms, the outermost one or more electrons get easily detached from the parent atom and start drifting along and so give rise to a flow of electrons.




     This continuous flow of electrons contitutes an electric current. It is found that those substances whose atoms have theis outermost orbit incomplete act as good conductors of electricity i.e. they permit an easy detachment of their outermost electrons and offer very little hindrance to their flow through their atoms. Such substances are know as good conductors. But substance whose electrons are rigidly held to their atoms are termed as bad conductors. In their case, a very large force (i.e. potential different) is required to detach their electrons and even then the number of electrons detached and set drifting is comparatively small. Material like germanium, silicon and silicon carbide etc. Whose resistances at ordinary temperatures lie in between those of typical metals and typical insulators are called semi-conductors. suppose that in conductor:
    'n' is the number of free electron available per m^3 of the conductor material.
    'v' is the axial drift velocity  (m/s)
    'dt' is time (s)
so the distance would be counted by:

If 'A' is are of cross section of the conductors, the volume would be counted by: 
 
so we will know the number of electrons containing in conductor using this formula: 
Obviously, all this electrons will cross the normal section in time dt. If 'e' is the charge of each electron, then total charge (dq) which cross the section time in 'dt' .
since current is the rate of flow of charge, it is given as: 
current density can be counted by: 
assuming a normal current density of 
J = 1,55 x 106 A/m2
n= 1029 for a copper conductor, and 
e = 1,6 x 1019coulomb, 

we get 1,55 x 106 = 1029x1,6x10-19x v; so v = 9,7 x 10-5m/s = 0,58 cm/min

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