PHYSICS

  LASER “Light Amplification by Stimulated Emission of Radiation”  Characteristics of Laser:  1. High ly coh erent:   All the electrons which are excited to the higher energy level jump to same lower energy level at the same time during lasing process(lasing actio n). 2. Laser is monochromatic:   The light consists single wavelength 3.Laser consists high intensity. They can travel through any longer distance with out dispersion.  4. It propagates in the form a narrow beam.  laser can penetrate through water. 5. Laser consists directionality :   ordinary light propagates in all directions but laser propagates along one direction only. 6. Laser is not absorbed by water.    ordinary light propagates in all directions but laser propagates along one direction only. 7. Laser produces heat and increase the temperature of the substance on which it incidents. A very high temperature about 100K can be produced using lasers.                   Applications of Lasers:   1

    ANTI  FERROMAGNETISM  In anti-ferromagnetic substances the spin of dipoles (direction of magnetic moment )of neighborhood domain is equal and opposite in direction.  Examples are Mns, MnO4, FeO, FeCl2 etc.  The susceptibility of anti-ferromagnetism is positive. These materials exhibit a very low external magnetic field.  The susceptibility decreases with temperature of the material.  They obey Curie’s law. It means if their temperature is increased, they behave as para magnetic substances beyond Curie temperature.                                        FERRIMAGNETISM (OR FERRITES)  FERRI MAGNETIC MATERIALS ARE ALSO CALLED FERRITES.  These materials produce strong external filed and hence they are used to prepare strong electromagnets.  The direction of magnetic moment of neighborhood domain is not equal but it is in opposite direction. The examples are MnFe2O4, FeFe2O4 CuFe2O4 etc.                       Applications of ferrites:  They are used in the

WEISS THEORY OF FERRO MAGNETISM Ferro magnetic substances have strong magnetic properties such as permeability and susceptibility. The reason for these properties is every atom in the Ferro substance is like a small magnet. Hence every  atom is a dipole. According to Weiss theory a group of small atomic magnets form a region called domain. Within a domain the dipole direction of group of atomic magnets is same. But the direction of dipoles for  all domains is random. In the normal condition i.e. in the absence of external magnetic field all the domains form a closed chain. When the substance is placed in an external field and the field is increased then the substance is  magnetized. The dipole arrangement of domains gradually is aligned along the direction of external field. Then the domain chain is broken and slowly the substance gets magnetized. The Ferro substance attains  crystal structure. In this way the Ferro substance possess an internal magnetic field and hence mag

   HYSTERESIS CURVE ( I – H CURVE) or B-H CURVE Hysteresis is the phenomenon of lagging intensity of magnetization with  external field  during the process of magnetization of a Ferro specimen. The graph which is drawn between intensity of magnetization ( I ) and the external field H during the  process of magnetization of a ferromagnetic specimen is known as HYSTERESIS CURVE OR I-H CURVE. We can draw the graph for magnetic flux density (B) developed inside the Ferro specimen instead of I. Hence it is also called B-H curve. In this process a ferromagnetic specimen is placed in an external magnetic field. The field strength H can be  varied. The intensity of magnetization ( I ) or the magnetic flux density ( B )developed with in the Ferro  specimen is observed by varying the external field H. 1. In this process the external field is first increased it is observed that the intensity of magnetization of  Ferro specimen is also increased.  At a point of external field H, the v

CONDUCTION THROUGH PURE SEMICONDUCTORS CONCEPT OF HOLE FORMATION A pure semiconductor is called intrinsic semiconductor. The conductivity through a pure semiconductor at room temperature is zero because it does not contain any free electron at room temperature. The conductivity is produced through a pure semiconductor if its temperature is increased. A semiconductor like Ge or Si has crystalline structure Germanium and silicon has 4 unpaired valency electrons. Each atom of Ge or Si has valency equal to 4.  These four valency electrons form four weak covalent bonds with four nearest electrons of neighborhood atoms.  At room temperature all the four electrons are  complete and no free electron is available. This is continued throughout the crystal lattice. Hence there will be no conduction of charges through a pure semiconductor at room temperature. W hen the temperature of substance is increased, a few valency electrons get energized to come out of their orbit.  Hence th

P - Type and N – Type semiconductors The conduction through a pure or intrinsic semiconductor is very low. The reason is the number of free electrons is equal to the number of holes. To increase the conductivity through the semiconductor either free electrons or holes must be greater in number. Through the process of doping the conductivity of semiconductor is increased. Doping It is the process of adding impurities like the atoms of other group elements to a pure or intrinsic semiconductor. There are two methods of doping. 1.Adding the atoms of 3rd group element 2.Adding the atoms of 5th group element. We choose 3rd and 5th group elements to dope a semiconductor because the valency of a semiconductor like germanium and silicon is 4.  In the process of doping for every 1000 atoms of germanium or silicon one atom of 3rd group or 5th group element is added by diffusion method. This type of doping is moderate. Note: After adding the impurities the semiconductor wi

Classification of  Intrinsic and Extrinsic semiconductors semiconductors are the substances which do not conduct current at room temperature. conduction takes place through a semiconductor  when the temperature of the semiconductor  is slightly increased. germanium and silicon are the examples of semiconductors. semiconductors are classified into two types. 1. pure semiconductor ( intrinsic semiconductor) 2.impure semiconductor(extrinsic semiconductor) Intrinsic semiconductors 1. An intrinsic semiconductor is extremely pure semiconductor. 2.It consists purely the atoms of semiconductor only. 3.The examples are pure germanium and silicon. 4.At room temperature intrinsic semiconductor  does not contain free electrons and holes. The valency of germanium or silicon is 4. Hence every atom of germanium or silicon forms. four covalent bonds with 4 neighbor atoms. at room temperature all 4 bonds are completely filled. 5.Free electrons are rel

CLASSIFICATOIN OF  MAGNETIC SUBSTANCS ALL THE SUBSTANCES ARE CLASSIFIED INTO THREE TYPES BASED ON THEIR MAGNETIC PROPERTIES. THEY ARE 1.  DIA, 2. PARA AND 3. FERRO MAGNETIC SUBSTANCE . 1  Dia substances are repelled      by the magnet.     Para substances are feebly  attracted by the magnet.     Ferro substances are strongly attracted   by the magnet  2 Example of dia substances:  Bi, Zn, Ag, Cu, Au . Example of para substances Al, Pt, Mn, CuCl2,O2. Example of ferro magnetic substances : Fe,Co, Fe2O4,Ni 3 Susceptibility of dia substances is negative.         Susceptibility of  para magnetic substances is positive, low. Susceptibility of ferro substances is  positive , high 4 Permeability of  dia magnetic substances is  less than 1.       Permeability of  para magnetic substances is greater than1         Permeability of ferro magnetic substances is  greater than1 5. When a dia substance is placed in the form of liqui

Curie-weiss in ferro magnetism Ferromagnetic substances follow curie Weiss law. This law describes the change in susceptibility of ferromagnetic substances with their temperature. Susceptibility of a magnetic substance is equal to the ratio of intensity of magnetization to the external field applied when it is placed in an external magnetic field. This is denoted by a symbol                       χ = I / H It varies with the absolute temperature of the magnetic substance. According to curie Weiss law the susceptibility of ferro substances is inversely proportional to the absolute temperature.                           χ ∝ 1/ T It is observed that above a temperature called curie temperature susceptibility of ferromagnetic substance will be very less and so that the ferro substance loses its high intensive properties and behave as para magnetic substance. Curie temperature is denoted by Tc. The value of curie temperature varies with the substance. Examples :

Langevin theory on  para magnetism Para magnetic  substances have similar properties as Ferro magnetic substances  but with less intensity. The example  are aluminium, platinum, sodium, manganese , calcium, etc . Para magnetic substances have permanent magnetic dipole  moment . The dipoles are randomly arranged in the absence of external magnetic field. When an external field is applied the dipoles of para substances align in the direction of external field. The alignment of dipoles in the direction of external field depends on two factors. 1.   The magnitude of external field 2.    The temperature ( thermal agitation) of the atoms or molecules of the para substance Let the number of atoms or molecules per unit volume of para substance is equal to   N. The dipole moment of each atomic or molecular magnet = M And the angle between the direction of field and the dipole of pare substance = ϴ Then the potential energy of the dipole in the direction of external

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